In this article, we will look at how glucose is oxidized. Carbohydrates are compounds of the polyhydroxycarbonyl type, as well as their derivatives. Characteristic features are the presence of aldehyde or ketone groups and at least two hydroxyl groups.
According to their structure, carbohydrates are divided into monosaccharides, polysaccharides, oligosaccharides.
Monosaccharides
Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed. Depending on which group is present in the composition - aldehyde or ketone, aldoses (these include galactose, glucose, ribose) and ketoses (ribulose, fructose) are isolated.
Oligosaccharides
Oligosaccharides are carbohydrates that have in their composition from two to ten residues of monosaccharide origin, connected through glycosidic bonds. Depending on the number of monosaccharide residues, disaccharides, trisaccharides, and so on are distinguished. What is formed when glucose is oxidized? This will be discussed later.
Polysaccharides
Polysaccharidesare carbohydrates that contain more than ten monosaccharide residues interconnected by glycosidic bonds. If the composition of the polysaccharide contains the same monosaccharide residues, then it is called a homopolysaccharide (for example, starch). If such residues are different, then with a heteropolysaccharide (for example, heparin).
What is the importance of glucose oxidation?
Functions of carbohydrates in the human body
Carbohydrates perform the following main functions:
- 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 energy needs of a person are satisfied. As a result of the oxidation of one gram of carbohydrates, 16.9 kJ are released.
- Reserve. Glycogen and starch are a form of nutrient storage.
- Structural. Cellulose and some other polysaccharide compounds form a strong framework in plants. Also, they, in combination with lipids and proteins, are a component of all cell biomembranes.
- Protective. Acid heteropolysaccharides play the role of a biological lubricant. They line the surfaces of the joints that touch and rub against each other, the mucous membranes of the nose, the digestive tract.
- Anticoagulant. A carbohydrate such as heparin has an important biological property, namely, it prevents blood clotting.
- 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 dietary 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 the glucose oxidation process. In this process, energy is released, which is subsequently stored in ATP and NADH. One of its molecules breaks down 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 oxidizing agent is oxygen, but it is worth noting that the process of glycolysis can be carried out in the absence of oxygen. This type of glycolysis is called anaerobic.
Anaerobic type glycolysis is a stepwise process of glucose oxidation. With this glycolysis, glucose oxidation does not occur completely. Thus, during the oxidation of glucose, only one molecule of pyruvate is formed. In terms of energy benefits, anaerobic glycolysis is less beneficial than aerobic. However, if oxygen enters the cell, then anaerobic glycolysis can be converted into aerobic, which is the complete oxidation of glucose.
Mechanism of glycolysis
Glycolysis breaks down six-carbon glucose into two molecules of three-carbon pyruvate. The whole process is divided into five preparatory stages and five more, during which ATP is storedenergy.
Thus, glycolysis proceeds in two stages, each of which is divided into five stages.
Stage 1 of the glucose oxidation reaction
- The first stage. The first step is glucose phosphorylation. Saccharide activation occurs by phosphorylation at the sixth carbon atom.
- Second stage. There is a process of isomerization of glucose-6-phosphate. At this stage, glucose is converted to fructose-6-phosphate by catalytic phosphoglucoisomerase.
- Third stage. Phosphorylation of fructose-6-phosphate. At this stage, the formation of fructose-1,6-diphosphate (also called aldolase) occurs under the influence of phosphofructokinase-1. It is involved in accompanying the phosphoryl group from adenosine triphosphoric acid to the fructose molecule.
- The fourth stage. At this stage, the cleavage of aldolase occurs. As a result, two triose phosphate molecules are formed, in particular ketoses and eldoses.
- The fifth stage. Isomerization of triose phosphates. At this stage, glyceraldehyde-3-phosphate is sent to the next stages of glucose breakdown. In this case, the transition of dihydroxyacetone phosphate to the form of glyceraldehyde-3-phosphate occurs. 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 then phosphorylated to diphosphoglycerate-1, 3.
- Seventh stage. This step involves the transfer of the phosphate group from 1,3-diphosphoglycerate to ADP. The end result of this step is 3-phosphoglycerateand ATP.
Stage 2 - complete oxidation of glucose
- The eighth stage. At this stage, the transition of 3-phosphoglycerate to 2-phosphoglycerate is carried out. 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.
- The tenth stage. There is a transfer of phosphates obtained as a result of the previous steps into PEP and ADP. Phosphoenulpyrovate is transferred to ADP. Such a chemical reaction is possible in the presence of magnesium (Mg) and potassium (K) ions.
Under aerobic conditions, the whole process comes to CO2 and H2O. The equation for glucose oxidation looks like this:
S6N12O6+ 6O2 → 6CO2+ 6H2O + 2880 kJ/mol.
Thus, there is no accumulation of NADH in the cell during the 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 that is the final electron acceptor that is transferred by NADH to the respiratory chain.
In the process of calculating the energy balance of the glycolytic reaction, it must be taken into account that each step of the second stage is repeated twice. From this we can conclude that two ATP molecules are spent in the first stage, and 4 ATP molecules are formed during the second stage by phosphorylation.substrate type. This means that as a result of the oxidation of each glucose molecule, the cell accumulates two ATP molecules.
We looked at the oxidation of glucose by oxygen.
Anaerobic glucose oxidation pathway
Aerobic oxidation is an oxidation process in which energy is released and which proceeds 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 the intermediate reaction of substrate oxidation.
The aerobic dichotomous type glucose oxidation process is the main pathway 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 splitting of the glucose molecule into water and carbon dioxide. The released energy will then be stored in ATP. This process can be roughly divided into three stages:
- 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 pathway for glucose breakdown.
- The process of formation of acetyl-CoA as a result of oxidative decarboxylation of pyruvic acid. This reaction takes place in cellular mitochondria.
- The process of 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 oxidized by enzyme complexes of the respiratory chain. As a result, ATP is formed when glucose is oxidized.
Formation of coenzymes
Coenzymes, which are formed at the second and third stages of aerobic glycolysis, will be oxidized directly in the mitochondria of cells. In parallel with this, 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 through the mitochondrial membranes. Hydrogen is transferred from cytoplasmic NADH to cellular mitochondria via shuttle cycles. Among these cycles, the main one can be distinguished - malate-aspartate.
Then, with the help of cytoplasmic NADH, oxaloacetate is reduced to malate, which, in turn, enters the cellular mitochondria and is then oxidized to reduce mitochondrial NAD. Oxaloacetate returns to the cell cytoplasm as aspartate.
Modified forms of glycolysis
Glycolysis may additionally be accompanied by the release of 1, 3 and 2, 3-biphosphoglycerates. At the same time, 2,3-biphosphoglycerate under the influence of biological catalysts can return to the glycolysis process, and then change its form to 3-phosphoglycerate. These enzymes play a variety of roles. For example, 2, 3-biphosphoglycerate, found in hemoglobin, promotes the transfer of oxygen to tissues, while contributing to the dissociation and decrease in the affinity of oxygen and red blood cells.
Conclusion
Many bacteria can change the form of glycolysis at its various stages. In this case, it is possible to reduce their total number or modify these stages as a result of the action of various enzymatic compounds. Some of the anaerobes have the ability to decompose carbohydrates in other ways. Most thermophiles have only two glycolytic enzymes, in particular enolase and pyruvate kinase.
We looked at how glucose is oxidized in the body.
