This article will examine how glucose oxidizes. Carbohydrates are polyhydroxycarbonyl type compounds, as well as their derivatives. Characteristic features are the presence of aldehyde or ketone groups and at least two hydroxyl groups.
In 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 are isolated (these include galactose, glucose, ribose) and ketoses (ribulose, fructose).
Oligosaccharides
Oligosaccharides are carbohydrates, which contain from two to ten residues of monosaccharide origin, connected by glycosidic bonds. Depending on the amount of monosaccharide residues, disaccharides, trisaccharides and so on are distinguished. What is formed during glucose oxidation? This will be discussed later.
Polysaccharides
Polysaccharides are carbohydrates that contain more than ten monosaccharide residues interconnected by glycosidic bonds. If the polysaccharide contains the same monosaccharide residues, then it is called a homopolysaccharide (for example, starch). If such residues are different, then a heteropolysaccharide (for example, heparin).
What does glucose oxidation matter?
The 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 human energy needs are satisfied. The oxidation of one gram of carbohydrate releases 16.9 kJ.
- Reserve. Glycogen and starch are a form of accumulation of nutrients.
- Structural. Cellulose and some other polysaccharide compounds form a strong skeleton in plants. Also, they, in combination with lipids and proteins, are a component of all cellular biomembranes.
- Protective. For acidic heteropolysaccharides, the role of a biological lubricant is assigned. They line the surfaces of joints that touch and rub against each other, mucous membranes of the nose, digestive tract.
- Antigoagulant. A carbohydrate such as heparin has an important biological property, namely, it prevents blood coagulation.
- Carbohydrates are a carbon source needed for the synthesis of proteins, lipids and nucleic acids.
For the body, the main source of carbohydrates is dietary carbohydrates - sucrose, starch, glucose, lactose). Glucose can be synthesized in the body 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 up into two pyruvate molecules.
The process of glycolysis occurs under the influence of various enzymatic substances, that is, catalysts of a biological nature. The most important oxidizing agent is oxygen, but it is worth noting that the glycolysis process can be carried out in the absence of oxygen. This type of glycolysis is called anaerobic.
Anaerobic glycolysis is a stepwise process of glucose oxidation. With this glycolysis, glucose oxidation does not occur completely. Thus, only one pyruvate molecule is formed during glucose oxidation. In terms of energy benefits, anaerobic glycolysis is less beneficial than aerobic. However, if oxygen enters the cell, then conversion of anaerobic glycolysis to aerobic, which is the complete oxidation of glucose, can occur.
Glycolysis mechanism
In the process of glycolysis, six-carbon glucose breaks down into two three-carbon pyruvate molecules. The whole process is divided into five preparatory stages and five more, during which energy is stored in the ATP.
Thus, glycolysis proceeds in two stages, each of which is divided into five stages.
Stage No. 1 reaction of glucose oxidation
- First stage. At the first stage, glucose phosphorylation occurs. Activation of the saccharide occurs by phosphorylation at the sixth carbon atom.
- Second phase. The process of isomerization of glucose-6-phosphate. At this stage, glucose is converted to fructose-6-phosphate by the action of catalytic phosphoglucoisomerase.
- The 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 the accompaniment of the phosphoryl group from adenosine triphosphoric acid to the fructose molecule.
- The fourth stage. At this stage, the aldolase is cleaved. As a result, two triosophosphate 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. This transition is carried out under the action of enzymes.
- Sixth stage. The oxidation process 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 of a phosphate group from 1,3-diphosphoglycerate to ADP. The final result of this step is 3-phosphoglycerate and ATP.
Stage No. 2 - complete oxidation of glucose
- The eighth stage. At this stage, 3-phosphoglycerate is converted to 2-phosphoglycerate. The transition process is carried out under the influence of an enzyme such as phosphoglyceratmutase. This chemical reaction of glucose oxidation proceeds with the obligatory presence of magnesium (Mg).
- The ninth stage. At this stage, 2-phosphoglycerate is dehydrated.
- The tenth stage. There is a transfer of phosphates obtained as a result of the previous stages, in FEP and ADP. The transfer to ADP is phosphoenylated. Such a chemical reaction is possible in the presence of magnesium (Mg) and potassium (K) ions.
Under aerobic conditions, the whole process reaches CO2 and H2O. The equation for glucose oxidation looks like this:
6 12 6 + 6 2 → 6 2 + 6 2 + 2880 kJ / mol.
Thus, NADH does not accumulate in the cell during the formation of lactate from glucose. This means that such a process is anaerobic, and it can occur in the absence of oxygen. It is oxygen that is the final acceptor of electrons that are transferred by NADH to the respiratory chain.
In the process of calculating the energy balance of the glycolytic reaction, it is necessary to take into account that each stage of the second stage is repeated twice. From this we can conclude that at 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 glucose molecule, the cell accumulates two ATP molecules.
We examined the oxidation of glucose by oxygen.
Anaerobic glucose oxidation pathway
Aerobic oxidation is the oxidation process in which energy is released and which proceeds in the presence of oxygen, which acts as the final hydrogen acceptor in the respiratory chain. The donor of hydrogen molecules is the reduced form of coenzymes (FADN2, 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 occur in all tissues and organs of the human body. The result of this reaction is the breakdown of a glucose molecule into water and carbon dioxide. The released energy will be accumulated in the ATP. This process can be 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 acetyl CoA formation as a result of oxidative decarboxylation of pyruvic acid. This reaction occurs in cellular mitochondria.
- The process of oxidation of acetyl-CoA in the Krebs cycle. The reaction proceeds in cellular mitochondria.
At each stage of this process, reduced forms of coenzymes are formed that are oxidized by enzymatic complexes of the respiratory chain. As a result, ATP is formed during glucose oxidation.
Coenzyme Education
Coenzymes that are formed 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 through the membranes of mitochondria. Hydrogen is transferred from the cytoplasmic NADH to cell mitochondria through shuttle cycles. Among these cycles, the main one can be distinguished - malate aspartate.
Then, with the help of the cytoplasmic NADH, oxaloacetate is reduced to malate, which, in turn, penetrates into the cellular mitochondria and then oxidizes with the restoration of mitochondrial NAD. Oxaloacetate is returned to the cell cytoplasm as aspartate.
Modified forms of glycolysis
The course of glycolysis may additionally be accompanied by the release of 1,3 and 2,3-bisphosphoglycerates. In this case, 2,3-bisphosphoglycerate under the influence of biological catalysts can return to the glycolysis process, and then change its shape to 3-phosphoglycerate. These enzymes play a variety of roles. For example, 2,3-bisphosphoglycerate, located in hemoglobin, promotes the transfer of oxygen to tissues, while promoting dissociation and lowering the affinity of oxygen and red blood cells.
Conclusion
Many bacteria can change the course 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 exposure to various enzyme compounds. Some of the anaerobes have the ability to other ways to break down carbohydrates. Most thermophiles have only two glycolysis enzymes, in particular enolase and pyruvate kinase.
We examined how the oxidation of glucose in the body proceeds.