14-Mar-2013 | No comments | Lolita Okolnova

Nutrition is a very complex, multi-stage process. And a lot depends on it too. The essence of digestion is the conversion of nutrients into energy necessary for the life of the body.

Digestive system

human

In the human body, food is processed both mechanically and chemically.

Organs of the digestive system

Traditionally, the organs of the digestive system are divided into 3 groups - according to the stages of food processing:

1. Mechanical processing - these are organs up to the stomach: the oral cavity, pharynx and esophagus;
2. Chemical treatment - stomach, glands:, small and large intestines;
3. Organs to excrete the remains of digestion from the system.

Digestion in oral cavity

Digestion starts from the mouth itself.

Mechanical grinding of food is carried out with the help of teeth, and VERY important role is played by salivary glands.

The composition of saliva:

• saliva has an alkaline environment, tk. contains alkali metal salts, i.e. affects those bacteria that enter the mouth with food;
• about 90% - water, saliva softens food;
• enzymes - are part of saliva and break down to monomers. Active enzyme that breaks down carbohydrates amylase.

Digestion begins in the oral cavity, and it begins with the breakdown of carbohydrates.

Pharynx And esophagus- due to muscle contractions, they move food down to the stomach.

human stomach

- a hollow muscular organ located in the left hypochondrium.

In the stomach, food is exposed to intense action in the first place, gastric juice.
Composition of gastric juice contains hydrochloric acid - HCl. How does such a powerful acid not dissolve the walls of the stomach?

From the inside, this organ of the digestive system is lined with a fairly thick mucous membrane. It forms numerous folds, thereby increasing the surface area.

This is how the walls of the human stomach look from the inside - a huge number of folds ...

If it is depleted for any reason, then the acid begins to act corrosively, and then it is called gastritis, which can develop into a stomach ulcer.

Also gastric juice contains enzymes.

The main digestive enzymes of gastric juice are pepsin and lipase.

Break down in the stomach protein and partially fat components food.

Absorption of the received nutrients takes place in the stomach.

Human small intestine

After the stomach, food enters the small intestine. This is where most of the digestion takes place.

Fats are digested in the small intestine.

The small intestine is the longest organ of the digestive system.

At the very beginning of the small intestine, immediately after the stomach, there is a section called duodenum (its length is equal to the thickness of 12 human fingers) .

The common intestine opens into the duodenum bilious duct And pancreatic duct.

It is in the duodenum that the process of intestinal digestion begins. Another important function of the duodenum is to initiate and regulate the secretion of pancreatic enzymes and bile, depending on the acidity and chemical composition food slurry entering it.

In the small intestine there is a thick layer of mucous membrane, plus there is also a huge amount of intestinal villi - they absorb nutrients.

It is interesting that there are organisms in the human intestine -. They are called intestinal microflora.

There are a lot of functions, the bottom line is that if these bacteria somehow die in a person, then human digestion is practically reduced to zero. This threatens with serious diseases outside the digestive system.

Colon

This is the very end of the digestive tract where water is absorbed and stool is formed. The end of the large intestine is the rectum, which in turn ends in the anus.

Thus, dissimilation - energy exchange occurs in digestive system in the following way:

• break down in the mouth
• break down in the stomach
• split in the stomach and in the small intestine of the digestive system.

human digestive systemis regulated not only chemically - with the help of enzymes and hormones, but also with the help of

Lesson topic: Digestion in the mouth. swallowing.

Lesson motto:"Who chews well, he lives long."

Tasks:

• Educational:
  • to form in students new anatomical and physiological concepts about nutrients, digestion, the structure and functions of the digestive organs, enzymes, digestive glands, absorption, and hygienic conditions for normal digestion.
  • develop the ability to experiment, work with a book, substantiate the rules of digestive hygiene.
• Educational:
  • for physical and hygienic education, explain the hygienic conditions of normal nutrition, prove the harm of smoking and drinking alcohol, the dependence of human health and performance on the prevention and treatment of gastrointestinal diseases.
• Educational:
  • using active, problem-search methods of teaching, questions for reflection and independent work with a textbook, to develop creative thinking, speech, and cognitive abilities of students.

Equipment: tab. "Scheme of the structure of the digestive organs", "Unconditioned salivary reflex", tab. "Conditioned reflex salivation".

Laboratory equipment for demonstration of experience: 2 pieces of starched gauze, matches, cotton wool, a Petri dish (or a regular saucer) with iodine and a glass of clean water.

The main content of the lesson:

1. Digestion in the oral cavity:
- the role of teeth in the mechanical processing of food;
- salivary glands and their functions (general characteristics)
2. Hygienic rules for the care of teeth and oral cavity.
3. Chemical processing of food in the oral cavity. Enzymes of saliva and the specifics of their action (laboratory work).
4. Reflex regulation of salivation (scheme of unconditioned salivary reflex; examples of conditioned reflex salivation).
5. Swallowing.

The main stages of the lesson:

1. Mobilizing and activating the beginning of the lesson. Creating a problematic situation by posing the question “What is health? Why do people say hello?
2. Frontal search conversation to resolve a problematic issue.
3. Knowledge update. Checking knowledge on the previous topic.
4. Explanation of the main material. Teacher's story, frontal filling of the table. Notes in a notebook.
5. Partial reinforcement.
6. Laboratory work. Heuristic (partial search method). Explanation of the purpose of the saliva experiment (expected result not reported).
7. A brief briefing on how the experiment is performed and what to do at the same time.
8. Organization independent work, study of the results of the experiment, design of notebooks (short report and conclusion).
9. Generalization and consolidation.
10. Operational diagnostics of the quality of training using “are the statements true”.
11. Concluding the lesson with an appeal to the motto: "He who chews well, he lives long."

DURING THE CLASSES

1. Updating knowledge

A. What is health? Why are they saying hello? (Conversation with students)
B. What is the importance of digestion?
Student's answer: "For chemical and mechanical processing of food"

Today the purpose of our lesson:

1) reveal the importance of mechanical and chemical processing of food in the oral cavity;
2) get acquainted with enzymes that break down salivary substances into simpler ones in the oral cavity.

You have to find out how and what happens to food in the oral cavity, to investigate the effect of enzymes on starch.

2. Survey

1. Work at the blackboard.

Bring in line.

Writing on the blackboard: meat, fish, milk, bread, vermicelli, fats, carbohydrates, vegetables, fruits.

2. Collect the digestive tract on a magnetic board (fig. in the textbook).

3. Write the sequence of the digestive tract.

Student record.

Mouth--> pharynx--> esophagus--> stomach--> small intestine--> large intestine--> rectum--> anus.

Parallel work with the class

Repetition of basic biological concepts (along the chain) term - definition.

Products, nutrition, digestion, enzymes, organ, tissue, organism, cell, esophagus, nutrients, anatomy, biology, hygiene, physiology.

The guys have finished their work at the blackboard - they voice their answers.
Summarizes the repetition of homework and the transition to a new topic.
Issues for discussion.
What path must the product take in order to be absorbed by the body and reach each cell?
What nutrients are included in food?
Proteins, fats, carbohydrates (student answer).
Where does the breakdown of these substances take place? (students answer).
What substances are these substances broken down into?
Proteins are amino acids
Fats - glycerin
Carbohydrates are starch.

Teacher: Today it is necessary to consider the breakdown of carbohydrates.

3. New theme

Writing in the notebook of the topic of the lesson.

Explanation of the material.

Issues for discussion.

• Why does the sight of a cut lemon cause salivation?
• Why is it not recommended to talk while eating?

(Answers vary).

The teacher works at the blackboard, the students write in notebooks.
What happens in the oral cavity?

Filling in the table:

Organs	Structural features	Functions
1. Mucous membrane	epithelial tissue	Protects the mouth, cavity from damage
2. Teeth	Alveolar - sit in the cells of the jaw Crown, Neck, Root. 3 2 1 2 2 1 2 3	Bite off (cutters). Tearing (fangs). They grind (indigenous). Mechanical processing of food.
3. Language	Attached to the bottom of the oral cavity, consists of a cross-striped muscle tissue covered with taste buds.	Approbation.
4. Salivary glands	3 pairs of salivary glands; glandular epithelium.	Produces saliva containing: a) lysocin; b) amylase.

4. Fixing

1. What happens in the oral cavity?

• Approbation of food (38 - 52 C).
• Mechanical processing of food.
• Wetting with saliva.
• Disinfection.
• Chemical processing of food.
• Formation of a food bolus.
• Ingestion.

2. Laboratory work.

"The action of saliva on starch" using a tubeless test with saliva.
Before the lesson, students are given two pieces of starched gauze, matches, cotton wool, a glass of clean water on their desks.
Students briefly talk about digestive enzymes, the breakdown of starch in the mouth, and swallowing.
As a result of this conversation, students should repeat the general properties of enzymes:
1) Enzymes are catalysts and therefore can speed up certain processes.
2) Enzymes act only on certain substrates.
3) Enzymes are able to act only under certain temperature conditions and in a certain environment: acidic, alkaline, neutral.

4) Enzymes - proteins, when boiled, they are destroyed and lose their enzymatic properties.

Properties of digestive enzymes:

1) Salivary enzymes act on salivary carbohydrates, they convert starch into glucose. Starch is insoluble, it cannot be absorbed into the blood, but glucose is absorbed.

2) Saliva enzymes act on starch. They break down these substances into products that can be absorbed into the blood and lymph.

The task. Prove that saliva enzymes are capable of decomposing starch.

The results of the experiment in a notebook.

Output(make notes).

3. Are the statements true:

1) In the oral cavity, only mechanical processing of food occurs. (-)
2) Saliva is released into the oral cavity only during meals. (-)
3) Saliva enzymes break down starch into glucose. (+)
4) Saliva is produced by three pairs of salivary glands. (+)
5) Enzymes slow down the digestion process. (-)
6) The breakdown of carbohydrates begins in the oral cavity. (+)
7) The epiglottis prevents food from entering the Airways. (+)
8) The salivary glands produce enzymes that break down carbohydrates. (+)
9) Lysozyme corrodes enamel. (-)
10) Each jaw has 4 incisors. (+)

5. Summary of the lesson

6. Homework

Carbohydrates are digested in the mouth by salivary enzymes. 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 a small amount. The active center of α-amylase contains Ca 2+ ions. Na + ions activate the enzyme.

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

The main site of carbohydrate digestion is the duodenum, where it is excreted as part of the pancreatic juice. α- amylase. This enzyme completes the breakdown of starch and glycogen, initiated by salivary amylase, to 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 from food is carried out in the area of ​​the brush border of epithelial cells (enterocytes) small intestine. Disaccharidases are integral proteins of enterocyte microvilli. They form a polyenzymatic 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 the D-galactose and D-glucose.

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

Milk intolerance in 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. Impaired tolerance symptoms develop very quickly after birth . Feeding lactose-free food leads to the disappearance of symptoms.

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

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

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

Nb! Glucose enters 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.

Two mechanisms of glucose entry into cells are known: facilitated diffusion and secondary active transport associated with the transfer of Na + ions. Fig.5.1. The 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 structural feature of which is a long polypeptide chain that forms 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- terminal sections of the carrier are turned inside 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 moving glucose into the cell. The carriers of this family contain 492-524 amino acid residues and differ in their affinity for glucose. Each transporter appears to perform specific functions.

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

Below, in tab. 5.1. some properties of monosaccharide carriers are given.

Table 5.1. Characterization of glucose transporters in animals
			Main places of education
secondary active transport
	Glucose absorption		Small intestine, kidney tubules
	Glucose absorption		renal tubules
accelerated diffusion
			Placenta, blood-brain barrier, brain, red blood cells, kidneys, large intestine, other organs
	Glucose sensor in B cells; transport from epithelial cells of the kidneys and intestines		Islet cells, liver, small intestine epithelium, kidneys
	Use of glucose by cells under physiological conditions		Brain, placenta, kidneys, other organs
	Insulin-stimulated glucose uptake		Skeletal and cardiac muscle, adipose tissue, other tissues
	Fructose transport		Small intestine, spermatozoa

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 provides, together with sodium ions, the movement (symport) of glucose into the enterocyte. Sodium ions are then actively, with the participation of Na + -K + -ATPase, removed from the enterocyte, 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 the lymphatic system and the pulmonary circulation. Excess glucose is stored in the liver in the form of glycogen.

NB! The exchange of glucose 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 of which is 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 hesokinase isoenzymes ( hexokinase 4), there are important differences between hexokinase and glucokinase: 1) hexokinase is able to phosphorylate not only glucose, but also other hexoses (fructose, galactose, mannose), while glucokinase activates only glucose; 2) hexokinase is present in all tissues, glucokinase - 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.

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

Since the glucokinase reaction is insulin-dependent, it is possible to prescribe fructose 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 for the conversion of glucose and in the synthesis of other monosaccharides necessary for the cell. The place that this reaction occupies in glucose metabolism allows it to be considered the key reaction of carbohydrate metabolism.

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

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

Rice. 5.11. Scheme of localization of enzymatic systems of digestion of carbohydrates

5.2.1. Digestion of carbohydrates by mouth and pancreas -amylase ( -1,4-glycosidase). Dietary polysaccharides, namely starch (consists of a linear amylose polysaccharide, in which glucosyl residues are linked by -1,4-glycosidic bonds, and amylopectin, a branched polysaccharide, where -1,6-glycosidic bonds are also found) , begin to hydrolyze already in the oral cavity after wetting with saliva containing the hydrolytic enzyme -amylase (-1,4-glycosidase) (EC 3.2.1.1), which cleaves 1,4-glycosidic bonds in starch, but does 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 some 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 coma, where amylase activity can persist for some time until the pH in the entire piece becomes acidic. In the gastric juice there are no enzymes that break down carbohydrates, only a 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 pancreatic secretion containing bicarbonates HCO 3 - and having a pH of 7.5-8.0. In the secret of the pancreas, pancreatic amylase is found, which hydrolyzes -1,4-glycosidic bonds in starch and dextrins with the formation of maltose disaccharides (in this carbohydrate, two glucose residues are linked by -1,4-glycosidic bonds) and isomaltose (in this carbohydrate, two glucose residue located at the branching sites in the starch molecule and linked by α-1,6-glycosidic bonds). Oligosaccharides are also formed containing 8–10 glucose residues linked by both -1,4-glycosidic and -1,6-glycosidic bonds.

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

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

The maltose, isomaltose and triose sugars formed in the upper intestine are further hydrolyzed in the small intestine by specific glycosidases. Dietary disaccharides, sucrose and lactose, are also hydrolyzed by specific disaccharidases in the small intestine.

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

Cleaving glycosidic bonds in disaccharides, enzymes (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 sucrase-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 α-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. The sucrose 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 the family of 31 glycosyl hydrolases. Being an exoglycosidase, the enzyme acts from the reducing end, it can also break down maltose, acting as maltase in this reaction (in this case, the glycoamylase complex hydrolyzes the remaining 20% ​​of the maltose oligo- and polysaccharides formed during digestion). The complex includes two catalytic subunits with slight differences in substrate specificity. The complex is most active 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. Tregalase. This enzyme (EC 3.2.1.28) is a glycosidase complex that hydrolyzes 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 carbons.

As a result of the action of glycosyl hydrolases, monosaccharides are formed from food carbohydrates as a result of the action of glycosyl hydrolases: glucose, fructose, galactose in a large amount, 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 the concentration gradient, and glucose  against the concentration gradient (secondary active transport), therefore, the larger the gradient, the more glucose will be transferred to the enterocytes. With a decrease in the concentration of Na + in the extracellular fluid, 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 by the mechanism of secondary active transport.

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

Due to active transport, enterocytes can absorb glucose at its low concentration in the intestinal lumen. At a high concentration of glucose, it enters the cells by facilitated diffusion with the help of special carrier proteins (transporters). In the same way, fructose is transferred into the epithelial cells.

Monosaccharides enter the blood vessels from enterocytes mainly by facilitated diffusion. Half of the glucose through the capillaries of the villi through the portal vein is transported 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. The glucose transporter (transporter) from erythrocytes (GLUT1), as seen in 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 outer side of the membrane there is a glucose binding site. When glucose is bound, the conformation of the carrier changes, and the monosaccharide binding site becomes open inside the cell. Glucose passes into the cell, separating from the carrier protein.

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

GLUT 1, localized in the brain, placenta, kidneys, large intestine, erythrocytes, supplies glucose to the brain.

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

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

GLUT 4 transports glucose into 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 tolerate fructose.

All carriers can be located both in the cytoplasmic

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

vesicles in cells and in the 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, fuse with it, and GLUT 4 is incorporated into the membrane, after which the transporter facilitates the diffusion of glucose into the cell. After a decrease in the concentration of insulin in the blood, the transporters return to the cytoplasm again and the transport of glucose into the cell stops.

Various disorders have been identified in the work of glucose transporters. With a hereditary defect in carrier proteins, non-insulin-dependent diabetes mellitus develops. In addition to protein defects, there are other disorders caused by: 1) a defect in the transmission of the insulin signal about the movement of the transporter to the membrane, 2) a defect in the movement of the transporter, 3) a defect in the inclusion of the protein in the membrane, 4) a violation of the lacing from the membrane.

5.2.8. Insulin. This compound is a hormone secreted by the β-cells of the islets of Langerhans of 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 interconnected by two disulfide bonds: A7-B7, A20-B19. Inside 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: but porcine insulin, T-conformation, b human insulin, R-conformation (A-chain is shown red color, B-chain  yellow) (according to )

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

Mammalian insulins have a great homology in primary structure with human insulin: for example, in pig insulin there is only one substitution - instead of threonine at the carboxyl end of the B-chain there is alanine, in bovine insulin there are three other amino acid residues in comparison with human insulin. Most often, substitutions occur at positions 8, 9, and 10 of the A chain, but they do not significantly affect 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 the 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 that enters the lumen of the endoplasmic reticulum and directs the movement of the growing polypeptide. At the end of synthesis, the signal peptide, 24 amino acid residues long, is cleaved from preproinsulin to form proinsulin, which contains 86 amino acid residues and is transferred to the Golgi apparatus, where further maturation of insulin occurs in tanks. The spatial structure of proinsulin is shown in fig. 5.16.

In the process of prolonged maturation, under the action 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 C-peptide consisting of 31 amino acid residues being cleaved. 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 specificity similar to carboxypeptidase B, i.e., hydrolyzes peptide bonds, the imino group which belongs to the main amino acid (Fig. 5.17 and 5.18).

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

Insulin and C-peptide in equimolar amounts enter the secretory granules, where insulin, interacting with the zinc ion, forms dimers and hexamers. Secretory granules, merging with the plasma membrane, secrete insulin and C-peptide into the extracellular fluid as a result of exocytosis. The half-life of insulin in blood plasma is 3–10 min, that of C-peptide is about 30 min. Insulin undergoes breakdown by the action of the enzyme insulinase, this process takes place 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 protein genes involved in the metabolism of the main energy carriers. Glucose can directly bind to transcription factors, which has a direct effect on the rate of gene expression. A secondary effect on the secretion of insulin and glucagon is possible, when the release of insulin from secretory granules activates the transcription of insulin mRNA. But the secretion of insulin depends on the concentration of Ca 2+ ions and decreases with their deficiency even at a high concentration of glucose, which activates the synthesis of insulin. In addition, it is inhibited by adrenaline when it binds to  2 receptors. Stimulators of insulin secretion are growth hormones, cortisol, estrogens, hormones of the gastrointestinal tract (secretin, cholecystokinin, gastric inhibitory peptide).

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

The secretion of insulin by β-cells of the islets of Langerhans in response to an increase in the concentration of glucose in the blood is realized 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 cleavage of arginine residues at the N-terminus of the A-chain and C-terminus B-chains under the action of carboxypeptidase E (cleaved off arginine residues are shown in circles). As a result of processing, in addition to insulin, a C-peptide is formed (according to)

1) glucose is transported into -cells by the GLUT 2 carrier protein;

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

3) under the action of ATP, potassium ion channels are closed and the membrane is depolarized;

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

5) an increase in the level of calcium in the cell activates phospholipase C, which cleaves 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 increase the transport of glucose across the cell membrane. Stimulation with insulin leads to an increase in the rate of glucose entry into the cell by 20–40 times. When stimulated with insulin, there is a 5–10-fold increase in the content of glucose transport proteins in plasma membranes with a simultaneous decrease by 50–60% of their content in the intracellular pool. The required amount of energy in the form of ATP is required 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 of glucose is required to move glucose transporters. Translocation of glucose transporters to the cell membrane is observed as early as a few minutes after the interaction of insulin with the receptor, and further stimulatory effects of insulin are required to accelerate or maintain the process of cycling of transporter proteins.

Insulin, like other hormones, acts on cells through the corresponding receptor protein. The insulin receptor is a complex integral cell membrane protein consisting of two -subunits (130 kDa) and two -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 interacting with the hormone and linked to each other by disulfide bridges between cysteines 524 and the Cys682, Cys683, Cys685 triplet of both α-subunits (see Fig. 5.19, but), and two transmembrane -subunits exhibiting tyrosine kinase activity linked 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: but modular structure of the insulin receptor. Above - α-subunits linked by disulfide bridges Cys524, Cys683-685 and consisting of six domains: two containing leucine repeats L1 and L2, a cysteine-rich CR region, and three type III fibronectin domains Fn o , Fn 1 , ID (introduction domain) . Below - -subunits associated with the -subunit by the Cys647Cys872 disulfide bridge and consisting of seven domains: three fibronectin domains ID, Fn 1 and Fn 2 ST; b spatial arrangement of the receptor, one dimer is shown in color, the other is white, A  activating loop opposite the hormone binding site, X (red)  C-terminal part of the -subunit, X (black)  N-terminal part of the -subunit , yellow balls 1,2,3 - disulfide bonds between cysteine ​​residues at positions 524, 683-685, 647-872 (according to )

acid residues and consists of six domains: two domains L1 and L2 containing leucine repeats, a cysteine-rich CR region where the insulin binding site is located, and three type III fibronectin domains Fn o , Fn 1 , Ins (introduction 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 TM domain, a JM domain adjacent to the membrane, a TK tyrosine kinase domain, and a C-terminal CT . Two insulin binding sites were found on the receptor: one with high affinity, the other with low affinity. To conduct a hormone signal into the cell, insulin must bind to a high affinity site. This center is formed when insulin binds from the L1, L2, and CR domains of one -subunit and the fibronectin domains of another, while the arrangement of -subunits is opposite to each other, as shown in Fig. 5.19, from.

In the absence of insulin interaction with the center of high affinity of the receptor, -subunits are moved away from -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 -subunit with phosphorylation sites on another - sub-unit (Figure 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 α- and β-subunits from approaching, the activating loops of TK domains interact with tyrosine phosphorylation sites on the opposite TK domain, transphosphorylation of β-subunits occurs at seven tyrosine residues: Y1158 , Y1162, Y1163 of the activating loop (this is a kinase regulatory domain), Y1328, Y1334 of the ST domain, Y965, Y972 of the JM domain (Fig. 5.20, but), which leads to an increase in the tyrosine kinase activity of the receptor. At position 1030 of the TK there is a lysine residue included in the catalytic active center - 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 addition of insulin to such a receptor has no effect on cell metabolism and proliferation. Phosphorylation of some serine-threonine residues, on the contrary, reduces the 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 transducers and transcription activators are discussed in detail in Part 4 "Biochemical basis of defense reactions").

IRS-1 is a cytoplasmic protein that binds to the phosphorylated tyrosines of the insulin receptor TK with its SH2 domain and is phosphorylated by the tyrosine kinase of the receptor immediately after insulin stimulation. The degree of phosphorylation of the substrate depends on 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 may be the cause of 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. Simplified scheme of structural changes in the binding of insulin to the insulin receptor: but change in receptor conformation as a result of hormone binding at the high affinity center leads to displacement of the protrusion, convergence of subunits and transphosphorylation of TK domains; b in the absence of insulin interaction with the high affinity binding site on the insulin receptor, the protrusion (cam) prevents the approach of - and -subunits and transphosphorylation of 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 TK, set 1 and set 2 - amino acid sequences of -subunits that form a place of high affinity of insulin to 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 PHI-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 action of insulin. The interaction of insulin with the receptor leads to phosphorylation of the insulin receptor substrate (IRS) that binds PI-3-kinase (PI3K), which catalyzes the synthesis of the phosphatidylinositol-3,4,5-triphosphate phospholipid (PtdIns(3,4,5)P3). The latter compound, by binding plextrin domains (PH), mobilizes protein kinases PDK1, PDK2, and PKV to the cell membrane. PDK1 phosphorylates RKB at Thr308, activating it. Phosphorylated RKV associates with GLUT4-containing vesicles, causing their translocation to the plasma membrane, leading to increased glucose transport into muscle and fat cells (according to )

Stimulated by phosphorylated IRS-1, phospholipase C hydrolyzes the cell membrane phospholipid phosphatidylinositol-4,5-diphosphate to form two second messengers: inositol-3,4,5-triphosphate 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 a change in the activity of cellular systems.

Phosphorylated IRS-1 also activates PHI-3-kinase, which catalyzes the phosphorylation of 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.

PHI-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 attaches to IRS-1 with high affinity. Phosphatidylinositol derivatives formed in the membrane, phosphorylated at position 3, bind proteins containing the so-called plextrin (PH) domain (the domain exhibits a high affinity for phosphatidylinositol-3-phosphates): protein kinase PDK1 (phosphatidylinositide-dependent kinase), protein kinase B (PKV).

Protein kinase B (PKB) consists of three domains: N-terminal plextrin, central catalytic, and C-terminal regulatory. The plectrin domain is required for RKV activation. By binding with the help of the plextrin domain near the cell membrane, PKV approaches the protein kinase PDK1, which through

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

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

The effect on the metabolism of glucose 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 the 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. Diagram of glycogen synthesis

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

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

5.2.9. Glucagon. In response to a decrease in the concentration of glucose in the blood, the -cells of the islets of Langerhans of the pancreas produce the "hunger hormone" - glucagon, which is a polypeptide of molecular weight 3485 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 a rapid mobilization of potential energy sources.

Rice. 5.23. The structure of human proglucagon and tissue-specific processing of proglucagon into proglucagon-derived peptides: glucagon and MPGF (mayor proglucagon fragment) are formed from proglucagon in the pancreas; Glycentin, oxyntomodulin, GLP-1 (a peptide derived from proglucagon), GLP-2, two intermediate peptides (intervening peptide - IP), GRPP - glicentin-related pancreatic polypeptide (polypeptide from pancreas - a derivative of glycentine) (according to )

The hormone is synthesized by -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.

To bind glucagon to the receptors of responding cells, the integrity of its 1-27 sequence 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, the 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, while 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 inhibited 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 activates adenylate cyclase, which catalyzes the formation of cAMP, by means of a G-protein, which, in turn, leads to the 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-diphosphatase. The net effect of glucagon in the liver is to increase the production of glucose.

In fat cells, the hormone also, using the adenylate cyclase cascade, activates the hormone-sensitive triacylglycerol lipase, stimulating lipolysis. Glucagon increases the secretion of catecholamines by the adrenal medulla. By participating in the implementation of reactions such as "fight or flight", 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, insulin, glucagon, and adrenaline; in the muscles, insulin and adrenaline. Switching of the processes of synthesis or breakdown of glycogen occurs during the transition from the absorptive period to the postabsorptive period or when the state of rest changes 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 occur. These hormones regulate the processes of glycogen synthesis and breakdown by influencing 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 by 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 but and inactive glycogen synthase) and dephosphorylated (inactive phosphorylase b and active glycogen synthase) (Figures 5.24 and 5.25). Phosphorylation is carried out by a kinase catalyzing 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 casein kinase (CKII) phosphorylation. Insulin, glucose, and glucose-6-phosphate activate phosphoprotein phosphatase, while glucagon and epinephrine (epinephrine) inhibit it. Insulin inhibits the action of glycogen synthase kinase 3 (according to)

cAMP-dependent protein kinase A (PKA) phosphorylates phosphorylase kinase, turning 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 protein kinase pp90S6, 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. A change in the concentration of glucose in the blood also changes the relative concentrations of hormones: insulin and glucagon. The ratio of the concentration of insulin to the concentration of glucagon 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 mentioned 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 by synthesis from other substances - gluconeogenesis. From glycogen, glucose-1-phosphate is formed, which isomerizes into glucose-6-phosphate, which is hydrolyzed by the action of glucose-6-phosphatase to form free glucose that can leave the cell into the blood (Fig. 5.26).

The action of adrenaline on hepatocytes is similar to the action of glucagon in the case of the use of  2 receptors and is due to phosphorylation and activation of glycogen phosphorylase. In the case of the interaction of adrenaline with  1 -receptors of the plasma membrane, the 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 rises and the influence of insulin predominates. Insulin reduces the concentration of glucose in the blood, activates, by phosphorylation via the Ras pathway, cAMP phosphodiesterase, which hydrolyzes this second messenger with the formation of AMP. Insulin also activates via the Ras pathway phosphoprotein phosphatase of glycogen granules, which dephosphorylates and activates glycogen synthase and inactivates 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, glycogen breakdown 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 the further conversion of glucose-6-phosphate, formed from glycogen, ATP is synthesized, which is necessary for the implementation of intensive 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 glycogen breakdown occurs due to allosteric activation of glycogen phosphorylase b with the help of AMP and orthophosphate formed during ATP hydrolysis.

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

With 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 is attenuated, 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 calmodulin protein, in this case acting as one of the kinase subunits. The muscle phosphorylase kinase has the structure  4  4  4  4. Only the -subunit has catalytic properties, - and -subunits, being regulatory, are phosphorylated at serine residues using PKA, the -subunit is identical to the calmodulin protein (discussed in detail in Section 2.3.2, 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, muscle glycogen synthesis also occurs. Glucose enters muscle cells with the help of GLUT 4 carrier proteins (their mobilization into the cell membrane under the action of insulin is discussed in detail in Section 5.2.4.3 and in Fig. 5.21). The influence of insulin on the synthesis of glycogen in the muscles is also carried out through the dephosphorylation of glycogen synthase and glycogen phosphorylase.

5.2.11. Non-enzymatic glycosylation of proteins. One of the types 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 resulting from the multistep interaction of sugars with proteins are called advanced glycation 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 of the carbonyl group of glucose by the -amino group of lysine or the guanidinium group of arginine occurs, resulting in the formation of a labile Schiff base - N-glycosylimine (Fig. 5.29). The formation of the Schiff base is a relatively fast and reversible process.

Next comes the rearrangement N-glycosylimine with the formation of 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. Diagram of protein glycation. The open form of carbohydrate (glucose) reacts with the -amino group of lysine to form a Schiff base, which undergoes a rearrangement of Amadori to ketoamine through the intermediate formation of enolamine. Amadori rearrangement is accelerated if aspartate and arginine residues are located near the lysine residue. Ketoamine can further give a variety of products (glycation end products - AGE). 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. Modifications of lysine residues in proteins at an early stage of glycation, apparently, are facilitated by the presence of histidine, lysine, or arginine residues in the immediate vicinity of the reacting amino group, which carry out acid- the main catalysis of the process, as well as aspartate residues, pulling a proton from the second carbon atom of the sugar. Ketoamine can bind another carbohydrate residue at the imino group to form a double-glycated lysine, which turns into 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 glycation end products (AGEs). Recently, data have appeared on the direct participation in the formation of AGEs of α‑dicarbonyl compounds (glyoxal, methylglyoxal, 3‑deoxyglucozone), which are 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 the composition of proteins with glucose (Fig. 5.30). Specific reductases and sulhydryl compounds (lipoic acid, glutathione) are able to transform reactive dicarbonyl compounds into inactive metabolites, which is reflected in a decrease in the formation of glycation end 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 crosslinks, which are responsible for the 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 crosslinks.

Among AGEs characterized N ε ‑carboxymethyllysine (CML) and N ε -carboxyethyllysine (CEL), bis(lysyl)imidazole adducts (GOLD - glyoxal-lysyl-lysyl-dimer, MOLD - methylglyoxal-lysyl-lysyl-dimer, DOLD - deoxyglucoson-lysyl-lysyl-dimer), imidazolones (G-H, MG‑H and 3DG‑H), pyrraline, argpyrimidine, pentosidine, crosslin, and vesperlysin. 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 )

end products of glycation. For example, pentosidine and carboxymethyl lysine (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 protein surface, to 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. Pentosidin is a cross-link (a product of cross-linking) between the Amadori product and an arginine residue in any position of the protein, it is formed from ascorbate, glucose, fructose, ribose, found in the brain tissues of patients with Alzheimer's disease, in the skin and blood plasma of diabetic patients.

Glycation end products can promote free-radical oxidation, change in charge on the protein surface, irreversible cross-linking between different parts of the protein, which

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

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

Glycation end products (AGEs) also affect the expression of some 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, as well as in the liver and spleen, where they are most abundant, 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 the nuclear factor NF-kB, which regulates the expression of many genes that respond to various damages.

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 attachment 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 both to a violation of the structure and functions when glucose is attached 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. Currently, approaches are being studied to prevent the effect of glycation on long-lived proteins using pharmacological and genetic interventions.