A. Aerobic and anaerobic oxidation of glucose. Aerobic oxidation of pyruvate (oxidative decarboxylation of pyruvic acid) Aerobic oxidation of glucose
The breakdown of carbohydrates under aerobic conditions can occur in a direct (apotomic or pentose) way or in an indirect (dichotomous) way.
Dichotomous (Greek dicha - into two parts, tome-section) oxidation of carbohydrates proceeds according to the equation:
C6H12O6+6O2 = 6 CO2+b H2O+686 kcal
This path is the main one in the formation of energy. The first stages of this pathway coincide with the anaerobic oxidation of glucose. The divergence of pathways begins at the stage of formation of pyruvic acid, which in animal tissues is decarboxylated by the oxidative pathway. Glycolysis is a sequence of enzymatic reactions leading to the conversion of glucose into pyruvate with the simultaneous formation of ATP. Under aerobic conditions, pyruvate penetrates into mitochondria, where it is completely oxidized to CO2 and H2O. If the oxygen content is insufficient, as may be the case in an actively contracting muscle, pyruvate is converted to lactate. Anaerobic glycolysis is a complex enzymatic process of glucose breakdown that occurs in human and animal tissues without oxygen consumption. The end product of glycolysis is lactic acid. During glycolysis, ATP is produced. The overall equation of glycolysis can be represented as follows:
Under anaerobic conditions, glycolysis is the only process in the animal body that supplies energy. It is thanks to glycolysis that the human and animal body can perform a number of physiological functions for a certain period in conditions of oxygen deficiency. In cases where glycolysis occurs in the presence of oxygen, we speak of aerobic glycolysis. The first enzymatic reaction of glycolysis is phosphorylation, i.e. transfer of an orthophosphate residue to glucose by ATP. The reaction is catalyzed by the enzyme hexokinase:
The second reaction of glycolysis is the conversion of glucose-6-phosphate under the action of the enzyme glucose-6-phosphate isomerase into fructose 6-phosphate:
The third reaction is catalyzed by the enzyme phosphofructokinase; the resulting fructose-6-phosphate is phosphorylated again due to the second ATP molecule:
The fourth reaction of glycolysis is catalyzed by the enzyme aldolase. Under the influence of this enzyme, fructose-1,6-bisphosphate is split into two phosphotrioses:
The fifth reaction is the isomerization reaction of triose phosphates. Catalyzed by the enzyme triosephosphate isomerase:
The formation of glyceraldehyde-3-phosphate completes the first stage of glycolysis. The second stage is the most difficult and important. It includes a redox reaction (glycolytic oxidoreduction reaction) coupled with substrate phosphorylation, during which ATP is formed. As a result of the sixth reaction, glyceraldehyde-3-phosphate, in the presence of the enzyme glyceraldehyde phosphate dehydrogenase, the coenzyme NAD and inorganic phosphate, undergoes a peculiar oxidation to form 1,3-bisphosphoglyceric acid and the reduced form of NAD (NADH). This reaction is blocked by iodine or bromoacetate and proceeds in several stages:
The seventh reaction is catalyzed by phosphoglycerate kinase, which transfers an energy-rich phosphate residue (phosphate group at position 1) to ADP to form ATP and 3-phosphoglyceric acid (3-phosphoglycerate):
The eighth reaction is accompanied by intramolecular transfer of the remaining phosphate group, and 3-phosphoglyceric acid is converted to 2-phosphoglyceric acid (2-phosphoglycerate).
The ninth reaction is catalyzed by the enzyme enolase, in which 2-phosphoglyceric acid, as a result of the elimination of a water molecule, becomes phosphoenolpyruvic acid (phosphoenolpyruvate), and the phosphate bond at position 2 becomes highly energetic:
The tenth reaction is characterized by the cleavage of a high-energy bond and the transfer of a phosphate residue from phosphoenolpyruvate to ADP (substrate phosphorylation). Catalyzed by the enzyme pyruvate kinase:
As a result of the eleventh reaction, pyruvic acid is reduced and lactic acid is formed. The reaction occurs with the participation of the enzyme lactate dehydrogenase and the coenzyme NADH, formed in the sixth reaction:
The biological significance of the glycolysis process lies primarily in the formation of energy-rich phosphorus compounds. In the first stages of glycolysis, 2 ATP molecules are consumed (hexokinase and phosphofructokinase reactions). In subsequent reactions, 4 ATP molecules are formed (phosphoglycerate kinase and pyruvate kinase reactions). Thus, the energy efficiency of glycolysis under anaerobic conditions is 2 ATP molecules per glucose molecule.
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Aerobic oxidation carbohydrates. Decay carbohydrates V aerobic under conditions, it can follow a direct (apotomy or pentose) path and an indirect (dichotomous) path. -
Aerobic oxidation carbohydrates. Decay carbohydrates V aerobic conditions can follow a direct (apotomic or pentotic) path and. -
Glycolysis is the simplest form of biol. energy storage mechanism carbohydrates in ATP.
With an energetically more favorable aerobic oxidation from one molecule of glucose... -
Oxidative phosphorylation would be more accurately called phosphorylation in the respiratory chain.
Aerobic oxidation carbohydrates. -
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The main route of energy production in the body is aerobic oxidation of carbohydrates. In this case, glucose in the presence of oxygen is oxidized to CO 2 and H 2 O with the release of a large amount of energy, part of which goes to the synthesis of 38-39 ATP molecules.
The aerobic process proceeds according to the following scheme:
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + 680 kcal.
Aerobic oxidation of glucose can occur in two ways - direct and indirect.
In the direct pathway of glucose oxidation (synonyms: apotomic or pentose cycle), sequential elimination of each of its 6 carbon atoms from a glucose molecule occurs with the formation of one molecule of CO 2 and H 2 O during one cycle. The breakdown of the entire glucose molecule occurs over 6 repeating cycles . This process predominates in red blood cells, the lactating mammary gland, the adrenal cortex, and the lens of the eye; in the liver and kidneys it is a by-product of the breakdown of carbohydrates.
A feature of this process is the formation of pentoses, which go towards the construction of RNA and DNA, the release of energy (36 ATP molecules) and the accumulation of NADPH 2 -coenzyme dehydrogenases, which are involved in the synthesis of cholesterol, fatty acids, activation of folic acid, etc.
In the liver and kidneys, another pathway of glucose oxidation predominates, which is called indirect, or dichotomous (see Scheme 3). During this process, a glucose molecule is first split into two phosphotriose molecules (a process similar to the anaerobic breakdown of carbohydrates) with the subsequent formation of pyruvic acid. Pyruvic acid is converted to acetyl-CoA as a result of oxidative decarboxylation
The latter enters the Kreos cycle, where it is gradually oxidized to CO 2 and H 2 O and releases a large amount of energy.
During the “indirect” oxidation of one glucose molecule, 680 kcal of energy is released, from which 38-39 ATP molecules are formed (see Diagram 3).
The breakdown of carbohydrates also occurs in yeast cells and various microorganisms, but the final products differ depending on the type of microbe and yeast. Thus, the formation of ethyl alcohol occurs in yeast cells.
The mechanism of alcoholic fermentation of glucose was revealed by the work of I. M. Manasseina, E. Bukhner, A. N. Lebedeva and other authors. Under the action of enzymes in yeast cells, the previously discussed process of breakdown of glucose or glycogen to pyruvic acid occurs. The latter undergoes decarboxylation to form acetaldehyde, which is reduced to ethyl alcohol:
Thus, the end products of alcoholic fermentation are CO 2 and ethyl alcohol.
Lactic acid bacteria convert carbohydrates into lactic acid, butyric acid bacteria into butyric acid, etc.
When studying fermentation L. Pasteur drew attention to the fact that with an excess of oxygen, the process of glycolysis is inhibited. This fact is called Pasteur effect. There is no explanation for him yet. There are various hypotheses, but none of them can explain it with a sufficient degree of accuracy.
Research O. Warburg It was found that in embryonic tissue and tissues of malignant tumors, oxygen does not inhibit glycolysis. The formation of lactic acid in the presence of oxygen is called "aerobic glycolysis".
Under aerobic conditions, glucose is oxidized to CO 2 and H 2 O. The overall equation is:
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + 2880 kJ/mol.
This process includes several stages:
Aerobic glycolysis . In it, 1 glucose is oxidized to 2 PVC, with the formation of 2 ATP (first 2 ATP are consumed, then 4 are formed) and 2 NADH 2;
Conversion of 2 PVK into 2 acetyl-CoA with the release of 2 CO 2 and the formation of 2 NADH 2;
CTK. It oxidizes 2 acetyl-CoA with the release of 4 CO 2, the formation of 2 GTP (yielding 2 ATP), 6 NADH 2 and 2 FADH 2;
Oxidative phosphorylation chain. In it, 10 (8) NADH 2, 2 (4) FADH 2 are oxidized with the participation of 6 O 2, while 6 H 2 O is released and 34 (32) ATP is synthesized.
As a result of aerobic oxidation of glucose, 38 (36) ATP is formed, of which: 4 ATP in reactions of substrate phosphorylation, 34 (32) ATP in reactions of oxidative phosphorylation. The efficiency of aerobic oxidation will be 65%.
Anaerobic oxidation of glucose
Glucose catabolism without O2 occurs in anaerobic glycolysis and PFS (PFP).
During anaerobic glycolysis 1 glucose is oxidized to 2 molecules of lactic acid with the formation of 2 ATP (first 2 ATP are consumed, then 4 are formed). Under anaerobic conditions, glycolysis is the only source of energy. The overall equation is: C 6 H 12 O 6 + 2H 3 PO 4 + 2ADP → 2C 3 H 6 O 3 + 2ATP + 2H 2 O.
During PFP Pentoses and NADPH 2 are formed from glucose. During PFS Only NADPH 2 is formed from glucose.
GLYCOLYSIS
Glycolysis is the main pathway for the catabolism of glucose (as well as fructose and galactose). All its reactions take place in the cytosol.
Aerobic glycolysis is the process of oxidation of glucose to PVC, occurring in the presence of O 2.
Anaerobic glycolysis is the process of oxidation of glucose to lactate, occurring in the absence of O 2.
Anaerobic glycolysis differs from aerobic glycolysis only in the presence of the last 11 reactions; the first 10 reactions are common to them.
Stages of glycolysis
In any glycolysis, 2 stages can be distinguished:
Stage 1 is preparatory, it consumes 2 ATP. Glucose is phosphorylated and broken down into 2 phosphotrioses;
Stage 2 is associated with ATP synthesis. At this stage, phosphotrioses are converted to PVC. The energy of this stage is used for the synthesis of 4 ATP and the reduction of 2NADH 2, which under aerobic conditions is used for the synthesis of 6 ATP, and under anaerobic conditions they reduce PVA to lactate.
Energy balance of glycolysis
Thus, the energy balance of aerobic glycolysis is:
8ATP = -2ATP + 4ATP + 6ATP (from 2NADH 2)
Energy balance of anaerobic glycolysis:
2ATP = -2ATP + 4ATP
General reactions of aerobic and anaerobic glycolysis
1. Hexokinase (hexokinase II, ATP: hexose-6-phosphotransferase) in muscles phosphorylates mainly glucose, less fructose and galactose. Km<0,1 ммоль/л. Ингибитор глюкозо-6-ф, АТФ. Активатор адреналин. Индуктор инсулин.
Glucokinase (hexokinase IV, ATP: glucose-6-phosphotransferase) phosphorylates glucose. Km - 10 mmol/l, active in the liver and kidneys. Glucose-6-ph is not inhibited. Insulin inducer. Hexokinases carry out phosphorylation of hexoses.
2. Phosphohexose isomerase (glucose-6ph-fructose-6ph-isomerase) carries out aldo-ketoisomerization of open forms of hexoses.
3. Phosphofructokinase 1 (ATP: fructose-6ph-1-phosphotransferase) carries out phosphorylation of fructose-6ph. The reaction is irreversible and the slowest of all glycolysis reactions, determining the rate of all glycolysis. Activated by: AMP, fructose-2,6-df (a powerful activator, formed with the participation of phosphofructokinase 2 from fructose-6ph), fructose-6-ph, Fn. Inhibited by: glucagon, ATP, NADH 2, citrate, fatty acids, ketone bodies. Insulin response inducer.
4. Aldolaza A (fructose-1,6-ph: DAP-lyase). Aldolases act on open forms of hexoses, have 4 subunits, and form several isoforms. Most tissues contain Aldolase A. The liver and kidneys contain Aldolase B.
5. Phosphotriose isomerase (DAP-PHA isomerase).
6. 3-PHA dehydrogenase (3-PHA: NAD+ oxidoreductase (phosphorylating)) consists of 4 subunits. Catalyzes the formation of a high-energy bond in 1,3-PGA and the reduction of NADH 2, which are used under aerobic conditions for the synthesis of 8 (6) ATP molecules.
7. Phosphoglycerate kinase (ATP: 3PGA-1-phosphotransferase). Carries out substrate phosphorylation of ADP with the formation of ATP.
In the following reactions, the low-energy phosphoester is converted to high-energy phosphate.
8. Phosphoglycerate mutase (3-PGA-2-PGA isomerase) transfers the phosphate residue to PGA from position 3 to position 2.
9. Enolase (2-PHA: hydro-lyase) splits off a water molecule from 2-PHA and forms a high-energy bond with phosphorus. Inhibited by F - ions.
10. Pyruvate kinase (ATP: PVK-2-phosphotransferase) carries out substrate phosphorylation of ADP to form ATP. Activated by fructose-1,6-df, glucose. Inhibited by ATP, NADH 2, glucagon, adrenaline, alanine, fatty acids, Acetyl-CoA. Inducer: insulin, fructose.
The resulting enol form of PVK is then non-enzymatically converted to a more thermodynamically stable keto form. This reaction is the last for aerobic glycolysis.
Further catabolism of 2 PVK and the use of 2 NADH 2 depends on the availability of O 2 .
At the first stage, glucose is split into 2 trioses:
Thus, at the first stage of glycolysis, 2 molecules of ATP are spent on activating glucose and 2 molecules of 3-phosphoglyceraldehyde are formed.
In the second stage, 2 molecules of 3-phosphoglyceraldehyde are oxidized to two molecules of lactic acid.
The significance of the lactate dehydrogenase reaction (LDH) is to oxidize NADH 2 to NAD under oxygen-free conditions and allow the glycerophosphate dehydrogenase reaction to occur.
The overall equation of glycolysis: glucose + 2ADP + 2H 3 PO 4 → 2 lactate + 2ATP + 2H 2 O
Glycolysis occurs in the cytosol. Its regulation is carried out by key enzymes - hexokinase, phosphofructokinase And pyruvate kinase. These enzymes are activated by ADP and NAD and inhibited by ATP and NADH 2 .
The energy efficiency of anaerobic glycolysis comes down to the difference between the number of ATP molecules consumed and the number of ATP molecules produced. 2 ATP molecules are consumed per glucose molecule in the hexokinase reaction and the phosphofructokinase reaction. 2 molecules of ATP are formed per molecule of triose (1/2 glucose) in the glycerokinase reaction and pyruvate kinase reaction. For a molecule of glucose (2 trioses), 4 molecules of ATP are formed, respectively. Total balance: 4 ATP – 2 ATP = 2 ATP. 2 ATP molecules accumulate ≈ 20 kcal, which is about 3% of the energy of complete oxidation of glucose (686 kcal).
Despite the relatively low energy efficiency of anaerobic glycolysis, it has an important biological significance in that it the only one a method of generating energy in oxygen-free conditions. In conditions of oxygen deficiency, it ensures the performance of intense muscle work and the beginning of muscle work.
In children anaerobic glycolysis is very active in fetal tissues under conditions of oxygen deficiency. It remains active during the neonatal period, gradually giving way to aerobic oxidation.
Further conversion of lactic acid.
- With an intensive supply of oxygen under aerobic conditions, lactic acid is converted into PVA and, through acetyl CoA, is included in the Krebs cycle, providing energy.
- Lactic acid is transported from muscles to the liver, where it is used for glucose synthesis - the Cori cycle.
Measles cycle
- At high concentrations of lactic acid in tissues, it can be excreted through the kidneys to prevent acidosis.
In the presence of oxygen (under aerobic conditions), most animal cells obtain energy due to the complete destruction of nutrients (lipids, amino acids and carbohydrates), that is, due to oxidative processes. In the absence of oxygen (anaerobic conditions), the cell can synthesize ATP (ATP) only through the glycolytic breakdown of glucose. Although this breakdown of glucose, resulting in the formation of lactate, provides little energy for ATP synthesis, this process is critical for the survival of cells in the absence or lack of oxygen.
IN aerobic conditions(in the diagram on the left) ATP is formed almost exclusively due to oxidative phosphorylation (see). Fatty acids in the form of acylcarnitine they enter the mitochondrial matrix (see), where they undergo β-oxidation to form acyl-CoA (see). Glucose in the cytoplasm it is converted into pyruvate by glycolysis (see). Pyruvate is transported into the mitochondrial matrix, where it is decarboxylated by the pyruvate dehydrogenase complex (see) to form acetyl-CoA. Reducing equivalents released during glycolysis are transported into the mitochondrial matrix by the malate shuttle. Acetyl residues formed from fatty acids are oxidized to CO 2 in the citrate cycle (see). Degradation amino acids also leads to acetyl residues or products that are directly included in the citrate cycle (see). In accordance with the energy needs of the cell, reducing equivalents are transferred by the respiratory chain to oxygen (see). This releases chemical energy, which, by creating a proton gradient, is used for the synthesis of ATP (see).
In the absence of oxygen, that is under anaerobic conditions(in the diagram on the right), the picture changes completely. Since there are not enough electron acceptors for the respiratory chain, NADH + H + and QH 2 cannot be re-oxidized. As a result, not only mitochondrial ATP synthesis stops, but almost the entire metabolism in the mitochondrial matrix. The main reason for this stop is the high concentration of NADH, which inhibits the citrate cycle and pyruvate dehydrogenase (see). The process of β-oxidation and the functioning of the malate shuttle, which depend on the presence of free NAD +, also stop. Since energy can no longer be obtained from the degradation of amino acids, the cell becomes completely energy dependent on the consumption of glucose at glycolysis. In this case, a prerequisite is the constant oxidation of the resulting NADH + H +. Since this process can no longer occur in mitochondria, in animal cells operating under anaerobic conditions, pyruvate is reduced to lactate, which enters the blood. Processes of this type are called fermentation(cm. ). The production of ATP during these processes is insignificant: during the formation of lactate, only 2 ATP molecules are produced per glucose molecule.
In order to estimate the number of ATP molecules formed in the aerobic state, it is necessary to know the so-called P/O ratio, that is, the molar ratio of synthesized ATP (P) and water (O). During the transfer of two electrons from NADH to O 2, about 10 protons and only 6 molecules of ubiquinol (QH 2) are transported into the intermembrane space. To synthesize ATP, ATP synthase requires three H + ions, so the maximum possible P/O ratio is approximately 3 or, respectively, 2 (for ubiquinol). It must, however, be taken into account that during the transition of metabolites into the matrix and the exchange of mitochondrial ATP 4- for cytoplasmic ADP 3-, protons are also consumed in the intermembrane space. Therefore, during the oxidation of NADH, the P/O ratio is most likely 2.5, and during the oxidation of QH 2 - 1.5. If, based on these values, we calculate the energy balance of aerobic glycolysis, it turns out that oxidation one molecule of glucose accompanied by synthesis of 32 ATP molecules.