Complete oxidation of glucose. The reaction of glucose oxidation

In this article, we will look at how glucose oxidation occurs. Carbohydrates are compounds of polyhydroxycarbonyl type, as well as their derivatives. Characteristic signs - the presence of aldehyde or ketone groups and at least two groups of hydroxyl.

By its structure carbohydrates are divided into monosaccharides, polysaccharides, oligosaccharides.

Monosaccharides

Monosaccharides are the simplest carbohydrates that can not be hydrolyzed. Depending on which group is present in the composition - aldehyde or ketone, isolates aldoses (these include galactose, glucose, ribose) and ketoses (ribulose, fructose).

Oligosaccharides

Oligosaccharides are carbohydrates that have from two to ten monosaccharide residues connected by glycosidic bonds. Depending on the amount of monosaccharide residues, disaccharides, trisaccharides and so on are distinguished. What happens when glucose is oxidized? This will be discussed later.

Polysaccharides

Polysaccharides are carbohydrates that contain more than ten monosaccharide residues linked together by glycosidic bonds. If the same polysaccharide contains the same monosaccharide residues, it is called homopolysaccharide (for example, starch). If such residues are different - then heteropolysaccharide (for example, heparin).

What is the significance of glucose oxidation?

Functions of carbohydrates in the human body

Carbohydrates perform the following main functions:

1. Energy. The most important function of carbohydrates, as they serve as the main source of energy in the body. As a result of their oxidation, more than half of the human energy needs are met. As a result of oxidation of one gram of carbohydrates, 16.9 kJ is released.
2. Backup. Glycogen and starch are a form of accumulation of nutrients.
3. Structural. Cellulose and some other polysaccharide compounds form a strong core in plants. Also, in combination with lipids and proteins, they are a component of all cellular biomembranes.
4. Protective. For acidic heteropolysaccharides, the role of a biological lubricant is assigned. They lining the surfaces of the joints that touch and rub against each other, the mucous membranes of the nose, and the digestive tract.
5. Anticoagulant. A carbohydrate, like heparin, has an important biological property, namely, it prevents blood clotting.
6. Carbohydrates are a source of carbon necessary for the synthesis of proteins, lipids and nucleic acids.

For the body the main source of carbohydrates are food carbohydrates - sucrose, starch, glucose, lactose). Glucose can be synthesized in the body itself from amino acids, glycerol, lactate and pyruvate (gluconeogenesis).

Glycolysis

Glycolysis is one of three possible forms of glucose oxidation. In this process, energy is released, which is subsequently stored in ATP and NADH. One of its molecules breaks up into two molecules of pyruvate.

The process of glycolysis occurs under the action of a variety of enzymatic substances, that is, catalysts of a biological nature. The most important oxidizer is oxygen, but it is worth noting that the process of glycolysis can be carried out in the absence of oxygen. A similar kind of glycolysis is called anaerobic.

Glycolysis of anaerobic type is a stepwise process of glucose oxidation. With this glycolysis, glucose oxidation does not take place completely. Thus, during the oxidation of glucose, only one pyruvate molecule is formed. In terms of energy benefits, anaerobic glycolysis is less beneficial than aerobic glycolysis. However, if oxygen enters the cell, anaerobic glycolysis may turn into aerobic glycolysis, which is complete oxidation of glucose.

Mechanism of glycolysis

In the process of glycolysis the decomposition of six-carbon glucose into two molecules of the three-carbon pyruvate occurs. The entire process is divided into five preparatory stages and five more, during which the ATF stores energy.

Thus, glycolysis proceeds in two stages, each of which is divided into five stages.

Stage 1 of the reaction of glucose oxidation

• First step. At the first stage, glucose is phosphorylated. Activation of saccharide occurs by phosphorylation on the sixth carbon atom.
• Second phase. There is a process of isomerization of glucose-6-phosphate. At this stage, glucose is converted to fructose-6-phosphate by catalytic phosphogluco isomerase.
• The third stage. Phosphorylation of fructose-6-phosphate. At this stage, fructose-1,6-diphosphate (also called aldolase) is produced by phosphofructokinase-1. It participates in the accompaniment of a phosphoryl group from adenosine triphosphate to a fructose molecule.
• The fourth stage. At this stage, the aldolase is cleaved. As a result, two molecules of triose phosphate, in particular ketoses and eldoses, are formed.
• The fifth stage. Isomerization of triisophosphates. At this stage, glyceraldehyde-3-phosphate is sent to the next stages of glucose cleavage. In this case, the dihydroxyacetone phosphate is converted to the form of glyceraldehyde-3-phosphate. This transition is carried out under the action of enzymes.
• The sixth stage. The process of oxidation of glyceraldehyde-3-phosphate. At this stage, the molecule is oxidized and its subsequent phosphorylation to diphosphoglycerate-1,3.
• Seventh stage. This step involves the transfer from the 1,3-diphosphoglycerate of the phosphate group to ADP. In the final result of this stage, 3-phosphoglycerate and ATP are formed.

Stage 2 - complete oxidation of glucose

• Eighth stage. At this stage, the transition of 3-phosphoglycerate to 2-phosphoglycerate. The transition process is carried out under the action of an enzyme, such as phosphoglycerate mutase. This chemical reaction of glucose oxidation proceeds with the obligatory presence of magnesium (Mg).
• The ninth stage. At this stage dehydration of 2-phosphoglycerate occurs.
• Tenth stage. There is a transfer of phosphates, obtained as a result of the previous stages, to FEP and ADP. The transfer to ADP is phosphoenepiroval. Such a chemical reaction is possible in the presence of magnesium (Mg) and potassium (K) ions.

Under aerobic conditions, the whole process reaches CO ₂ and H ₂ O. The glucose oxidation equation looks like this:

C ₆ H ₁₂ O ₆ + 6 O ₂ → 6 CO ₂ + 6H ₂ O + 2880 kJ / mol.

Thus, in the cell there is no accumulation of NADH in the process of formation of lactate from glucose. This means that such a process is anaerobic, and it can proceed in the absence of oxygen. It is oxygen - the final electron acceptor, which are transferred to the NADH in the respiratory chain.

In the process of calculating the energy balance of the glycolytic reaction, it must be taken into account that each stage of the second stage is repeated twice. From this it can be concluded that in the first stage two ATP molecules are wasted, and during the second stage 4 ATP molecules are formed by phosphorylation of the substrate type. This means that as a result of the oxidation of each molecule of glucose, the cell accumulates two ATP molecules.

We examined the oxidation of glucose by oxygen.

Anaerobic pathway of glucose oxidation

Aerobic oxidation is the process of oxidation, in which energy is released and which takes place in the presence of oxygen, which acts as the final acceptor of hydrogen in the respiratory chain. The donor of hydrogen molecules is the reduced form of coenzymes (FADH2, NADH, NADPH), which are formed during an intermediate reaction of substrate oxidation.

The process of glucose oxidation of aerobic dichotomous type is the main way of glucose catabolism in the human body. This type of glycolysis can be carried out in all tissues and organs of the human body. The result of this reaction is the cleavage of the glucose molecule to water and carbon dioxide. The allocated energy will be accumulated in ATP. This process can be divided into three stages:

1. The process of converting a glucose molecule into a pair of pyruvic acid molecules. The reaction occurs in the cell cytoplasm and is a specific way of glucose disintegration.
2. The process of formation of acetyl-CoA as a result of oxidative decarboxylation of pyruvic acid. This reaction occurs in cellular mitochondria.
3. Oxidation of acetyl-CoA in the Krebs cycle. The reaction takes place in cellular mitochondria.

At each stage of this process, reduced forms of coenzymes are formed, which are oxidized by enzyme complexes of the respiratory chain. As a result, ATP is formed during the oxidation of glucose.

Formation of coenzymes

The coenzymes that form in the second and third stages of aerobic glycolysis will be oxidized directly in the mitochondria of the cells. In parallel, NADH, which was formed in the cell cytoplasm during the reaction of the first stage of aerobic glycolysis, does not have the ability to penetrate the membranes of the mitochondria. Hydrogen is transferred from cytoplasmic NADH to cellular mitochondria through shuttle cycles. Among such cycles it is possible to single out the main one - malate-aspartate.

Then, with the help of cytoplasmic NADH, oxaloacetate is reduced to malate, which in turn penetrates into the cellular mitochondria and then is oxidized with the reduction of mitochondrial NAD. Oxaloacetate returns to the cytoplasm of the cell in the form of aspartate.

Modified forms of glycolysis

The flow of glycolysis can additionally be accompanied by the release of 1,3 and 2,3-bisphosphoglycerates. In this case, 2,3-bisphosphoglycerate can be returned to the glycolysis process under the influence of biological catalysts, and then change its form to 3-phosphoglycerate. These enzymes play a variety of roles. For example, 2,3-bisphosphoglycerate, located in hemoglobin, promotes the transition of oxygen into tissues, thereby promoting dissociation and a decrease in the affinity of oxygen and red blood cells.

Conclusion

Many bacteria can change the forms of glycolysis at various stages. It is possible to reduce their total number or to modify these steps as a result of the action of various enzyme compounds. Some of the anaerobes have the ability to other ways of decomposing carbohydrates. Most of the thermophiles have only two glycolytic enzymes, in particular enolase and pyruvate kinase.

We examined how the oxidation of glucose in the body.