Under the influence of amylase, they are broken down in the oral cavity. Nb! Digestion of carbohydrates begins in the mouth. Composition and functions of saliva

For many people, food is one of the few joys in life. Food should indeed be a pleasure, but... the physiological meaning of nutrition is much broader. Few people think about how amazingly food from our plate is converted into energy and building material, so necessary for the constant renewal of the body.

Our food is presented different products, which consist of proteins, carbohydrates, fats and water. Ultimately, everything we eat and drink is broken down in our body into universal, smallest components under the influence of digestive juices (a person secretes up to 10 liters of them per day).

The physiology of digestion is a very complex, energy-consuming, remarkably organized process, consisting of several stages of processing food passing through the digestive tract. It can be compared to a well-regulated conveyor belt, on the well-coordinated operation of which our health depends. And the occurrence of “failures” leads to the formation of many forms of diseases.

Knowledge is a great power that helps prevent any violations. Knowledge of how our digestive system works should help us not only enjoy food, but also prevent many diseases.

I will act as a guide on a fascinating sightseeing tour, which I hope will be useful to you.

So, our various foods of plant and animal origin go through a long journey before (after 30 hours) the final products of its breakdown enter the blood and lymph and are integrated into the body. The process of digesting food is ensured by unique chemical reactions and consists of several stages. Let's look at them in more detail.

Digestion in the mouth

The first stage of digestion begins in oral cavity, where food is crushed/chewed and processed by a secretion called saliva. (Up to 1.5 liters of saliva are produced daily.) In fact, the digestion process begins even before food touches our lips, since the very thought of food already fills our mouth with saliva.

Saliva is a secretion secreted by three paired salivary glands. It is 99% water and contains enzymes, the most important of which is alpha-amylase, which is involved in the hydrolysis/breakdown of carbohydrates. That is, of all food components (proteins, fats and carbohydrates), only carbohydrates begin to hydrolyze in the oral cavity! Salivary enzymes do not act on either fats or proteins. The process of carbohydrate breakdown requires an alkaline environment!

The composition of saliva also includes: lysozyme, which has bactericidal properties and serves as a local protective factor for the oral mucosa; and mucin, a mucus-like substance that forms a smooth, chewable bolus of food that is easy to swallow and transport through the esophagus into the stomach.

Why is it important to chew your food well? Firstly, in order to grind it well and moisten it with saliva, and start the digestion process. Secondly, in Eastern medicine, teeth are associated with energy channels (meridians) passing through them. Chewing activates the movement of energy through the channels. The destruction of certain teeth indicates problems in the corresponding organs and systems of the body.

We don't think about the saliva in our mouth and don't notice its absence. We often walk around for a long time with a feeling of dry mouth. And saliva contains a lot chemical substances, necessary for good digestion and preservation of the oral mucosa. Its release depends on pleasant, familiar smells and tastes. Saliva provides the taste of food. Molecules broken down in saliva reach 10,000 taste buds on the tongue, which can detect and distinguish sweet, sour, bitter, spicy and even in new food. salty tastes. This allows you to perceive food as a pleasure, an enjoyment of tastes. Without moisture we cannot taste. If the tongue is dry, then we don’t feel like we’re eating. Without saliva we cannot swallow.

Therefore, it is so important for healthy digestion to eat food in a calm environment, not “on the run,” in beautiful dishes, tasty prepared. It is important, without rushing and without being distracted by reading, talking or watching TV, to chew your food slowly, enjoying the variety taste sensations. It is important to eat at the same time, as this promotes secretory regulation. It is important to drink enough plain water at least 30 minutes before meals and an hour after meals. Water is necessary for the formation of saliva and other digestive juices, and the activation of enzymes.

Difficult to maintain in the oral cavity alkaline balance, if a person constantly eats something, especially sweets, which always leads to acidification of the environment. After eating, it is recommended to rinse your mouth and/or chew something that tastes bitter, such as cardamom seeds or parsley.

And I also want to add about hygiene, cleaning teeth and gums. It was, and still is, a tradition among many peoples to brush their teeth with twigs and roots, which often have a bitter, astringent taste. And tooth powders also taste bitter. Bitter and astringent tastes are cleansing, have a bactericidal effect, and increase the secretion of saliva. While the sweet taste, on the contrary, promotes the growth of bacteria and stagnation. But manufacturers of modern toothpastes (especially sweet children's ones) simply add antimicrobials and preservatives, and we turn a blind eye to it. In our area, the pine taste is bitter, tart/astringent. If children are not accustomed to sweet tastes, they will normally accept unsweetened toothpaste.

Let's get back to digestion. As soon as food enters the mouth, preparation for digestion begins in the stomach: hydrochloric acid is released and enzymes are activated gastric juice.

Digestion in the stomach

Food does not stay in the mouth for long, and after it has been crushed by the teeth and processed by saliva, it passes through the esophagus into the stomach. Here it can stay for up to 6-8 hours (especially meat), digesting under the influence of gastric juices. The normal volume of the stomach is about 300 ml (about the size of a fist), however, after a large meal or frequent overeating, especially at night, its size can increase many times.

What does gastric juice consist of? First of all, from of hydrochloric acid, which begins to be produced as soon as something is in the oral cavity (this is important to keep in mind), and creates an acidic environment necessary for the activation of gastric proteolytic (protein-breaking) enzymes. Acid corrodes tissue. The mucous membrane of the stomach constantly produces a layer of mucus that protects against acid and mechanical damage coarse food components (when food is not sufficiently chewed and processed with saliva, when they snack on dry food on the go, simply swallowing). The formation of mucus and lubrication also depends on whether we drink enough plain water. During the day, about 2-2.5 liters of gastric juice are secreted, depending on the quantity and quality of food. During meals, gastric juice is secreted into maximum quantity and differs in acidity and enzyme composition.

Hydrochloric acid in pure form- this is a powerful aggressive factor, but without it the digestion process in the stomach will not occur. The acid promotes the transition of the inactive form of the gastric juice enzyme (pepsinogen) to the active form (pepsin), and also denatures (destroys) proteins, which facilitates their enzymatic processing.

So, proteolytic (protein-breaking) enzymes act mainly in the stomach. This is a group of enzymes that are active in different pH environments of the stomach (at the beginning of the digestion stage the environment is very acidic, at the exit from the stomach it is least acidic). As a result of hydrolysis, a complex protein molecule is divided into simpler components - polypeptides (molecules consisting of several amino acid chains) and oligopeptides (a chain of several amino acids). Let me remind you that the final product of protein breakdown is an amino acid - a molecule capable of absorption into the blood. This process occurs in the small intestine, and in the stomach it is carried out preparatory stage breaking down the protein into pieces.

In addition to proteolytic enzymes, gastric secretions contain an enzyme - lipase, which takes part in the breakdown of fats. Lipase works only with emulsified fats found in dairy products and is active in childhood. (You shouldn’t look for proper/emulsified fats in milk; they are also found in ghee, which no longer contains protein).

Carbohydrates in the stomach are not digested or processed because... the corresponding enzymes are active in an alkaline environment!

What else is interesting to know? Only in the stomach, thanks to the secretion component (Castle factor), does the transition of the inactive form of vitamin B12 supplied with food into the digestible form occur. The secretion of this factor may decrease or stop due to inflammatory damage to the stomach. Now we understand that it is not food enriched with vitamin B12 (meat, milk, eggs) that is important, but the condition of the stomach. It depends: on sufficient mucus production (this process is influenced increased acidity due to excessive consumption of protein products, and even in combination with carbohydrates, which, when left in the stomach for a long time, begin to ferment, which leads to acidification); from insufficient water consumption; from taking medications that both reduce acidity and dry out the gastric mucosa. This vicious circle can be broken with properly balanced food, drinking water and eating routine.

The production of gastric juice is regulated by complex mechanisms, which I will not dwell on. I just want to remind you that one of them ( unconditioned reflex) we can observe when juices begin to flow just from the thought of a familiar delicious food, from odors, from the onset of the usual meal time. When something enters the oral cavity, the release of hydrochloric acid with maximum acidity immediately begins. Therefore, if after this food does not enter the stomach, the acid corrodes the mucous membrane, which leads to its irritation, erosive changes, even ulcerative processes. Don’t similar processes occur when people chew gum or smoke on an empty stomach, when they take a sip of coffee or other drink and run away in a hurry? We don’t think about our actions until “thunder strikes”, until it really hurts, because the acid is real...

The secretion of gastric juices is affected by the composition of food:

• fatty foods inhibit gastric secretion, as a result, food is retained in the stomach;
• the more protein, the more acid: consuming difficult-to-digest proteins (meat and meat products) increases the secretion of hydrochloric acid;
• carbohydrates in the stomach do not undergo hydrolysis; an alkaline environment is needed to break them down; carbohydrates that remain in the stomach for a long time increase acidity due to the fermentation process (therefore, it is important not to eat protein foods with carbohydrates).

The result of our incorrect attitude to nutrition is disturbances in the acid-base balance in digestive tract and the appearance of diseases of the stomach and oral cavity. And here again, it is important to understand that it is not drugs that reduce acidity or alkalize the body that will help maintain health and healthy digestion, but a conscious attitude towards what we are doing.

In the next article we will look at what happens to food in the small and large intestines.

In the oral cavity, carbohydrates are digested by an enzyme in saliva α-amylase. The enzyme cleaves internal α(1→4)-glycosidic bonds. In this case, products of incomplete hydrolysis of starch (or glycogen) are formed - dextrins. Maltose is also formed in small quantities. The active center of α-amylase contains Ca 2+ ions. The enzyme is activated by Na + ions.

In gastric juice, the digestion of carbohydrates is inhibited, since amylase is inactivated in an acidic environment.

The main site of carbohydrate digestion is duodenum, where it is secreted as part of pancreatic juice α- amylase. This enzyme completes the breakdown of starch and glycogen, begun by salivary amylase, into maltose. Hydrolysis of the α(1→6)-glycosidic bond is catalyzed by the intestinal enzymes amylo-1,6-glucosidase and oligo-1,6-glucosidase .

Digestion of maltose and disaccharides supplied with food occurs in the area of ​​the brush border of epithelial cells (enterocytes) small intestine. Disaccharidases are integral proteins of enterocyte microvilli. They form a multienzyme complex consisting of four enzymes, the active centers of which are directed into the intestinal lumen.

1M altaza(-glucosidase) hydrolyzes maltose for two molecules D-glucose.

2. Lactase(-galactosidase) hydrolyzes lactose on D-galactose and D-glucose.

3. Isomaltase/Sucrase(dual-acting enzyme) has two active centers located in different domains. The enzyme hydrolyzes sucrose before D-fructose and D-glucose, and with the help of another active center the enzyme catalyzes hydrolysis isomaltose up to two molecules D-glucose.

Milk intolerance by some people, manifested by abdominal pain, bloating (flatulence) and diarrhea, is due to a decrease in lactase activity. There are three types of lactase deficiency.

1. Hereditary lactase deficiency. Symptoms of impaired tolerance develop very quickly after birth . Feeding a lactose-free diet results in resolution of symptoms.

2. Low primary lactase activity(gradual decrease in lactase activity in susceptible individuals). In 15% of children in Europe and 80% of children in the countries of the East, Asia, Africa, and Japan, the synthesis of this enzyme gradually stops as they grow older and adults develop milk intolerance, accompanied by the above symptoms. Fermented milk products are well tolerated by such people.

2. Low secondary lactase activity. Indigestibility of milk is often a consequence of intestinal diseases (tropical and non-tropical forms of sprue, kwashiorkor, colitis, gastroenteritis).

Symptoms similar to those described for lactase deficiency are characteristic of deficiencies of other disaccharidases. Treatment is aimed at eliminating the corresponding disaccharides from the diet.

Nb! Glucose penetrates into the cells of different organs by different mechanisms

The main products of complete digestion of starch and disaccharides are glucose, fructose and galactose. Monosaccharides enter the blood from the intestine, overcoming two barriers: the brush border membrane facing the intestinal lumen and the basolateral membrane of the enterocyte.

There are two known mechanisms for the entry of glucose into cells: facilitated diffusion and secondary active transport associated with the transfer of Na + ions. Fig.5.1. Structure of the glucose transporter

Glucose transporters (GLUTs), which provide a mechanism for its facilitated diffusion through cell membranes, form a family of related homologous proteins, a characteristic feature of the structure of which is a long polypeptide chain forming 12 transmembrane helical segments (Fig. 5.1). One of the domains located on the outer surface of the membrane contains an oligosaccharide. N- And C- the terminal sections of the transporter face the inside of the cell. The 3rd, 5th, 7th, and 11th transmembrane segments of the transporter appear to form a channel through which glucose enters the cell. A change in the conformation of these segments ensures the process of glucose movement into the cell. Transporters of this family contain 492-524 amino acid residues and differ in their affinity for glucose. Each transporter appears to perform specific functions.

Transporters that provide secondary sodium ion-dependent active transport of glucose from the intestine and renal tubules (NGLT) differ significantly in amino acid composition from the GLUT family of transporters, although they are also built from twelve transmembrane domains.

Below, in the table. 5.1. Some properties of monosaccharide transporters are given.

The transition of glucose and other monosaccharides into the enterocyte is facilitated by GLUT 5, located in the apical membrane of the enterocyte (facilitated diffusion along the concentration gradient) and SGLT 1, which ensures the movement (symport) of glucose into the enterocyte together with sodium ions. Sodium ions are then actively removed from the enterocyte, with the participation of Na + -K + -ATPase, which maintains a constant gradient of their concentration. Glucose leaves the enterocyte through the basolateral membrane with the help of GLUT 2 along a concentration gradient.

Absorption of pentoses occurs by simple diffusion.

The overwhelming amount of monosaccharides enters the portal circulatory system and the liver, a small part - into lymphatic system and pulmonary circulation. In the liver, excess glucose is stored “in reserve” in the form of glycogen.

N.B.! Glucose metabolism in the cell begins with its phosphorylation

P
The entry of glucose into any cell begins with its phosphorylation. This reaction solves several problems, the main ones of which are the “capture” of glucose for intracellular use and its activation.

The phosphorylated form of glucose does not pass through the plasma membrane, becomes the “property” of the cell and is used in almost all pathways of glucose metabolism. The only exception is the recovery path (Fig. 5.2.).

The phosphorylation reaction is catalyzed by two enzymes: hexokinase and glucokinase. Although glucokinase is one of the four isoenzymes of hesokinase ( hexokinase 4), there are important differences between hexokinase and glucokinase: 1) hexokinase is capable of phosphorylating not only glucose, but also other hexoses (fructose, galactose, mannose), while glucokinase activates only glucose; 2) hexokinase is present in all tissues, glucokinase is present in hepatocytes; 3) hexokinase has a high affinity for glucose ( TO M< 0,1 ммоль/л), напротив, глюкокиназа имеет высокую К M (около 10 ммоль/л), т.е. ее сродство к глюкозе мало и фосфорилирование глюкозы возможно только при массивном поступлении ее в клетки, что в физиологических условиях происходит на высоте пищеварения в печеночных клетках. Активирование глюкокиназы препятствует резкому увеличению поступления глюкозы в общий кровоток; в перерывах между приемами пищи для включения глюкозы в обменные процессы вполне достаточно гексокиназной активности. При диабете из-за низкой активности глюкокиназы (синтез и активность которой зависят от инсулина) этот механизм не срабатывает, поэтому глюкоза не задерживается в печени и вызывает гипергликемию.

The glucose-6-phosphate formed in the reaction is considered an allosteric inhibitor hexokinase (but not glucokinase).

Since the glucokinase reaction is insulin-dependent, fructose can be prescribed instead of glucose to diabetic patients (fructose is phosphorylated by hexokinase directly into fructose-6-phosphate).

Glucose-6-phosphate is used in the mechanisms of glycogen synthesis, in all oxidative pathways of glucose conversion and in the synthesis of other monosaccharides necessary for the cell. The place this reaction occupies in glucose metabolism allows it to be considered a key reaction in carbohydrate metabolism.

The hexokinase reaction is irreversible (G = -16.7 kJ/mol), therefore, to convert glucose-6-phosphate into free glucose, the enzyme glucose-6-phosphate phosphatase is present in the liver and kidney cells, catalyzing the hydrolysis of glucose-6-phosphate. The cells of these organs can thereby supply glucose to the blood and provide other cells with glucose.

The oral cavity includes the vestibule and the mouth itself. The vestibule is formed by the lips, the outer side of the cheeks, teeth and gums. The lips are covered on the outside with a thin layer of epithelium, on the inside they are lined with mucous membrane, which is a continuation inside cheeks They tightly cover the teeth and are attached to the gums using the upper and lower frenulum.

The mouth is formed by:

• buccal mucosa;
• incisors, canines, large and small molars;
• gums;
• language;
• soft and hard palate.

Rice. 1. Structure of the oral cavity.

More details about the structure of the oral cavity are presented in the table.

Rice. 2. Internal structure tooth

Functions

The main functions of the oral cavity in the process of digestion:

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• taste recognition;
• grinding solid food;
• imparting body temperature to incoming products;
• formation of a food bolus;
• breakdown of sugars;
• protection against penetration of pathogenic microorganisms.

The main function of digestion in the human oral cavity is performed by saliva. The salivary glands, located in the mucous membrane, moisten the food with the help of secreted saliva and the tongue, forming a food bolus.
There are three pairs of large glands:

• parotid;
• submandibular;
• sublingual.

Rice. 3. Location of the salivary glands.

Saliva is 99% water. The remaining percentage is biological active substances, exhibiting different properties.
Saliva contains:

• lysozyme - antibacterial enzyme;
• mucin - a viscous protein substance that binds food particles into a single lump;
• amylase and maltase - enzymes that break down starch and other complex sugars.

Enzymes are protein compounds that accelerate chemical reactions. They are a catalyst in the breakdown of food.

In small quantities, saliva contains other enzyme catalysts, as well as organic salts and trace elements.

Digestion

A brief description of how digestion occurs in the oral cavity is as follows:

• the food piece enters the cavity through the incisors;
• due to the masticatory muscles that hold the jaw, the chewing process begins;
• molars grind food, which is abundantly moistened with saliva;
• the cheeks, tongue and hard palate roll up a food bolus;
• The soft palate and tongue push the prepared food into the pharynx.

Food entering the oral cavity irritates receptors for various purposes (temperature, tactile, olfactory), which respond by producing saliva, gastric juice, and bile.

What have we learned?

The oral cavity has great importance during the digestion process. Through the cheeks, teeth, and tongue, incoming food is crushed and moved to the pharynx. Food moistened with saliva softens and sticks together into a single food bolus. Enzymes in saliva begin digestion by breaking down starch and other sugars.

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Only monosaccharides are absorbed in the intestine: glucose, galactose, fructose. Therefore, oligo- and polysaccharides entering the body with food must be hydrolyzed by enzyme systems to form monosaccharides. In Fig. 5.11 schematically depicts the localization of enzymatic systems involved in the digestion of carbohydrates, which begins in the oral cavity with the action of oral α-amylase and then continues in different parts of the intestine with the help of pancreatic α-amylase, sucrase-isomaltase, glycoamylase, β-glycosidase (lactase), trehalase complexes.

Rice. 5.11. Scheme of localization of enzyme systems for carbohydrate digestion

5.2.1. Digestion of carbohydrates using the mouth and pancreas -amylase ( -1,4-glycosidases). Polysaccharides received from food, namely starch (consists of the linear polysaccharide amylose, in which glucosyl residues are linked by α-1,4-glycosidic bonds, and amylopectin, a branched polysaccharide, where α-1,6-glycosidic bonds are also found) , begin to hydrolyze already in the oral cavity after wetting with saliva containing the hydrolytic enzyme α-amylase (α-1,4-glycosidase) (EC 3.2.1.1), which breaks down 1,4-glycosidic bonds in starch, but not acting on 1,6-glycosidic bonds.

In addition, the contact time of the enzyme with starch in the oral cavity is short, so starch is partially digested, forming large fragments - dextrins and a little maltose disaccharide. Disaccharides are not hydrolyzed by salivary amylase.

When entering the stomach in an acidic environment, salivary amylase is inhibited, the digestion process can only occur inside the food bolus, where amylase activity can persist for some time until the pH of the entire piece becomes acidic. Gastric juice does not contain enzymes that break down carbohydrates; only slight acid hydrolysis of glycosidic bonds is possible.

The main site of hydrolysis of oligo- and polysaccharides is the small intestine, in different parts of which certain glycosidases are secreted.

In the duodenum, the contents of the stomach are neutralized by the secretion of the pancreas, containing HCO 3 bicarbonates and having a pH of 7.5-8.0. Pancreatic amylase is found in the secretion of the pancreas, which hydrolyzes -1,4-glycosidic bonds in starch and dextrins to form the disaccharides maltose (in this carbohydrate two glucose residues are linked by an -1,4-glycosidic bond) and isomaltose (in this carbohydrate there are two glucose residues located at branching sites in the starch molecule and linked by α-1,6-glycosidic bonds). Oligosaccharides containing 8–10 glucose residues linked by both α-1,4-glycosidic and α-1,6-glycosidic bonds are also formed.

Both amylases are endoglycosidases. Pancreatic amylase also does not hydrolyze -1,6-glycosidic bonds in starch and -1,4-glycosidic bonds that connect glucose residues in the cellulose molecule.

Cellulose passes through the intestines unchanged and serves as a ballast substance, giving food volume and facilitating the digestion process. In the large intestine, under the influence of bacterial microflora, cellulose can be partially hydrolyzed to form alcohols, organic acids and CO 2, which can act as stimulants of intestinal motility.

Maltose, isomaltose and triosesaccharides formed in the upper intestine are further hydrolyzed in the small intestine under the action of specific glycosidases. Dietary disaccharides, sucrose and lactose, are also hydrolyzed by specific disaccharidases of the small intestine.

In the intestinal lumen, the activity of oligo- and disaccharidases is low, but most enzymes are associated with the surface of epithelial cells, which in the intestine are located on finger-shaped projections - villi and themselves, in turn, are covered with microvilli; all these cells form a brush border, increasing the contact surface of hydrolytic enzymes with their substrates.

Enzymes that break down glycosidic bonds in disaccharides (disaccharidases) are grouped into enzyme complexes located on the outer surface of the cytoplasmic membrane of enterocytes: sucrase-isomaltase, glycoamylase, -glycosidase.

5.2.2. Sucrase-isomaltase complex. This complex consists of two polypeptide chains and is attached to the surface of the enterocyte using a transmembrane hydrophobic domain located in the N-terminal part of the polypeptide. The sucrose-isomaltase complex (EC 3.2.1.48 and 3.2.1.10) cleaves -1,2- and -1,6-glycosidic bonds in sucrose and isomaltose.

Both enzymes of the complex are also capable of hydrolyzing -1,4-glycosidic bonds in maltose and maltotriose (a trisaccharide containing three glucose residues and formed during the hydrolysis of starch).

Although the complex has a fairly high maltase activity, hydrolyzing 80% of the maltose formed during the digestion of oligo- and polysaccharides, its main specificity is still the hydrolysis of sucrose and isomaltose, the rate of hydrolysis of glycosidic bonds in which is greater than the rate of hydrolysis of bonds in maltose and maltotriose. In this case, the sucrase subunit is the only intestinal enzyme that hydrolyzes sucrose. The complex is localized mainly in the jejunum; in the proximal and distal parts of the intestine, the content of the sucrase-isomaltase complex is insignificant.

5.2.3. Glycoamylase complex. This complex (EC 3.2.1.3 and 3.2.1.20) hydrolyzes -1,4-glycosidic bonds between glucose residues in oligosaccharides. The amino acid sequence of the glycoamylase complex has 60% homology with the sequence of the sucrase-isomaltase complex. Both complexes belong to family 31 of glycosyl hydrolases. Being an exoglycosidase, the enzyme acts from the reducing end and can also break down maltose, acting in this reaction as maltase (in this case, the glycoamylase complex hydrolyzes the remaining 20% ​​of the maltose formed during the digestion of oligo- and polysaccharides). The complex includes two catalytic subunits that have slight differences in substrate specificity. The complex exhibits the greatest activity in the lower parts of the small intestine.

5.2.4.  -Glycosidase complex (lactase). This enzyme complex hydrolyzes the -1,4-glycosidic bonds between galactose and glucose in lactose.

The glycoprotein is associated with the brush border and is unevenly distributed throughout the small intestine. With age, lactase activity decreases: it is maximum in infants, in adults it is less than 10% of the level of enzyme activity isolated in children.

5.2.5. Trehalase. This enzyme (EC 3.2.1.28) is a glycosidase complex that hydrolyzes the bonds between monomers in trehalose, a disaccharide found in fungi and consisting of two glucosyl residues linked by a glycosidic bond between the first anomeric carbon atoms.

From food carbohydrates, as a result of the action of glycoside hydrolases, monosaccharides are formed: glucose, fructose, galactose in large quantities, and to a lesser extent mannose, xylose, arabinose, which are absorbed by the epithelial cells of the jejunum and ileum and transported through the membranes of these cells using special mechanisms.

5.2.6. Transport of monosaccharides across the membranes of intestinal epithelial cells. The transfer of monosaccharides into the cells of the intestinal mucosa can be carried out by facilitated diffusion and active transport. In the case of active transport, glucose is transported across the membrane along with the Na + ion by one carrier protein, and these substances interact with different parts of this protein (Fig. 5.12). The Na + ion enters the cell along a concentration gradient, and glucose - against the concentration gradient (secondary active transport), therefore, the greater the gradient, the more glucose will be transferred into the enterocytes. As the concentration of Na + in the extracellular fluid decreases, the supply of glucose decreases. The Na + concentration gradient underlying active symport is provided by the action of Na + , K + -ATPase, which works as a pump pumping Na + out of the cell in exchange for the K + ion. In the same way, galactose enters enterocytes through the mechanism of secondary active transport.

Rice. 5.12. Entry of monosaccharides into enterocytes. SGLT1 is a sodium-dependent glucose/galactose transporter in the membrane of epithelial cells; Na + , K + -ATPase on the basolateral membrane creates a gradient of sodium and potassium ion concentrations necessary for the functioning of SGLT1. GLUT5 transports predominantly fructose across the membrane into the cell. GLUT2 on the basolateral membrane transports glucose, galactose and fructose out of the cell (according to)

Thanks to active transport, enterocytes can absorb glucose when its concentration is low in the intestinal lumen. At a high concentration of glucose, it enters cells through facilitated diffusion with the help of special carrier proteins (transporters). Fructose is transported into epithelial cells in the same way.

Monosaccharides enter the blood vessels from enterocytes mainly through facilitated diffusion. Half of the glucose is transported through the capillaries of the villi along the portal vein to the liver, half is delivered by the blood to the cells of other tissues.

5.2.7. Transport of glucose from blood to cells. The entry of glucose from the blood into cells is carried out by facilitated diffusion, i.e., the rate of glucose transport is determined by the gradient of its concentrations on both sides of the membrane. In muscle cells and adipose tissue, facilitated diffusion is regulated by the pancreatic hormone insulin. In the absence of insulin, the cell membrane does not contain glucose transporters. Glucose carrier protein (transporter) from erythrocytes (GLUT1), as can be seen from Fig. 5.13, is a transmembrane protein consisting of 492 amino acid residues and having a domain structure. Polar amino acid residues are located on both sides of the membrane, hydrophobic ones are localized in the membrane, crossing it several times. On the outside of the membrane there is a glucose binding site. When glucose binds, the conformation of the transporter changes, and the monosaccharide binding site becomes open into the cell. Glucose moves into the cell by being separated from the carrier protein.

5.2.7.1. Glucose transporters: GLUT 1, 2, 3, 4, 5. Glucose transporters are found in all tissues, of which there are several varieties, numbered in the order of their discovery. Five types of GLUTs have been described, having a similar primary structure and domain organization.

GLUT 1, localized in the brain, placenta, kidneys, large intestine, and red blood cells, supplies glucose to the brain.

GLUT 2 transports glucose from organs that secrete it into the blood: enterocytes, liver, and transports it to the β-cells of the islets of Langerhans in the pancreas.

GLUT 3 is found in many tissues, including the brain, placenta, kidneys, and ensures the flow of glucose to the cells of the nervous tissue.

GLUT 4 transports glucose to muscle cells (skeletal and cardiac) and adipose tissue, and is insulin-dependent.

GLUT 5 is found in the cells of the small intestine and may also transport fructose.

All transporters can be located both in the cytoplasmic

Rice. 5.13. Structure of the protein carrier (transporter) of glucose from erythrocytes (GLUT1) (according to)

vesicles of cells, and in plasma membrane. In the absence of insulin, GLUT 4 is located only inside the cell. Under the influence of insulin, vesicles are transported to the plasma membrane, merge with it and GLUT 4 is incorporated into the membrane, after which the transporter facilitates the diffusion of glucose into the cell. After the insulin concentration in the blood decreases, the transporters return to the cytoplasm and the transport of glucose into the cell stops.

Various disturbances have been identified in the functioning of glucose transporters. With a hereditary defect in transporter proteins, insulin-independent diabetes mellitus develops. In addition to protein defects, there are other disorders caused by: 1) a defect in the transmission of the insulin signal to move the transporter to the membrane, 2) a defect in transporter movement, 3) a defect in protein incorporation into the membrane, 4) a violation of detachment from the membrane.

5.2.8. Insulin. This compound is a hormone secreted by the β-cells of the islets of Langerhans in the pancreas. Insulin is a polypeptide consisting of two polypeptide chains: one contains 21 amino acid residues (chain A), the other contains 30 amino acid residues (chain B). The chains are connected to each other by two disulfide bonds: A7-B7, A20-B19. Within the A chain there is an intramolecular disulfide bond between the sixth and eleventh residues. The hormone can exist in two conformations: T and R (Fig. 5.14).

Rice. 5.14. Spatial structure of the monomeric form of insulin: A- porcine insulin, T-conformation, b human insulin, R-conformation (A-chain is shown red color, B-chain  yellow) (according to )

The hormone can exist in the form of a monomer, dimer and hexamer. In the hexameric form, insulin is stabilized by zinc ion, which forms coordination bonds with His10 of the B chain of all six subunits (Fig. 5.15).

Mammalian insulins have great homology in their primary structure with human insulin: for example, in pig insulin there is only one substitution - instead of threonine, there is alanine at the carboxyl end of the B-chain; in bovine insulin there are three other amino acid residues in comparison with human insulin. The most common substitutions occur in positions 8, 9 and 10 of chain A, but they do not have a significant effect on the biological activity of the hormone.

Substitutions of amino acid residues in the positions of disulfide bonds, hydrophobic residues in the C- and N-terminal regions of the A-chain and in the C-terminal regions of the B-chain are very rare, which indicates the importance of these regions in the manifestation of the biological activity of insulin. The Phe24 and Phe25 residues of the B-chain and the C- and N-terminal residues of the A-chain take part in the formation of the active center of the hormone.

Rice. 5.15. Spatial structure of insulin hexamer (R 6) (according to)

5.2.8.1. Biosynthesis of insulin. Insulin is synthesized as a precursor, preproinsulin, containing 110 amino acid residues, on polyribosomes in the rough endoplasmic reticulum. Biosynthesis begins with the formation of a signal peptide, which penetrates the lumen of the endoplasmic reticulum and directs the movement of the growing polypeptide. At the end of synthesis, a signal peptide of 24 amino acid residues is cleaved from preproinsulin to form proinsulin, which contains 86 amino acid residues and is transferred to the Golgi apparatus, where further insulin maturation occurs in the cisterns. The spatial structure of proinsulin is shown in Fig. 5.16.

During long-term maturation, under the influence of serine endopeptidases PC2 and PC1/3, first the peptide bond between Arg64 and Lys65 is cleaved, then the peptide bond formed by Arg31 and Arg32 is hydrolyzed, with the cleavage of the C-peptide consisting of 31 amino acid residues. The conversion of proinsulin to insulin, containing 51 amino acid residues, ends with the hydrolysis of arginine residues at the N-terminus of the A chain and the C-terminus of the B chain under the action of carboxypeptidase E, which exhibits a specificity similar to carboxypeptidase B, i.e., hydrolyzes peptide bonds, imino group which belongs to the main amino acid (Fig. 5.17 and 5.18).

Rice. 5.16. Presumable spatial structure of proinsulin in a conformation that promotes proteolysis. Red balls highlight amino acid residues (Arg64 and Lys65; Arg31 and Arg32), the peptide bonds between which undergo hydrolysis as a result of proinsulin processing (according to)

Insulin and C-peptide in equimolar quantities enter secretory granules, where insulin, interacting with zinc ion, forms dimers and hexamers. Secretory granules fuse with the plasma membrane and secrete insulin and C-peptide into the extracellular fluid by exocytosis. The half-life of insulin in blood plasma is 3–10 minutes, and that of C-peptide is about 30 minutes. Insulin is broken down by the enzyme insulinase, a process that occurs in the liver and kidneys.

5.2.8.2. Regulation of insulin synthesis and secretion. The main regulator of insulin secretion is glucose, which regulates the expression of the insulin gene and the genes of proteins involved in the metabolism of basic energy carriers. Glucose can directly bind to transcription factors, which has a direct effect on the rate of gene expression. There may be a secondary effect on insulin and glucagon secretion when the release of insulin from secretory granules activates the transcription of insulin mRNA. But insulin secretion depends on the concentration of Ca 2+ ions and decreases with their deficiency, even at a high concentration of glucose, which activates insulin synthesis. In addition, it is inhibited by adrenaline when it binds to  2 receptors. Stimulators of insulin secretion are growth hormones, cortisol, estrogens, and gastrointestinal hormones (secretin, cholecystokinin, gastric inhibitory peptide).

Rice. 5.17. Synthesis and processing of preproinsulin (according to)

The secretion of insulin by the β-cells of the islets of Langerhans in response to an increase in the concentration of glucose in the blood occurs as follows:

Rice. 5.18. Processing of proinsulin into insulin by hydrolysis of the peptide bond between Arg64 and Lys65, catalyzed by serine endopeptidase PC2, and cleavage of the peptide bond between Arg31 and Arg32 by serine endopeptidase PC1/3, the conversion ends with the cleavage of arginine residues at the N-terminus of the A-chain and the C-terminus B-chains under the action of carboxypeptidase E (cleavable arginine residues are shown in circles). As a result of processing, in addition to insulin, C-peptide is formed (according to)

1) glucose is transported into β-cells by the GLUT 2 transporter protein;

2) in the cell, glucose undergoes glycolysis and is further oxidized in the respiratory cycle to form ATP; the intensity of ATP synthesis depends on the level of glucose in the blood;

3) under the influence of ATP, potassium ion channels close and the membrane depolarizes;

4) membrane depolarization causes the opening of voltage-dependent calcium channels and calcium entry into the cell;

5) an increase in the level of calcium in the cell activates phospholipase C, which breaks down one of the membrane phospholipids - phosphatidylinositol-4,5-diphosphate - into inositol-1,4,5-triphosphate and diacylglycerol;

6) inositol triphosphate, binding to receptor proteins of the endoplasmic reticulum, causes a sharp increase in the concentration of bound intracellular calcium, which leads to the release of pre-synthesized insulin stored in secretory granules.

5.2.8.3. Mechanism of action of insulin. The main effect of insulin on muscle and fat cells is to enhance the transport of glucose across the cell membrane. Stimulation by insulin leads to an increase in the rate of glucose entry into the cell by 20–40 times. When stimulated by insulin, there is a 5-10-fold increase in the content of glucose transport proteins in plasma membranes, while a simultaneous decrease of 50-60% in their content in the intracellular pool. The amount of energy required in the form of ATP is necessary mainly for the activation of the insulin receptor, and not for the phosphorylation of the transporter protein. Stimulation of glucose transport increases energy consumption by 20−30 times, while only a small amount is required to move glucose transporters. Translocation of glucose transporters to the cell membrane is observed within a few minutes after the interaction of insulin with the receptor, and further stimulating effects of insulin are necessary to accelerate or maintain the cycling process of transporter proteins.

Insulin, like other hormones, exerts its effect on cells through the corresponding receptor protein. The insulin receptor is a complex integral protein of the cell membrane, consisting of two α-subunits (130 kDa) and two α-subunits (95 kDa); the former are located entirely outside the cell, on its surface, the latter penetrate the plasma membrane.

The insulin receptor is a tetramer consisting of two extracellular α-subunits that interact with the hormone and are connected to each other by disulfide bridges between cysteines 524 and the triplet Cys682, Cys683, Cys685 of both α-subunits (see Fig. 5.19, A), and two transmembrane -subunits exhibiting tyrosine kinase activity, connected by a disulfide bridge between Cys647 () and Cys872. The polypeptide chain of the α-subunit with a molecular weight of 135 kDa contains 719 amino

Rice. 5.19. Structure of the insulin receptor dimer: A- modular structure of the insulin receptor. At the top are α-subunits connected by disulfide bridges Cys524, Cys683-685 and consisting of six domains: two containing leucine repeats L1 and L2, a cysteine-rich region CR and three fibronectin domains of type III Fn o, Fn 1, ID (incorporation domain) . Below - -subunits, connected to the -subunit by a disulfide bridge Cys647Cys872 and consisting of seven domains: three fibronectin domains ID, Fn 1 and Fn 2, a transmembrane domain TM, a membrane-adjacent domain JM, a tyrosine kinase domain TK, a C-terminal ST; b spatial arrangement of the receptor, one dimer is shown in color, the other is white, A is the activating loop opposite the hormone binding site, X (red) is the C-terminal part of the α-subunit, X (black) is the N-terminal part of the α-subunit , yellow balls 1,2,3 - disulfide bonds between cysteine ​​residues at positions 524, 683-685, 647-872 (according to)

acidic residues and consists of six domains: two domains L1 and L2 containing leucine repeats, the cysteine-rich region CR, where the insulin binding center is localized, and three fibronectin domains of type III Fno, Fn 1, Ins (insertion domain) (see Fig. 5.18). The -subunit includes 620 amino acid residues, has a molecular weight of 95 kDa and consists of seven domains: three fibronectin domains ID, Fn 1 and Fn 2, a transmembrane domain TM, a membrane-adjacent domain JM, a tyrosine kinase domain TK, and a C-terminal ST . There are two binding sites for insulin on the receptor: one with high affinity, the other with low affinity. To carry the hormone signal into the cell, insulin must bind to a high-affinity center. This center is formed by the binding of insulin from the L1, L2 and CR domains of one α-subunit and the fibronectin domains of another, while the arrangement of the α-subunits is opposite to each other, as shown in Fig. 5.19, With.

In the absence of insulin interaction with the high-affinity site of the receptor, the α-subunits are moved away from the β-subunits by a protrusion (cam), which is part of the CR domain, which prevents contact of the activating loop (A-loop) of the tyrosine kinase domain of one β-subunit with phosphorylation sites on the other β-subunit. sub-unit (Fig. 5.20, b). When insulin binds to the high-affinity center of the insulin receptor, the conformation of the receptor changes, the protrusion no longer prevents the approach of the α- and β-subunits, the activating loops of the TK domains interact with the tyrosine phosphorylation sites on the opposite TK domain, transphosphorylation of the β-subunits occurs at seven tyrosine residues: Y1158 , Y1162, Y1163 activating loop (this is a kinase regulatory domain), Y1328, Y1334 CT domain, Y965, Y972 JM domain (Fig. 5.20, A), which leads to an increase in tyrosine kinase activity of the receptor. At position 1030 of the TC there is a lysine residue that is part of the catalytic active site - the ATP-binding center. Replacement of this lysine with many other amino acids by site-directed mutagenesis abolishes the tyrosine kinase activity of the insulin receptor but does not impair insulin binding. However, the attachment of insulin to such a receptor does not have any effect on cellular metabolism and proliferation. Phosphorylation of some serine-threonine residues, on the contrary, reduces affinity for insulin and reduces tyrosine kinase activity.

Several insulin receptor substrates are known: IRS-1 (insulin receptor substrate), IRS-2, proteins of the STAT family (signal transducer and activator of transcription - signal carriers and transcription activators are discussed in detail by us in Part 4 “Biochemical basis of protective reactions”).

IRS-1 is a cytoplasmic protein that binds to phosphorylated tyrosines of the insulin receptor TK with its SH2 domain and is phosphorylated by the receptor tyrosine kinase immediately after stimulation with insulin. The degree of phosphorylation of the substrate determines the increase or decrease in the cellular response to insulin, the amplitude of changes in cells and sensitivity to the hormone. Damage to the IRS-1 gene can cause insulin-dependent diabetes. The IRS-1 peptide chain contains about 1200 amino acid residues, 20–22 potential tyrosine phosphorylation centers and about 40 serine-threonine phosphorylation centers.

Rice. 5.20. A simplified diagram of structural changes when insulin binds to the insulin receptor: A a change in the conformation of the receptor as a result of hormone binding at the high-affinity center leads to a displacement of the protrusion, bringing the subunits closer together and transphosphorylation of the TK domains; b in the absence of interaction of insulin with the high-affinity binding site on the insulin receptor, the protrusion (cam) prevents the approach of the α- and β-subunits and the transphosphorylation of the TK domains. A-loop - activating loop of the TK domain, numbers 1 and 2 in a circle - disulfide bonds between subunits, TK - tyrosine kinase domain, C - catalytic center of the TK, set 1 and set 2 - amino acid sequences of α-subunits that form the site of high affinity for insulin receptor (according to)

Phosphorylation of IRS-1 at several tyrosine residues gives it the ability to bind to proteins containing SH2 domains: tyrosine phosphatase syp, p85 subunit of PI-3-kinase (phosphatidylinositol 3-kinase), adapter protein Grb2, protein tyrosine phosphatase SH-PTP2, phospholipase C , GAP (activator of small GTP-binding proteins). As a result of the interaction of IRS-1 with similar proteins, multiple downstream signals are generated.

Rice. 5.21. Translocation of glucose transporter proteins GLUT 4 in muscle and fat cells from the cytoplasm to the plasma membrane under the influence of insulin. The interaction of insulin with the receptor leads to phosphorylation of the insulin receptor substrate (IRS), which binds PI-3-kinase (PI3K), which catalyzes the synthesis of the phospholipid phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P 3). The latter compound, by binding pleckstrin domains (PH), mobilizes the protein kinases PDK1, PDK2 and PKB to the cell membrane. PDK1 phosphorylates PKB at Thr308, activating it. Phosphorylated PKB associates with vesicles containing GLUT 4, causing their translocation into the plasma membrane, leading to increased transport of glucose into muscle and fat cells (according to)

Phospholipase C, stimulated by phosphorylated IRS-1, hydrolyzes the cell membrane phospholipid phosphatidylinositol 4,5-diphosphate to form two second messengers: inositol 3,4,5-trisphosphate and diacylglycerol. Inositol-3,4,5-triphosphate, acting on the ion channels of the endoplasmic reticulum, releases calcium from it. Diacylglycerol acts on calmodulin and protein kinase C, which phosphorylates various substrates, leading to changes in the activity of cellular systems.

Phosphorylated IRS-1 also activates PI-3-kinase, which catalyzes the phosphorylation of phosphatidylinositol, phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-diphosphate at position 3 to form phosphatidylinositol-3-phosphate, phosphatidylinositol-3,4-diphosphate and phosphatidylinositol, respectively. -3,4,5-triphosphate.

PI-3-kinase is a heterodimer containing regulatory (p85) and catalytic (p110) subunits. The regulatory subunit has two SH2 domains and an SH3 domain, so PI-3-kinase binds to IRS-1 with high affinity. Phosphatidylinositol derivatives formed in the membrane, phosphorylated at position 3, bind proteins containing the so-called pleckstrin (PH) domain (the domain exhibits high affinity for phosphatidylinositol-3-phosphates): protein kinase PDK1 (phosphatidylinositide-dependent kinase), protein kinase B (PKB).

Protein kinase B (PKB) consists of three domains: N-terminal pleckstrin, central catalytic, and C-terminal regulatory. The pleckstrin domain is required for activation of PKB. Having bound via the pleckstrin domain near the cell membrane, PKB approaches the protein kinase PDK1, which, through

its pleckstrin domain is also localized near the cell membrane. PDK1 phosphorylates Thr308 of the PKV kinase domain, resulting in PKV activation. Activated PKB phosphorylates glycogen synthase kinase 3 (at Ser9), causing inactivation of the enzyme and thereby the process of glycogen synthesis. PI-3-phosphate-5-kinase is also phosphorylated, acting on vesicles in which GLUT 4 transport proteins are stored in the cytoplasm of adipocytes, causing the movement of glucose transporters to the cell membrane, integration into it and transmembrane transfer of glucose into muscle and fat cells ( Fig. 5.21).

Insulin not only affects the flow of glucose into the cell with the help of GLUT 4 transporter proteins. It is involved in the regulation of the metabolism of glucose, fats, amino acids, ions, in the synthesis of proteins, and influences the processes of replication and transcription.

The influence on glucose metabolism in the cell is carried out by stimulating the process of glycolysis by increasing the activity of the enzymes involved in this process: glucokinase, phosphofructokinase, pyruvate kinase, hexokinase. Insulin, through the adenylate cyclase cascade, activates phosphatase, which dephosphorylates glycogen synthase, which leads to activation of glycogen synthesis (Fig. 5.22) and inhibition of the process of its breakdown. By inhibiting phosphoenolpyruvate carboxykinase, insulin inhibits the process of gluconeogenesis.

Rice. 5.22. Glycogen synthesis scheme

In the liver and adipose tissue, under the influence of insulin, fat synthesis is stimulated by activating the enzymes: acetylCoA carboxylase, lipoprotein lipase. At the same time, the breakdown of fats is inhibited, since insulin-activated phosphatase, dephosphorylating hormone-sensitive triacylglycerol lipase, inhibits this enzyme and the concentration of fatty acids circulating in the blood decreases.

In the liver, adipose tissue, skeletal muscles, and heart, insulin affects the transcription rate of more than a hundred genes.

5.2.9. Glucagon. In response to a decrease in the concentration of glucose in the blood, the α-cells of the islets of Langerhans of the pancreas produce the “hunger hormone” - glucagon, which is a polypeptide with a molecular weight of 3,485 Da, consisting of 29 amino acid residues.

The action of glucagon is opposite to the effects of insulin. Insulin promotes energy storage by stimulating glycogenesis, lipogenesis and protein synthesis, and glucagon, by stimulating glycogenolysis and lipolysis, causes rapid mobilization of potential energy sources.

Rice. 5.23. Structure of human proglucagon and tissue-specific processing of proglucagon into proglucagon-derived peptides: in the pancreas, glucagon and MPGF (mayor proglucagon fragment) are formed from proglucagon; in the neuroendocrine cells of the intestine and some parts of the central nervous system, glycentin, oxyntomodulin, GLP-1 (peptide derived from proglucagon), GLP-2, two intermediate peptides (intervening peptide - IP), GRPP - glycentin-related pancreatic polypeptide (polypeptide from pancreas - glycentin derivative) (according to)

The hormone is synthesized by the α-cells of the islets of Langerhans of the pancreas, as well as in the neuroendocrine cells of the intestine and in the central nervous system in the form of an inactive precursor - proglucagon (molecular weight 9,000 Da), containing 180 amino acid residues and undergoing processing using convertase 2 and forming several peptides of different lengths, including glucagon and two glucagon-like peptides (glucagon like peptide - GLP-1, GLP-2, glycentin) (Fig. 5.23). 14 of the 27 amino acid residues of glucagon are identical to those in the molecule of another hormone of the gastrointestinal tract - secretin.

For glucagon to bind to the receptors of cells responding to it, the integrity of its sequence 1–27 from the N-terminus is required. An important role in the manifestation of the effects of the hormone is played by the histidine residue located at the N-terminus, and in binding to receptors - fragment 20-27.

In blood plasma, glucagon does not bind to any transport protein; its half-life is 5 minutes; in the liver, it is destroyed by proteinases, and the breakdown begins with the cleavage of the bond between Ser2 and Gln3 and the removal of the dipeptide from the N-terminus.

Glucagon secretion is suppressed by glucose but stimulated by protein foods. GLP-1 inhibits glucagon secretion and stimulates insulin secretion.

Glucagon has an effect only on hepatocytes and fat cells that have receptors for it in the plasma membrane. In hepatocytes, by binding to receptors on the plasma membrane, glucagon, through the G protein, activates adenylate cyclase, which catalyzes the formation of cAMP, which, in turn, leads to activation of phosphorylase, which accelerates the breakdown of glycogen, and inhibition of glycogen synthase and inhibition of glycogen formation. Glucagon stimulates gluconeogenesis by inducing the synthesis of enzymes involved in this process: glucose-6-phosphatase, phosphoenolpyruvate carboxykinase, fructose-1,6-biphosphatase. The overall effect of glucagon in the liver is reduced to increased production of glucose.

In fat cells, the hormone also, using the adenylate cyclase cascade, activates hormone-sensitive triacylglycerol lipase, stimulating lipolysis. Glucagon increases the secretion of catecholamines by the adrenal medulla. By participating in the implementation of “fight or flight” reactions, glucagon increases the availability of energy substrates (glucose, free fatty acids) for skeletal muscles and increases blood supply to skeletal muscles by increasing the work of the heart.

Glucagon has no effect on skeletal muscle glycogen due to the almost complete absence of glucagon receptors in them. The hormone causes an increase in insulin secretion from pancreatic β-cells and inhibition of insulinase activity.

5.2.10. Regulation of glycogen metabolism. The accumulation of glucose in the body in the form of glycogen and its breakdown are consistent with the body's energy needs. The direction of glycogen metabolism processes is regulated by mechanisms dependent on the action of hormones: in the liver of insulin, glucagon and adrenaline, in the muscles of insulin and adrenaline. Switching processes of glycogen synthesis or breakdown occurs during the transition from the absorptive period to the post-absorptive period or when changing from a state of rest to physical work.

5.2.10.1. Regulation of glycogen phosphorylase and glycogen synthase activity. When the concentration of glucose in the blood changes, the synthesis and secretion of insulin and glucagon occurs. These hormones regulate the processes of glycogen synthesis and breakdown, affecting the activity of the key enzymes of these processes: glycogen synthase and glycogen phosphorylase through their phosphorylation-dephosphorylation.

Rice. 5.24 Activation of glycogen phosphorylase by phosphorylation of the Ser14 residue using glycogen phosphorylase kinase and inactivation by phosphatase catalyzing the dephosphorylation of the serine residue (according to)

Both enzymes exist in two forms: phosphorylated (active glycogen phosphorylase A and inactive glycogen synthase) and dephosphorylated (inactive phosphorylase b and active glycogen synthase) (Figs. 5.24 and 5.25). Phosphorylation is carried out by a kinase that catalyzes the transfer of a phosphate residue from ATP to a serine residue, and dephosphorylation is catalyzed by phosphoprotein phosphatase. Kinase and phosphatase activities are also regulated by phosphorylation-dephosphorylation (see Fig. 5.25).

Rice. 5.25. Regulation of glycogen synthase activity. The enzyme is activated by the action of phosphoprotein phosphatase (PP1), which dephosphorylates three phosphoserine residues near the C-terminus in glycogen synthase. Glycogen synthase kinase 3 (GSK3), which catalyzes the phosphorylation of three serine residues in glycogen synthase, inhibits glycogen synthesis and is activated by phosphorylation by casein kinase (CKII). Insulin, glucose and glucose-6-phosphate activate phosphoprotein phosphatase, while glucagon and adrenaline (epinephrine) inhibit it. Insulin inhibits the action of glycogen synthase kinase 3 (according to)

cAMP-dependent protein kinase A (PKA) phosphorylates phosphorylase kinase, converting it into an active state, which in turn phosphorylates glycogen phosphorylase. cAMP synthesis is stimulated by adrenaline and glucagon.

Insulin, through a cascade involving the Ras protein (Ras signaling pathway), activates the pp90S6 protein kinase, which phosphorylates and thereby activates phosphoprotein phosphatase. Active phosphatase dephosphorylates and inactivates phosphorylase kinase and glycogen phosphorylase.

Phosphorylation by PKA of glycogen synthase leads to its inactivation, and dephosphorylation by phosphoprotein phosphatase activates the enzyme.

5.2.10.2. Regulation of glycogen metabolism in the liver. Changing the concentration of glucose in the blood also changes the relative concentrations of the hormones: insulin and glucagon. The ratio of insulin concentration to glucagon concentration in the blood is called the “insulin-glucagon index.” In the post-absorptive period, the index decreases and the regulation of blood glucose concentration is influenced by the concentration of glucagon.

Glucagon, as described above, activates the release of glucose into the blood due to the breakdown of glycogen (activation of glycogen phosphorylase and inhibition of glycogen synthase) or through synthesis from other substances - gluconeogenesis. Glucose-1-phosphate is formed from glycogen, which is isomerized into glucose-6-phosphate, hydrolyzed under the action of glucose-6-phosphatase to form free glucose, which can exit the cell into the blood (Fig. 5.26).

The effect of adrenaline on hepatocytes is similar to the effect of glucagon in the case of β 2 receptors and is caused by phosphorylation and activation of glycogen phosphorylase. In the case of interaction of adrenaline with  1 receptors of the plasma membrane, transmembrane transmission of the hormonal signal is carried out using the inositol phosphate mechanism. In both cases, the process of glycogen breakdown is activated. The use of one or another type of receptor depends on the concentration of adrenaline in the blood.

Rice. 5.26. Scheme of glycogen phosphorolysis

During digestion, the insulin-glucagon index increases and the influence of insulin predominates. Insulin reduces the concentration of glucose in the blood and activates, through phosphorylation through the Ras pathway, phosphodiesterase cAMP, which hydrolyzes this second messenger to form AMP. Insulin also activates phosphoprotein phosphatase of glycogen granules through the Ras pathway, dephosphorylating and activating glycogen synthase and inactivating phosphorylase kinase and glycogen phosphorylase itself. Insulin induces the synthesis of glucokinase to accelerate the phosphorylation of glucose in the cell and its incorporation into glycogen. Thus, insulin activates the process of glycogen synthesis and inhibits its breakdown.

5.2.10.3. Regulation of glycogen metabolism in muscles. In the case of intense muscle work, the breakdown of glycogen is accelerated by adrenaline, which binds to  2 receptors and, through the adenylate cyclase system, leads to phosphorylation and activation of phosphorylase kinase and glycogen phosphorylase and inhibition of glycogen synthase (Fig. 5.27 and 5.28). As a result of further conversion of glucose-6-phosphate formed from glycogen, ATP is synthesized, which is necessary for intense muscle work.

Rice. 5.27. Regulation of glycogen phosphorylase activity in muscles (according to)

At rest, muscle glycogen phosphorylase is inactive, as it is in a dephosphorylated state, but the breakdown of glycogen occurs due to allosteric activation of glycogen phosphorylase b with the help of AMP and orthophosphate, formed during the hydrolysis of ATP.

Rice. 5.28. Regulation of glycogen synthase activity in muscles (according to)

During moderate muscle contractions, phosphorylase kinase can be activated allosterically (by Ca 2+ ions). Ca 2+ concentration increases with muscle contraction in response to a motor nerve signal. When the signal decays, a decrease in Ca 2+ concentration simultaneously “turns off” the kinase activity, thus

Ca 2+ ions are involved not only in muscle contraction, but also in providing energy for these contractions.

Ca 2+ ions bind to the protein calmodulin, which in this case acts as one of the kinase subunits. Muscle phosphorylase kinase has the structure  4  4  4  4. Only the -subunit has catalytic properties, the - and -subunits, being regulatory, are phosphorylated at serine residues with the help of PKA, the -subunit is identical to the protein calmodulin (discussed in detail in section 2.3.2 of part 2 “Biochemistry of movement”), binds four Ca 2+ ions, which leads to conformational changes, activation of the catalytic -subunit, although the kinase remains in a dephosphorylated state.

During digestion at rest, glycogen synthesis also occurs in the muscles. Glucose enters muscle cells with the help of GLUT 4 transport proteins (their mobilization in cell membrane under the influence of insulin is discussed in detail in section. 5.2.4.3 and in Fig. 5.21). Insulin also influences glycogen synthesis in muscles through dephosphorylation of glycogen synthase and glycogen phosphorylase.

5.2.11. Non-enzymatic glycosylation of proteins. One type of post-translational modification of proteins is the glycosylation of serine, threonine, asparagine, and hydroxylysine residues using glycosyltransferases. Since a high concentration of carbohydrates (reducing sugars) is created in the blood during digestion, non-enzymatic glycosylation of proteins, lipids and nucleic acids, called glycation, is possible. Products formed as a result of the multi-step interaction of sugars with proteins are called advanced glycosylation end-products (AGEs) and are found in many human proteins. The half-life of these products is longer than that of proteins (from several months to several years), and the rate of their formation depends on the level and duration of exposure to reducing sugar. It is assumed that many complications arising from diabetes, Alzheimer's disease, and cataracts are associated with their formation.

The glycation process can be divided into two phases: early and late. At the first stage of glycation, a nucleophilic attack occurs on the carbonyl group of glucose by the -amino group of lysine or the guanidinium group of arginine, resulting in the formation of a labile Schiff base - N-glycosylimine (Fig. 5.29). The formation of a Schiff base is a relatively rapid and reversible process.

Next comes a regrouping N‑glycosylimine to form the Amadori product – 1‑amino‑1‑deoxyfructose. The rate of this process is lower than the rate of formation of glycosylimine, but significantly higher than the rate of hydrolysis of the Schiff base,

Rice. 5.29. Scheme of protein glycation. The open form of the carbohydrate (glucose) reacts with the -amino group of lysine to form a Schiff base, which undergoes an Amadori rearrangement to ketoamine through the formation of an enolamine intermediate. The Amadori rearrangement is accelerated if aspartate and arginine residues are located near the lysine residue. Ketoamine can further produce a variety of products (advanced glycation end products - AGEs). The diagram shows the reaction with the second carbohydrate molecule to form diketoamine (according to)

therefore, proteins containing 1‑amino‑1‑deoxyfructose residues accumulate in the blood. Modification of lysine residues in proteins at the early stage of glycation is apparently facilitated by the presence of histidine, lysine or arginine residues in the immediate vicinity of the reacting amino group, which carry out acidic the main catalysis of the process, as well as aspartate residues, which withdraw a proton from the second carbon atom of the sugar. Ketoamine can bind another carbohydrate residue at the imino group to form doubly glycated lysine, which becomes diketoamine (see Fig. 5.29).

Late stage of glycation, including further transformations N‑glycosylimine and the Amadori product, a slower process leading to the formation of stable advanced glycation end products (AGEs). IN Lately Data have appeared on the direct participation in the formation of AGEs of α-dicarbonyl compounds (glyoxal, methylglyoxal, 3-deoxyglucosone) formed in vivo both during the degradation of glucose and as a result of transformations of the Schiff base during the modification of lysine in proteins with glucose (Fig. 5.30). Specific reductases and sulhydryl compounds (lipoic acid, glutathione) are capable of transforming reactive dicarbonyl compounds into inactive metabolites, which is reflected in a decrease in the formation of advanced glycation products.

Reactions of α-dicarbonyl compounds with ε-amino groups of lysine residues or guanidinium groups of arginine residues in proteins lead to the formation of protein cross-links, which are responsible for complications caused by protein glycation in diabetes and other diseases. In addition, as a result of sequential dehydration of the Amadori product at C4 and C5, 1-amino-4-deoxy-2,3-dione and -enedione are formed, which can also participate in the formation of intramolecular and intermolecular protein cross-links.

Among the AGEs characterized N ε ‑carboxymethyllysine (CML) and N ε -carboxyethyl lysine (CEL), bis(lysyl)imidazole adducts (GOLD - glyoxal-lysyl-lysyl-dimer, MOLD - methylglyoxal-lysyl-lysyl-dimer, DOLD - deoxyglucosone-lysyl-lysyl-dimer), imidazolones (G-H, MG‑H and 3DG‑H), pyrraline, argpyrimidine, pentosidine, crosslin and vesperlysine. In Fig. 5.31 shows some

Rice. 5.30. Scheme of protein glycation in the presence of D-glucose. The box shows the main precursors of AGE products resulting from glycation (according to)

advanced glycation end products. For example, pentosidine and carboxymethyllysine (CML), glycation end products formed under oxidative conditions, are found in long-lived proteins: skin collagen and lens crystallin. Carboxymethyllysine introduces a negatively charged carboxyl group into the protein instead of a positively charged amino group, which can lead to a change in the charge on the surface of the protein and a change in the spatial structure of the protein. CML is an antigen recognized by antibodies. The amount of this product increases linearly with age. Pentosidine is a cross-link (cross-link product) between the Amadori product and an arginine residue at any position of the protein, formed from ascorbate, glucose, fructose, ribose, found in the brain tissue of patients with Alzheimer's disease, in the skin and blood plasma of patients with diabetes.

Advanced glycation end products can promote free radical oxidation, a change in charge on the protein surface, and irreversible cross-linking between different regions of the protein, which

disrupts their spatial structure and functioning, making them resistant to enzymatic proteolysis. In turn, free radical oxidation can cause non-enzymatic proteolysis or protein fragmentation, lipid peroxidation.

The formation of advanced glycation end products on basement membrane proteins (type IV collagen, laminin, heparan sulfate proteoglycan) leads to its thickening, narrowing of the lumen of capillaries and disruption of their function. These disturbances of the extracellular matrix change the structure and function of blood vessels (decreased elasticity of the vascular wall, changes in the response to the vasodilatory effect of nitric oxide), and contribute to a more accelerated development of the atherosclerotic process.

Advanced glycation end products (AGEs) also influence the expression of certain genes by binding to specific AGE receptors localized on fibroblasts, T-lymphocytes, in the kidneys (mesangial cells), in the vascular wall (endothelium and smooth muscle cells), in the brain, and also in the liver and spleen, where they are detected in the greatest number, i.e. in tissues rich in macrophages, which mediate the transduction of this signal by increasing the formation of oxygen free radicals. The latter, in turn, activate the transcription of nuclear factor NF-kB, a regulator of the expression of many genes that respond to various damage.

One of the effective ways to prevent the undesirable consequences of non-enzymatic glycosylation of proteins is to reduce the calorie content of food, which is reflected in a decrease in the concentration of glucose in the blood and a decrease in the non-enzymatic addition of glucose to long-lived proteins, such as hemoglobin. A decrease in glucose concentration leads to a decrease in both protein glycosylation and lipid peroxidation. The negative effect of glycosylation is due to both the disruption of structure and function when glucose attaches to long-lived proteins, and the resulting oxidative damage to proteins caused by free radicals formed during the oxidation of sugars in the presence of transition metal ions. Nucleotides and DNA also undergo non-enzymatic glycosylation, which leads to mutations due to direct DNA damage and inactivation of repair systems, causing increased fragility of chromosomes. Approaches to preventing the effects of glycation on long-lived proteins through pharmacological and genetic interventions are currently being explored.

The initial process of food processing occurs in the oral cavity. In the oral cavity occurs: grinding of food; wetting it with saliva; formation of a food bolus.

Food remains in the mouth for 10-15 seconds, after which it is pushed into the pharynx and esophagus by muscle contractions of the tongue.

Food entering the mouth is an irritant to taste, tactile and temperature receptors located in the mucous membrane of the tongue and scattered throughout the mucous membrane of the oral cavity.

Impulses from the receptors along the centripetal fibers of the trigeminal, facial and glossopharyngeal nerves enter the nerve centers that reflexively stimulate secretion salivary glands, glands of the stomach and pancreas, bile secretion. Efferent influences also change the motor activity of the esophagus, stomach, proximal small intestine, affect the blood supply to the digestive organs, and reflexively increase the energy consumption necessary for processing and assimilation of food.

Those. Despite the short stay of food in the oral cavity (15-18 s), triggering effects come from its receptors on almost the entire digestive tract. Irritation of the receptors of the tongue, oral mucosa and teeth is especially important in the implementation of digestive processes in the oral cavity itself.

Chewing is one of the initial phases of the food absorption process, consisting of grinding, grinding and mixing food with saliva, i.e. in the formation of the food bolus.

Wetting and mixing with saliva is necessary for dissolution, without which it is impossible to evaluate the taste of food and its hydrolysis.

Chewing occurs due to contractions of the masticatory muscles, which move the lower jaw relative to the upper jaw. Facial muscles and tongue muscles also take part in the process.

A person has 2 rows of teeth. Each has incisors (2), canines (2), small (2) and large (3) molars. The incisors and canines bite off food, the small molars crush it, and the large molars grind it. The incisors can develop a pressure on food of 11-25 kg/cm2, molars - 29-90. The act of chewing is carried out reflexively, has a chain nature, automated and voluntary components.

The motor nuclei of the medulla oblongata, red nucleus, substantia nigra, subcortical nuclei and cerebral cortex take part in the regulation of chewing. The set of neurons that control chewing is called the chewing center. Impulses from it travel through the motor fibers of the trigeminal nerve to the masticatory muscles. They make movements lower jaw down, up, forward, backward and sideways. The muscles of the tongue, cheeks, and lips move the bolus of food in the oral cavity, serve and hold food between the chewing surfaces of the teeth. In the coordination of chewing, impulses from the proprioceptors of the masticatory muscles and mechanoreceptors of the oral cavity and teeth play an important role.

The study of the chewing process is complex: cinematic method, electromyographic method. The graphic method of registration is called: masticationography.

The masticatiograph consists of a rubber balloon placed in a special plastic case, which is attached to the lower jaw. The balloon is connected to a Marey capsule, the pen of which records the movements of the jaw on the kymograph drum. Masticationography distinguishes the following phases: rest, introduction of food into the mouth, indicative, main, formation of a food bolus.

Salivary glands.

Saliva is produced by three pairs of large glands ( parotid, submandibular and sublingual) and many small glands of the tongue, mucous membrane of the palate and cheeks . Saliva enters the oral cavity through the excretory ducts.

The saliva of the glands has a different consistency: the sublingual and submandibular glands secrete more viscous and thick saliva than the parotid gland. This difference is determined by the presence of a protein substance – mucin.

Mixed secretion (with mucin) is isolated:

submandibular glands

sublingual glands

glands in the mucous membrane of the tongue and palate.

Serous secretion (liquid saliva with a high concentration of sodium, potassium and high amylase activity) is isolated

parotid

small glands on the lateral surfaces of the tongue.

Mixed saliva has a pH of 5.8-7.4 (the saliva of the parotid glands has a pH<5,81). С увеличением скорости секреции рН слюны повышается до 7,8.

Mucin gives saliva a peculiar slimy appearance and slipperiness, making saliva-soaked food easier to swallow.

Saliva contains several enzymes: α-amylase, α-glucosidase.

Salivary enzymes are highly active, but complete breakdown of carbohydrates does not occur due to the short duration of food in the mouth. Hydrolysis of carbohydrates with the help of these enzymes continues inside the food bolus in the stomach. On the surface of the food bolus, the acidic environment (HCl0.01%) stops the action of enzymes.

Proteolytic enzymes of saliva are important for the sanitation of the oral cavity. For example, lysozyme is highly bactericidal; Proteinase - disinfectant effect.

The amount and composition of saliva are adapted to the type of food eaten, diet, and food consistency.

More viscous saliva is secreted for food substances, and the more of it, the drier the food. For rejected substances and bitterness - a significant amount of liquid saliva.

The saliva secreted for most food substances contains 4 times more mucin than the saliva secreted when so-called rejected substances (hydrochloric acid, bitters, etc.) are introduced into the mouth.

Methods for studying salivation.

In dogs: fistula of the excretory duct of the parotid gland or submandibular gland with a piece of mucous membrane.

In humans: using a capsule - the Lashley-Krasnogorsky funnel, which is applied to the excretory duct of the salivary gland.

Regulation of salivation.

Outside of food intake, a person secretes saliva at a rate of 0.24 ml/min, during chewing - 3-3.5 ml/min, with the introduction of citric acid (0.5 mmol) - 7.4 ml/min.

Eating food stimulates salivation both conditionally and unconditionally as a reflex.

The irritant of unconditioned salivary reflexes is food or rejected substances that act on the receptors of the oral cavity.

The time between (food intake) exposure to a stimulus and the onset of salivation is called the latent period. (1-30 sec.)

Impulses from the receptors enter the salivary center, located in the medulla oblongata (in the region of the nuclei of the glossopharyngeal nerve). When this area is irritated, abundant secretion of saliva with a different qualitative composition can be obtained.

Impulses to the salivary glands follow efferent parasympathetic and sympathetic nerve fibers.

Parasympathetic influences. Under the influence of acetylcholine released by the endings of postganglionic neurons, a large amount of liquid saliva with a high concentration of electrolytes and low mucin is released. They stimulate salivation and kinins, which dilate the blood vessels of the salivary glands.

Sympathetic influences. Norepinephrine, secreted by the endings of postganglionic neurons, causes the release of a small amount of thick saliva and enhances the formation of mucin and enzymes in the glands.

Simultaneous stimulation of the parasympathetic nerves enhances the secretory effect. Differences in secretion in response to different foods are explained by changes in the frequencies of impulses along the parasympathetic and sympathetic nerve fibers. These changes can be unidirectional or multidirectional.

Factors leading to inhibition of salivation: negative emotions; dehydration of the body; painful irritations, etc.

Decreased secretion of the salivary glands - hyposalivation.

Excessive salivation – hypersalivation.

Swallowing.

Chewing ends with swallowing - the transition of a bolus of food from the oral cavity to the stomach.

According to Magendie's theory, the act of swallowing is divided into 3 phases - oral voluntary; pharyngeal involuntary (fast); esophageal involuntary – long-term, slow.

1) A food bolus with a volume of 5-15 cm 3 is separated from the crushed and moistened food mass in the mouth. This lump is pressed against the hard palate by voluntary movements of the front and then the middle part of the tongue and transferred to the root of the tongue by the front arches.

2) As soon as the food bolus hits the root of the tongue, the act of swallowing goes into a fast involuntary phase, which lasts ~ 1 second. This act is a complex reflex and is regulated by the swallowing center in the medulla oblongata. Information to the swallowing center goes through afferent fibers of the trigeminal nerve, laryngeal nerves and glossopharyngeal nerve. From it, impulses along the efferent fibers of the trigeminal, glossopharyngeal, hypoglossal and vagus nerves go to the muscles that ensure swallowing. If you treat the root of the tongue and pharynx with a cocaine solution (turn off the receptors), then swallowing will not occur.

The center of swallowing is located in the medulla oblongata, in the region of the bottom of the fourth ventricle, slightly above the center of breathing. It is associated with the respiratory center, vasomotor and centers that regulate the activity of the heart. During the act of swallowing, breathing is held and heart rate increases.

A reflex contraction of the muscles that lift the soft palate occurs (which prevents food from entering the nasal cavity). By moving the tongue, the bolus of food is pushed into the pharynx. At the same time, contraction of the muscles occurs, displacing the hyoid bone and causing the larynx to rise, as a result of which the entrance to the respiratory tract is closed, which prevents the entry of food into them.

The transfer of the food bolus into the pharynx is facilitated by an increase in pressure in the oral cavity and a decrease in pressure in the pharynx. The raised root of the tongue and the arches tightly adjacent to it prevent the reverse movement of food into the oral cavity.

Following the entry of the food bolus into the pharynx, the muscles contract, narrowing its lumen above the food bolus, as a result of which it moves into the esophagus. This is facilitated by the pressure difference in the cavities of the pharynx and esophagus. Before swallowing, the pharyngoesophageal sphincter is closed; during swallowing, the pressure in the pharynx rises to 45 mmHg. Art., the sphincter opens, and the food bolus enters the beginning of the esophagus, where the pressure is no more than 30 mm Hg. Art.

The first two phases of the act of swallowing last about 1 s.

3) Movement of food through the esophagus.

The movement of the food bolus through the esophagus occurs (immediately, immediately) following the swallowing movement (automatically, reflexively).

The passage time for solid food is 8-9 seconds.

The passage time for liquid food is 1-2 seconds.

The contraction of the esophageal muscles has the character of a wave, occurring in the upper part of the esophagus and further along its entire length (peristaltic contractions). At the same time, the ring-shaped muscles of the esophagus sequentially contract, moving the food bolus. A wave of decreased tone (relaxation) moves in front of him. The speed of its movement is greater than the waves of contraction, and it reaches the stomach in 1-2 s.

The primary peristaltic wave caused by swallowing reaches the stomach. At the level of the intersection of the esophagus with the aortic arch, a secondary wave occurs. The secondary wave also propels the bolus of food to the cardiac part of the stomach. The average speed of its spread is 2-5 cm/s, covering a section of the esophagus of 10-30 cm in 3-7 s.

Regulation of esophageal motility is carried out by efferent fibers of the vagus and sympathetic nerves; The intramural nervous system plays a major role.

Outside of swallowing movements, the entrance to the stomach is closed by the lower esophageal sphincter. When the relaxation wave reaches the final part of the esophagus, the sphincter relaxes and the peristaltic wave carries the bolus of food into the stomach.

When the stomach is filled, the tone of the cardia increases, which prevents the contents from refluxing into the esophagus.

Parasympathetic fibers of the vagus nerve stimulate peristalsis of the esophagus and relax the cardia; sympathetic fibers inhibit the motility of the esophagus and increase the tone of the cardia.

In some pathological conditions, the tone of the cardia decreases, peristalsis of the esophagus is disrupted - the contents of the stomach can be thrown into the esophagus (heartburn).

A swallowing disorder is aerophagia - excessive swallowing of air. This excessively increases intragastric pressure, and the person experiences discomfort. Air is expelled from the stomach and esophagus, often with a characteristic sound (belching).