Digestion is a complex multi-stage physiological process, during which food (a source of energy and nutrients for the body) entering the digestive tract undergoes mechanical and chemical processing.
Features of the digestion process
Digestion of food includes mechanical (moistening and grinding) and chemical processing. The chemical process involves a series of successive stages of breaking down complex substances into simpler elements, which are then absorbed into the blood.
Types of coagulant curds and enzymes
There are three types of enzymes.
Chymosin produced by fermentation
The activation process occurs by a mono- or bimolecular reaction, depending on the enzyme and conditions. This indicates that in most cases at least 85% of the amino acids are required to be identical with immunochemical cross-reactions.The enzyme has mainly endopeptide activity and very little exopeptide activity, this is due to the fact that the active site is extensive and can contain seven amino acid residues. For this reason, it has complex specificity and the enzyme appears to be nonspecific. Some existing aspartic proteases have molecular variants containing more or less enzymatic compositions, the microheterogeneity being more or less expressed by the set of coagulant enzymes. Microheterogeneity causes glycolysis, phosphorylation, deamidation or partial proteolysis.
This happens when mandatory participation enzymes that accelerate processes in the body. Catalysts are produced and are part of the juices they secrete. The formation of enzymes depends on what environment is established in the stomach, oral cavity and other parts of the digestive tract at one time or another.
Having passed the mouth, pharynx and esophagus, food enters the stomach in the form of a mixture of liquid and crushed by teeth. This mixture, under the influence of gastric juice, turns into a liquid and semi-liquid mass, which is thoroughly mixed due to the peristalsis of the walls. Next it enters the duodenum, where it is further processed by enzymes.
Specific molecular aspects
It is characterized by high specificity of milk coagulation and, as a rule, low proteolytic activity. Quimogen, also called prochymosin, is converted into an active enzyme by acid treatment. This occurs through the pseudochymosin intermediate at pH 2, where the rate of activation is rapid, which converts to chymosin at high pH. They are characterized by a high degree of proteolytic activity and resistance to heat treatment. These enzymes are homologous but have different specificities. . Digestion of food occurs as a result of a reaction called hydrolysis, which involves the breakdown of certain substances with the participation of water molecules.
The nature of the food determines what kind of environment will be established in the mouth and stomach. Normal in oral cavity slightly alkaline environment. Fruits and juices cause a decrease in pH oral fluid(3.0) and the formation of an acidic environment. Products containing ammonium and urea (menthol, cheese, nuts) can cause the saliva reaction to become alkaline (pH 8.0).
Structure of the stomach
The stomach is a hollow organ in which food is stored, partially digested and absorbed. The organ is located in the upper half of the abdominal cavity. If you draw a vertical line through the navel and chest, then approximately 3/4 of the stomach will be to the left of it. In an adult, the stomach volume is on average 2-3 liters. When consuming a large amount of food, it increases, and if a person is starving, it decreases.
These hydrolysis reactions are catalyzed by enzymes commonly called hydrolytic enzymes. Digestive enzymes are biological catalysts released in the organs of the digestive system that promote chemical reactions that reduce molecules, smaller organic compounds, present in foods, allowing them to be absorbed and used by the body.
Digestive enzymes are named according to the substrate on which they act, whether carbohydrates, lipids or proteins. Protease carbohydrase Lipase Nuclease Maltase Amylase. . Enzymes are very large and complex protein molecules, which act as catalysts in biochemical reactions. On starch they act by releasing various products, including dextrins and gradually small polymers consisting of glucose units. Produced in saliva and the pancreas, amylase is also produced by various fungi, bacteria and vegetables.
The shape of the stomach can change in accordance with its filling with food and gases, as well as depending on the condition of neighboring organs: pancreas, liver, intestines. The shape of the stomach is also influenced by the tone of its walls.
The stomach is an extended part of the digestive tract. At the entrance there is a sphincter (pyloric valve) that allows food to pass from the esophagus into the stomach in portions. The part adjacent to the entrance to the esophagus is called the cardiac part. To the left of it is the fundus of the stomach. The middle part is called the “body of the stomach”.
Amylases are divided into two groups: endoamylases and exoamylases. Endoamylases catalyze random hydrolysis in the starch molecule. Exoamylases exclusively hydrolyze -1,4 glycosidic linkages such as α-amylase or both α-1,4 and α-1,6 linkages such as amyloglucosidase and glycosidase. Amylase, like all other enzymes, acts as a catalyst, meaning it is not altered by the reaction but facilitates it, reducing the amount of energy required to achieve it. Amylase digests starches by catalyzing hydrolysis, which is the destruction by the addition of one molecule of water.
Between the antrum (end) of the organ and the duodenum there is another pylorus. Its opening and closing are controlled by chemical irritants released from small intestine.
Features of the structure of the stomach wall
The wall of the stomach is lined with three layers. The inner layer is the mucous membrane. It forms folds, and its entire surface is covered with glands (about 35 million in total), which secrete gastric juice, digestive enzymes, intended for chemical processing of food. The activity of these glands determines what environment in the stomach - alkaline or acidic - will be established in a certain period.
Thus, starch plus water is formed in maltose. Other enzymes then break down the maltose into glucose, which is absorbed through the walls small intestine, and after being taken into the liver it is used as energy. In addition to the catalytic breakdown of starch molecules, fungal alpha-amylase is a multienzyme capable of performing more than 30 enzymatic functions, including the breakdown of fat and protein molecules. It is also capable of converting 450 times more starch into maltose than own weight. -Amylase catalyzes the hydrolysis of fats, converting them into glycerol and fatty acids, proteins into proteoses and starch derivatives into dextrin and simpler sugars.
The submucosa has a rather thick structure, penetrated by nerves and vessels.
The third layer is a powerful membrane, which consists of smooth muscle fibers necessary for processing and pushing food.
The outside of the stomach is covered with a dense membrane - the peritoneum.
It has an activity pH close to 7. Indications:? -Amylase accelerates and facilitates the digestion of starch, fats and proteins. Thus, it can increase the body's utilization of food and be used to treat pancreatic secretion deficiency and chronic pancreatic inflammation, among other benefits.
Contraindications: Should not be administered to patients with known hypersensitivity to the fungal enzyme. Adverse reactions: the possibility of allergic reactions in persons with hypersensitivity to the fungal enzyme. Lipases can be of plant, porcine or microbial origin, and the latter has a significant advantage. Helpful when production deficiency occurs in the pancreas, lipase is an enzyme whose supplementation may be beneficial in cases of indigestion, celiac disease, cystic fibrosis and Crohn's disease.
Gastric juice: composition and features
The main role at the stage of digestion is played by gastric juice. The glands of the stomach are varied in their structure, but the main role in the formation of gastric fluid is played by cells that secrete pepsinogen, hydrochloric acid and mucoid substances (mucus).
Lipase is responsible for the breakdown and absorption of fats in the intestines. An enzyme essential for the absorption and digestion of nutrients in the intestines, responsible for the breakdown of lipids, especially triglycerides, lipase allows the body to absorb food more easily by maintaining nutrients at appropriate levels. In the human body, lipase is produced mainly by the pancreas, but is also secreted by the oral cavity and stomach. Most people produce sufficient amounts of pancreatic lipase.
The use of lipase supplements may be desirable in cases chronic disorder stomach. In a study of 18 people, supplements containing lipase and other pancreatic enzymes were shown to reduce stomach printing, tearing, gas and discomfort after eating a high-fat meal. Because some of these symptoms are associated with irritable bowel syndrome, some people with this condition may experience improvement with the use of pancreatic enzymes.
Digestive juice is a colorless, odorless liquid and determines what kind of environment should be in the stomach. It has a pronounced acidic reaction. When conducting a study to detect pathologies, it is easy for a specialist to determine what kind of environment exists in an empty (fasting) stomach. It is taken into account that normally the acidity of juice on an empty stomach is relatively low, but when secretion is stimulated it increases significantly.
Research suggests that lipase may be beneficial in cases of celiac disease, a condition in which gluten from food causes damage to the intestinal tract. Symptoms include abdominal pain, weight loss and fatigue. In a study of 40 children with celiac disease, those who received pancreatic therapy showed a slight increase in weight compared to the placebo group. People with pancreatic insufficiency and cystic fibrosis often need lipase and other enzyme supplements. People with celiac disease, Crohn's disease, or digestive disorders may be deficient in pancreatic enzymes, including lipase.
A person who adheres to a normal diet produces 1.5-2.5 liters of gastric fluid during the day. The main process occurring in the stomach is the initial breakdown of proteins. Since gastric juice affects the secretion of catalysts for the digestion process, it becomes clear in what environment the stomach enzymes are active - in an acidic environment.
Indications: In cases of pancreatic enzyme deficiency, dyspepsia, cystic fibrosis and celiac disease, Crohn's disease. Contraindications: there are no references in reference books. Adverse Reactions: There are no reports of side effects using the dosage suggested above.
Precautions: Lipase should not be taken concomitantly with betaine hydrochloride or hydrochloric acid, which may destroy the enzyme. Interactions: Talk to your doctor if the patient is taking orlistat, as it interferes with the activity of lipase supplements, blocking their ability to break down fats.
Enzymes produced by glands of the gastric mucosa
Pepsin is the most important enzyme in digestive juice, involved in the breakdown of proteins. It is produced under the influence of hydrochloric acid from its predecessor, pepsinogen. The action of pepsin is about 95% of the splitting juice. Factual examples show how high its activity is: 1 g of this substance is enough to digest 50 kg of egg white and curdle 100,000 liters of milk in two hours.
It is an enzyme secreted by the pancreas that is involved in the degradation of proteins resulting from the action of gastric pepsin. The protease is secreted as a proenzyme and is activated by intestinal juice. It is administered along with other pancreatic amylases and propancin lipases when there is a decrease in pancreatic secretion.
Proteases are enzymes that break down peptide bonds between amino acids in proteins. This process is called proteolytic cleavage, a common mechanism for activating or inactivating enzymes primarily involved in digestion and blood clotting.
Mucin (stomach mucus) is a complex complex of protein substances. It covers the entire surface of the gastric mucosa and protects it from both mechanical damage and self-digestion, since it can weaken the effect of hydrochloric acid, in other words, neutralize it.
Lipase is also present in the stomach - Gastric lipase is inactive and mainly affects milk fats.
Proteases occur naturally in all organisms and represent 1-5% of their genetic content. These enzymes are involved in a wide range of metabolic reactions, from simple digestion of food proteins to highly regulated cascades. Proteases are found in a variety of microorganisms such as viruses, bacteria, protozoa, yeast and fungi. The inability of plant and animal proteases to meet the global demand for enzymes has led to increasing interest in proteases of microbial origin.
Microorganisms are an excellent source of proteases due to their great biochemical diversity and ease genetic manipulation. Numerous proteinases are produced by individual microorganisms, depending on the species, or even by different strains of the same species. Different proteinases can also be produced by the same strain by changing culture conditions.
Another substance that deserves mention is that it promotes the absorption of vitamin B12. internal factor Castle. Let us remind you that vitamin B 12 is necessary for the transport of hemoglobin in the blood.
The role of hydrochloric acid in digestion
Hydrochloric acid activates enzymes in gastric juice and promotes the digestion of proteins, as it causes them to swell and loosen. In addition, it kills bacteria that enter the body with food. Hydrochloric acid is released in small doses, regardless of the environment in the stomach, whether there is food in it or whether it is empty.
Dosage: The dose varies from 600 units to 500 units. Contraindications: Should not be administered to patients with known hypersensitivity to the bacterial enzyme. Side effects: possibility of allergic reactions in persons with hypersensitivity to the bacterial enzyme.
Take 1 to 2 capsules with each meal. Pepsinogen is an inactive form of the enzyme. This precursor is secreted by the gastric mucosa and must be treated with hydrochloric acid to be active. About 1% of pepsinogen can enter the bloodstream and may be a useful indicator of gastric disease. In particular, its values are taken into account with the purpose.
But its secretion depends on the time of day: it has been established that the minimum level gastric secretion observed between 7 and 11 am, and maximum at night. When food enters the stomach, acid secretion is stimulated due to increased activity vagus nerve, stomach distension and chemical exposure food components on the mucous membrane.
Pepsinogen and pepsin: biological role and protein digestion
Monitor the health and functionality of the gastric mucosa; Assess the risk of developing gastritis; Determine the proportion of those affected as a result of certain pathological conditions. Pepsin is secreted as a zymogen, that is, in an inactive form that acquires functional capacity only after a precise structural change. Specifically, hydrochloric acid secreted by the parietal cells of the stomach converts pepsinogen, its precursor to pepsin, through a proteolytic cut, resulting in the removal of about forty amino acids.
What environment in the stomach is considered standard, norm and deviations
When talking about the environment in the stomach of a healthy person, it should be taken into account that different parts of the organ have different acidity values. So, highest value is 0.86 pH, and the minimum is 8.3. The standard indicator of acidity in the body of the stomach on an empty stomach is 1.5-2.0; on the surface of the inner mucous layer the pH is 1.5-2.0, and in the depths of this layer - 7.0; in the final part of the stomach varies from 1.3 to 7.4.
Stomach diseases develop as a result of an imbalance of acid production and neiolysis and directly depend on the environment in the stomach. It is important that the pH values are always normal.
Prolonged hypersecretion of hydrochloric acid or inadequate acid neutralization leads to an increase in acidity in the stomach. In this case, acid-dependent pathologies develop.
Low acidity is characteristic of (gastroduodenitis) and cancer. The indicator for gastritis with low acidity is 5.0 pH or more. Diseases mainly develop with atrophy of the cells of the gastric mucosa or their dysfunction.
Gastritis with severe secretory insufficiency
The pathology occurs in mature and elderly patients. Most often, it is secondary, that is, it develops against the background of another disease that precedes it (for example, a benign stomach ulcer) and is the result of the environment in the stomach - alkaline, in this case.
The development and course of the disease is characterized by the absence of seasonality and a clear periodicity of exacerbations, that is, the time of their occurrence and duration are unpredictable.
Symptoms of secretory insufficiency
- Constant belching with a rotten taste.
- Nausea and vomiting during exacerbation.
- Anorexia (lack of appetite).
- Feeling of heaviness in the epigastric region.
- Alternating diarrhea and constipation.
- Flatulence, rumbling and transfusions in the stomach.
- Dumping syndrome: a feeling of dizziness after eating carbohydrate foods, which occurs due to the rapid entry of chyme from the stomach into the duodenum, with a decrease in gastric activity.
- Weight loss (weight loss is up to several kilograms).
Gastrogenic diarrhea can be caused by:
- poorly digested food entering the stomach;
- a sharp imbalance in the process of fiber digestion;
- accelerated gastric emptying in case of disruption of the closing function of the sphincter;
- violation of bactericidal function;
- pathologies of the pancreas.
Gastritis with normal or increased secretory function
This disease is more common in young people. It is of a primary nature, that is, the first symptoms appear unexpectedly for the patient, since before that he did not feel any pronounced discomfort and subjectively considered himself healthy. The disease occurs with alternating exacerbations and respites, without a pronounced seasonality. To accurately determine the diagnosis, you need to consult a doctor so that he can prescribe an examination, including an instrumental one.
In the acute phase, pain and dyspeptic syndromes predominate. Pain, as a rule, is clearly related to the environment in the human stomach at the time of eating. Pain occurs almost immediately after eating. Late fasting pain (some time after eating) is less common; a combination of both is possible.
Symptoms of increased secretory function
- The pain is usually moderate, sometimes accompanied by pressure and heaviness in the epigastric region.
- Late pain is intense.
- Dyspeptic syndrome is manifested by belching “sour” air, bad aftertaste in the mouth, disorders taste sensations, nausea, pain relief by vomiting.
- Patients experience heartburn, sometimes painful.
- Intestinal dyspepsia syndrome is manifested by constipation or diarrhea.
- Typically characterized by aggressiveness, mood swings, insomnia and fatigue.
E) gastrogenic insufficiency during gastrectomy, gastrectomy, atrophic gastritis.
2. Violation of parietal digestion due to deficiency of disaccharidases (congenital, acquired lactase or other disaccharidase deficiency), with disruption of the intracellular transport of food components as a result of the death of enterocytes (Crohn's disease, celiac enteropathy, sarcoidosis, radiation, ischemic and other enteritis).
3. Impaired lymph outflow from the intestines - obstruction of the lymphatic ducts with lymphangectasia, lymphoma, intestinal tuberculosis, carcinoid.
4. Combined disorders in diabetes mellitus, giardiasis, hyperthyroidism, hypogammaglobulinemia, amyloidosis, AIDS, sepsis.
All of the conditions listed above are, to one degree or another, indications for enzyme therapy.
Despite the variety of causes that cause digestive disorders, the most severe disorders are caused by diseases of the pancreas, which are accompanied by exocrine insufficiency. It occurs in diseases of the pancreas combined with insufficiency of its exocrine function (chronic pancreatitis, pancreatic fibrosis, etc.). Exocrine pancreatic insufficiency remains one of the most current problems V modern medicine. Every year in Russia, more than 500 thousand people go to medical institutions due to various pathologies of the pancreas, accompanied by exocrine insufficiency. In addition, even minor deviations in the chemical structure of food lead to the development of exocrine pancreatic insufficiency. In chronic pancreatitis, exocrine pancreatic insufficiency develops over late stages diseases due to the progressive loss of functionally active organ parenchyma and its atrophy. At the same time, clinical signs of maldigestion with loss of body weight come to the fore; systemic complications(immunodeficiency, infectious complications, neurological disorders, etc.). In some cases, patients with chronic pancreatitis pain symptom does not bother and the disease manifests itself as exocrine and/or endocrine insufficiency. A long-term history of chronic pancreatitis significantly increases the risk of developing pancreatic cancer. To date, it has been established that the main reason for the development chronic pancreatitis with exocrine insufficiency are toxic-metabolic effects on the pancreas. In developed countries, alcohol abuse is the main cause of the development of chronic pancreatitis, especially in combination with a high protein and fat content in the drinkers' diet. In 55–80% of patients with chronic pancretitis with exocrine pancreatic insufficiency, the etiology of the disease is determined by alcohol. There is also data indicating genetic predisposition to the development of chronic pancreatitis. Besides, in Lately Cigarette smoking has been implicated in the development of chronic pancreatitis. Clinical signs of exocrine pancreatic insufficiency include flatulence, steatorrhea, nausea, weight loss, muscle atrophy, deficit fat-soluble vitamins. The symptom of abdominal pain with exocrine pancreatic insufficiency can be caused not only by concomitant pancreatitis, but also by overstretching of the intestinal wall due to excessive accumulation of gases and accelerated passage of feces. According to some authors, the pain symptom in exocrine pancreatic insufficiency may be due to the fact that reduced secretion of pancreatic enzymes in exocrine insufficiency leads to hyperstimulation of the pancreas by high levels of cholecystokinin in the blood plasma and, consequently, to abdominal pain syndrome. To diagnose exocrine insufficiency, laboratory and instrumental research methods are also used. Scatological research to this day has not lost its relevance and is an accessible informative method for determining the presence of exocrine pancreatic insufficiency. With functional deficiency, polyfecal matter appears, feces acquire a grayish tint, have a “greasy” appearance, a fetid, putrid odor, steatorrhea, creatorrhea, and rarely amilorrhea appear. Scatological examination is not always informative in case of mild disorders of exocrine function. Determining the content of elastase-1 in feces is one of the modern methods for assessing the severity of exocrine pancreatic insufficiency, since pancreatic elastase does not change its structure as it passes through the gastrointestinal tract. Also indispensable methods for diagnosing the cause that led to the development of exocrine pancreatic insufficiency are ultrasonography pancreas, computed tomography, etc.
Therapy for digestive dysfunction is based on the use of enzyme preparations, the choice of which should be made taking into account the type, severity, reversibility of pathological changes and motor disorders of the gastrointestinal tract. Typically, enzyme preparations are multicomponent drugs, the basis of which is a complex of enzymes of animal, plant or fungal origin in pure form or in combination with auxiliary components (bile acids, amino acids, hemicellulase, simethicone, adsorbents, etc.).
IN clinical practice the choice and dosage of enzyme preparations are determined by the following main factors:
- composition and quantity of active digestive enzymes that ensure the breakdown of nutrients;
- release form of the drug: ensuring the resistance of enzymes to the action of hydrochloric acid; providing rapid release of enzymes in the duodenum; ensuring the release of enzymes in the range of 5–7 units. pH;
- well tolerated and no side effects;
- long shelf life.
Let's take a closer look at the drug Unienzyme with MPS with its unique complex enzyme composition (Table 1).
Three main criteria also apply to enzymes, which are also characteristic of inorganic catalysts. In particular, they remain relatively unchanged after the reaction, i.e., they are released again and can react with new substrate molecules (although side effects of environmental conditions on the activity of the enzyme cannot be ruled out). Enzymes exert their effect in negligibly small concentrations (for example, one molecule of the enzyme rennin, contained in the mucous membrane of the calf’s stomach, curdles about 10 6 molecules of milk caseinogen in 10 minutes at 37 ° C). The presence or absence of an enzyme or any other catalyst does not affect either the value of the equilibrium constant or the change in free energy (ΔG). Catalysts only increase the rate at which a system approaches thermodynamic equilibrium, without shifting the equilibrium point. Chemical reactions with a high equilibrium constant and a negative ΔG value are usually called exergonic. Reactions with a low equilibrium constant and a correspondingly positive ΔG value (they usually do not occur spontaneously) are called endergonic. To start and complete these reactions, an influx of energy from outside is required. However, in living systems, exergonic processes are coupled with endergonic reactions, providing the latter with the necessary amount of energy.
Enzymes, being proteins, have a number of properties characteristic of this class of organic compounds that differ from the properties of inorganic catalysts.
Thermal lability of enzymes
Since the rate of chemical reactions depends on temperature, reactions catalyzed by enzymes are also sensitive to changes in temperature. The rate of a chemical reaction increases by 2 times when the temperature increases by 10°C. However, due to the protein nature of the enzyme, thermal denaturation of the enzyme protein with increasing temperature will reduce effective concentration enzyme with a subsequent decrease in the reaction rate. Thus, up to approximately 45-50°C, the effect of increasing the reaction rate, predicted by the theory of chemical kinetics, prevails. Above 45°C, thermal denaturation of the enzyme protein and a rapid drop in the reaction rate become more important (Fig. 51).
Thus, thermolability, or sensitivity to increased temperature, is one of the characteristic properties of enzymes that sharply distinguishes them from inorganic catalysts. In the presence of the latter, the reaction rate increases exponentially with increasing temperature (see Fig. 51).
At 100°C, almost all enzymes lose their activity (the exception, obviously, is only one enzyme muscle tissue- myokinase, which can withstand heating up to 100°C). The optimal temperature for the action of most enzymes in warm-blooded animals is 37-40°C. At low temperatures (0° or below), enzymes, as a rule, are not destroyed (denatured), although their activity drops almost to zero. In all cases, the time of exposure to the appropriate temperature is important. Currently, for pepsin, trypsin and a number of other enzymes, the existence of a direct relationship between the rate of enzyme inactivation and the degree of protein denaturation has been proven. We also point out that the thermolability of enzymes is influenced to a certain extent by the concentration of the substrate, the pH of the medium, and other factors.
Dependence of enzyme activity on pH of the environment
Enzymes are usually most active within a narrow zone of hydrogen ion concentration, which for animal tissues corresponds mainly to the physiological pH values of the environment developed during evolution (pH 6.0-8.0). When depicted graphically, the bell-shaped curve has a specific point at which the enzyme exhibits maximum activity; this point is called the optimum pH of the environment for the action of this enzyme (Fig. 52). When determining the dependence of enzyme activity on the concentration of hydrogen ions, the reaction is carried out at different pH values of the medium, usually at an optimal temperature and in the presence of sufficiently high concentrations of the substrate. In table Table 17 shows the optimal pH limits for a number of enzymes.
From the table 17 it can be seen that the pH optimum for enzyme action lies within physiological values. The exception is pepsin, whose pH optimum is 2.0 (at pH 6.0 it is not active and stable). This is explained by the function of pepsin, since gastric juice contains free hydrochloric acid, creating an environment of approximately this pH value. On the other hand, the pH optimum of arginase lies in the highly alkaline zone (about 10.0); There is no such environment in liver cells; therefore, in vivo, arginase apparently does not function in its optimal pH zone.
According to modern concepts, the effect of changes in the pH of the environment on the enzyme molecule is to influence the state or degree of ionization of acidic and basic groups (in particular, the COOH group of dicarboxylic amino acids, the SH group of cysteine, the imidazole nitrogen of histidine, the NH 2 group of lysine, etc. ). At different pH values of the medium, the active center can be in a partially ionized or non-ionized form, which affects the tertiary structure of the protein and, accordingly, the formation of the active enzyme-substrate complex. In addition, the ionization state of substrates and cofactors is important.
Enzyme specificity
Enzymes have high specificity of action. In this property they often differ significantly from inorganic catalysts. Thus, finely ground platinum and palladium can catalyze the reduction (with the participation of molecular hydrogen) of tens of thousands of chemical compounds of various structures. The high specificity of enzymes is due, as mentioned above, to the conformational and electrostatic complementarity between the molecules of the substrate and the enzyme and the unique structure of the active center of the enzyme, providing “recognition”, high affinity and selectivity for the occurrence of one reaction among thousands of other chemical reactions occurring simultaneously in living cells.
Depending on the mechanism of action, enzymes with relative or group specificity and with absolute specificity are distinguished. Thus, for the action of some hydrolytic enzymes, the type of chemical bond in the substrate molecule is of greatest importance. For example, pepsin breaks down proteins of animal and plant origin, although they may differ significantly from each other in both chemical structure both amino acid composition and physicochemical properties. However, pepsin does not break down carbohydrates or fats. This is explained by the fact that the site of action of pepsin is the peptide CO-NH bond. For the action of lipase, which catalyzes the hydrolysis of fats into glycerol and fatty acid, such a place is the ester bond. Trypsin, chymotrypsin, peptidases, enzymes that hydrolyze α-glycosidic bonds (but not β-glycosidic bonds present in cellulose) in polysaccharides, etc. have similar group specificity. Typically, these enzymes are involved in the digestion process, and their group specificity is more likely everything has a certain biological meaning. Some intracellular enzymes also have similar relative specificity, for example hexokinase, which catalyzes the phosphorylation of almost all hexoses in the presence of ATP, although at the same time in cells there are enzymes specific for each hexose that perform the same phosphorylation.
Absolute specificity of action is the ability of an enzyme to catalyze the transformation of only a single substrate. Any changes (modifications) in the structure of the substrate make it inaccessible to the action of the enzyme. An example of such enzymes is arginase, which breaks down arginine in natural conditions (in the body), urease, which catalyzes the breakdown of urea, etc. (see Metabolism of simple proteins).
There is experimental evidence of the existence of so-called stereochemical specificity, due to the existence of optically isomeric L- and D-forms or geometric (cis- and trans-) isomers of chemical substances. Thus, oxidases of L- and D-amino acids are known, although only L-amino acids are found in natural proteins. Each type of oxidase acts only on its specific stereoisomer 1. (1 There is, however, a small group of enzymes - racemases, which catalyze a change in the steric configuration of the substrate. Thus, bacterial alanine racemase reversibly converts both L- and D-alanine into an optically inactive mixture of both isomers: DL-alanine (racemate).)
A clear example of stereochemical specificity is bacterial aspartate decarboxylase, which catalyzes the removal of CO 2 only from L-aspartic acid, converting it into L-alanine. Stereospecificity is exhibited by enzymes that catalyze and synthetic reactions. Thus, from ammonia and α-ketoglutarate, the L-isomer of glutamic acid, which is part of natural proteins, is synthesized in all living organisms. If a compound exists in the form of cis and trans isomers with different arrangements of groups of atoms around the double bond, then, as a rule, only one of these geometric isomers can serve as a substrate for the action of the enzyme.
For example, fumarase catalyzes the conversion of only fumaric acid (trans isomer), but does not act on maleic acid (cis isomer).
Thus, due to the specificity of action, enzymes ensure the occurrence of high speed only certain reactions from a huge variety of possible transformations in the microspace of cells and the whole organism, thereby regulating the intensity of metabolism.
Factors determining enzyme activity
Factors that determine the rate of reactions catalyzed by enzymes will be briefly discussed here, and questions about the activation and inhibition of enzyme action will be discussed in more detail.
As is known, the rate of any chemical reaction decreases with time, however, the curve of the progress of enzymatic reactions over time (see Fig. 53) does not have the general shape that is characteristic of homogeneous chemical reactions. A decrease in the rate of enzymatic reactions over time may be due to inhibition by reaction products, a decrease in the degree of saturation of the enzyme with the substrate (since the concentration of the substrate decreases as the reaction proceeds), and partial inactivation of the enzyme at a given temperature and pH of the environment.
In addition, one should take into account the rate of the reverse reaction, which may be more significant when the concentration of enzymatic reaction products increases. Taking these circumstances into account, when studying the rate of enzymatic reactions in tissues and biological fluids, the initial reaction rate is usually determined under conditions when the rate of the enzymatic reaction approaches linear (including when the substrate concentration is high enough to saturate).
EFFECT OF SUBSTRATE AND ENZYME CONCENTRATION
ON THE RATE OF ENZYMATIVE REACTION
From the above material it follows important conclusion that one of the most significant factors determining the rate of an enzymatic reaction is the concentration of the substrate. At a constant enzyme concentration, the reaction rate gradually increases, reaching a certain maximum (Fig. 54), when a further increase in the amount of substrate no longer affects the reaction rate or, in some cases, even inhibits it. As can be seen from the curve of the relationship between the rate of the enzymatic reaction and the concentration of the substrate, at low concentrations of the substrate there is a direct relationship between these indicators, but at high concentrations the reaction rate becomes independent of the concentration of the substrate; in these cases it is generally assumed that the substrate is in excess and the enzyme is completely saturated. The rate-limiting factor in the latter case is the concentration of the enzyme.
The rate of any enzymatic reaction directly depends on the concentration of the enzyme. In Fig. 55 shows the relationship between the rate of reaction and increasing amounts of enzyme in the presence of excess substrate. It can be seen that there is a linear relationship between these quantities, i.e. the reaction rate is proportional to the amount of enzyme present.
Any study of the properties of enzymes, any application of them in practical activities - in medicine and in national economy- is always associated with the need to know at what speed the enzymatic reaction proceeds. To understand and correctly evaluate the results of determining enzymatic activity, you need to clearly imagine on what factors the reaction rate depends and what conditions influence it. There are many such conditions. First of all, this is the ratio of the concentration of the reacting substances themselves: enzyme and substrate. Further, these are all sorts of features of the environment in which the reaction takes place: temperature, acidity, the presence of salts or other impurities that can both accelerate and slow down the enzymatic process, and so on.
The action of enzymes depends on a number of factors, primarily on temperature and the reaction of the environment (pH). The optimal temperature at which enzyme activity is highest is usually in the range of 37 – 50˚C. At lower temperatures, the rate of enzymatic reactions decreases, and at temperatures close to 0˚C it almost completely stops. As the temperature increases, the speed also decreases and finally stops completely. The decrease in enzyme intensity with increasing temperature is mainly due to the destruction of the protein included in the enzyme. Since proteins denature in the dry state much more slowly than when hydrated (in the form of a protein gel or solution), inactivation of enzymes in the dry state occurs much more slowly than in the presence of moisture. Therefore, dry bacterial spores or dry seeds can withstand heating to much higher temperatures. high temperatures, than seeds and spores are more moist.
For most currently known enzymes, an optimum pH has been determined at which they have maximum activity. This value is an important criterion for the characteristics of the enzyme. Sometimes this property of enzymes is used for their preparative separation. The presence of an optimum pH can be explained by the fact that enzymes are polyelectrolytes and their charge depends on the pH value. Sometimes accompanying substances can change the pH optimum, such as buffer solutions. In some cases, depending on the substrates, enzymes with weakly expressed specificity have several optima.
An important factor on which the action of enzymes depends, as Sørensen first established, is the active reaction of the environment - pH. Individual enzymes differ in the optimal pH value for their action. For example, pepsin contained in gastric juice is most active in a strongly acidic environment (pH 1 – 2); trypsin - a proteolytic enzyme secreted by the pancreas, has an optimum action in a slightly alkaline environment (pH 8 - 9); papain, an enzyme of plant origin, works optimally in a slightly acidic environment (pH 5 – 6).
It follows that the value (PH optimum) is a very sensitive sign for this enzyme. It depends on the nature of the substrate and the composition of the buffer solution and therefore is not a true constant. It is also necessary to keep in mind the properties of enzymes as protein bodies capable of acid-base denaturation. Acid-base denaturation can lead to irreversible changes in the structure of the enzyme with the loss of its catalytic properties.
The rate of any enzymatic process depends largely on the concentration of both substrate and enzyme. Typically, the reaction rate is directly proportional to the amount of enzyme, provided that the substrate content is within the optimum range or slightly higher. At a constant amount of enzyme, the rate increases with increasing substrate concentration. This reaction is subject to the law of mass action and is considered in the light of the Michaelis-Menton theory, that is,
V=K(F) ,
V - reaction speed
K - rate constant
F - enzyme concentration.
The presence of certain ions in the reaction medium can activate the formation of the active substrate of the enzyme complex, in which case the rate of the enzymatic reaction will increase. Such substances are called activators. In this case, substances that catalyze enzymatic reactions do not directly participate in them. The activity of some enzymes is significantly affected by the concentration of salts in the system, while other enzymes are not sensitive to the presence of ions. However, some ions are absolutely necessary for the normal functioning of some enzymes. Ions are known that inhibit the activity of some enzymes and are activators for others. Specific activators include metal cations: Na + , K + , Rb + , Cs + , Mg2 + , Ca2 + , Zn2 + , Cd2 + , Cr2 + , Cu2 + , Mn2 + , Co2 + , Ni2 + , Al3 + . It is also known that Fe2 + , Rb + , Cs + cations act as activators only in the presence of Mg; in other cases, these cations are not activators. In most cases, one or two ions can activate a particular enzyme. For example, Mg2 + - a common activator for many enzymes, acting on phosphorimated substrates, in almost all cases can be replaced by Mn2 +, although other metals cannot replace it. It should be noted that alkaline earth metals generally compete with each other, in particular, Ca2 + suppresses the activity of many enzymes activated by Mg2 + and Zn2 +. The reason for this is still unclear. The mechanism of influence of metal ions - activators can be different. First of all, the metal may be a component of the active site of the enzyme. But it can act as a connecting bridge between the enzyme and the substrate, holding the substrate at the active site of the enzyme. There is evidence that metal ions are capable of binding organic compound with proteins and, finally, one of the possible mechanisms of action of metals as activators is a change in the equilibrium constant of the enzymatic reaction. It has been proven that anions also affect the activity of a number of enzymes. For example, the influence of CI on the activity of A-amylase of animal origin is very great.
The action of enzymes also depends on the presence of specific activators or inhibitors. Thus, the pancreatic enzyme enterokinase converts inactive trypsinogen into active trypsin. Such inactive enzymes contained in cells and in the secretions of various glands are called proenzymes. An enzyme can be competitive or non-competitive. In competitive inhibition, the inhibitor and substrate compete with each other, trying to displace each other from the enzyme-substrate complex. The effect of a competitive inhibitor is removed by high concentrations of the substrate, while the effect of a non-competitive inhibitor remains under these conditions. The effect of specific activators and inhibitors on the enzyme is great importance to regulate enzymatic processes in the body.
Along with the existence of enzyme activators, a number of substances are known whose presence inhibits the catalytic action of enzymes or completely inactivates it. Such substances are usually called inhibitors. Inhibitors are substances that act in a certain chemical way on enzymes and, according to the nature of their action, can be divided into reversible and irreversible inhibitors. Reversible inhibition is characterized by an equilibrium between the enzyme and the inhibitor with a certain equilibrium constant. A system of this type is characterized by a certain degree of inhibition, depending on the concentration of the inhibitor, and the inhibition is achieved quickly and is then independent of time. When the inhibitor is removed by dialysis, enzyme activity is restored. Irreversible inhibition is primarily expressed in the fact that dialysis does not restore enzyme activity. And in contrast to reversible inhibition, it increases with time, so that complete inhibition of the catalytic activity of the enzyme can occur at a very low concentration of inhibitor. In this case, the effectiveness of the inhibitor does not depend on the equilibrium constant, but on the rate constant, which determines the proportion of the enzyme that is inhibited in this case.
Consequently, in relation to the pH of the environment, the digestive enzymes of fish mostly do not work under optimal conditions. This “deficiency” in the functioning of the digestive tract is compensated by the fact that digestion in fish occurs with constant mixing of food and enzymes gastrointestinal tract thanks to the peristalsis of the latter. Movements of the gastrointestinal tract are important not only for the constant movement of food along the tract, but also for mixing the enzyme with the substrate (food), for grinding the substrate and better saturating it with the enzyme.[...]
Fonck experimentally showed that fibrin is digested by pak-creatic juice approximately 2 times faster if digestion in test tubes is carried out with constant stirring compared to dark test tubes in which no stirring is performed. [...]
During the digestion process, there is a constant release of new portions of enzymes into the digestive tract, which, of course, enhances the digestive ability of the latter.[...]
Under natural conditions, the products of chemical interaction: enzyme and substrate are removed from the reaction sphere and thereby create conditions for a more complete effect of the enzyme on the substrate, i.e. there is no reverse inhibitory effect of the chemical reaction product on the original reacting substances.[...]
Each enzyme has its own specific activator, in the presence of which the enzyme becomes active. Pepsin has hydrochloric acid, trypsin has enterokinase and bile, lipase has chloride, magnesium and bile.[...]
Trypsin usually digests proteins in a slightly alkaline environment, but does not digest in an acidic environment. But it can digest fibrin in a slightly acidic environment if a significant amount of bile is added.[...]
As you can see, the activation of enzymes in the body can be carried out in different ways, and the final result of digestion, its completeness, depend not only on the enzyme itself, but also on the environment in which it operates, on those activators that are released into the digestive tract and, in addition, , also depend on the peristalsis of the digestive tract.[...]
So, the intensity of food digestion depends not only on its quality, but also on the enzyme itself. Let us assume that the concentration of the enzyme is sufficiently high and it acts on a specific substrate, then a favorable environment is also necessary for the successful digestion of food." If the environment is unfavorable for the action of the enzyme, then the enzyme may not act at all or have a weak effect on the substrate.
