Anaerobic oxidation of glucose
Main stages of this process
Glucose (a six-atom organic compound in the class of monosaccharides) can be metabolically transformed in the body by two ways – aerobic (with oxygen molecules) and anaerobic (without oxygen).
The main goal of these processes is the synthesis of ATP (adenosine triphosphate, which plays a key role in metabolism in the body). Anaerobic oxidation of glucose can be presented as a process that is the reverse of the photosynthesis process (this reverse process is usually called oxidation or breathing). Substances are formed from glucose and oxygen which are initial for photosynthesis – water and carbon dioxide.
Anaerobic oxidation of glucose
Anaerobic glycolysis is the oxidation process of glucose without oxygen, in which lactates are formed. In this process, besides lactic acid, 2 ATP molecules are formed (it is important that at the preparation stage, 2 ATP molecules are spent on the reaction, but in the course of glycolysis 4 ATP molecules form).
In general, both aerobic and anaerobic glycolysis take place almost identically – the only stage in which these two types of oxidation can be distinguished is the last (it depends on the presence or absence of oxygen in the oxidation process).
Steps of anaerobic oxidation of glucose
Anaerobic glycolysis is a process which includes 2 steps – activation and oxidation.
In the first step, the glucose molecule splits into trioses – to activate the glucose, 2 ATP molecules are expended. As a result, isomeric carbonyl compounds form (they have an identical elemental composition, but a different position of atoms by structure) – phospho-dihydroxyacetone and 3-phospho-glycerin aldehyde (the ketone can also be isomerized into an aldehyde, as a result forming only 2 molecules of aldehyde as a product of the process).
The second stage is the stage of oxidation of the formed molecules of 3-phospho-glycerin aldehyde with the formation of lactic acid (as 2 aldehyde molecules enter into the reaction, as a result 2 acid molecules form). The intermediate product may be pyruvic acid.
This is why the summary equation of anaerobic glycolysis is presented in this form:
C₆H₁₂O₆ + 2ADP + 2H₃PO₄ = 2ATP + 2C₃H₆O₃ + 2H₂O;
The energy balance of the reaction may be written down in the following form: 2 ATP= -2ATP + 4ATP (2 ATP molecules are expended, and 2 form).
Processes that lie at the basis of glycolysis
Anaerobic glycolysis is a many-stage process including 11 reactions. In the course of oxidation of glycolysis, these reactions take place at the first stage:
phosphorylation of glucose in reaction with ATP with formation of glucose-6-phosphate;
isomerization of glucose-6-phosphate to fructose-6-phosphate;
repeated phosphorylation of the product by ATP with formation of fructoto-1,6-disphopshate;
separation of the product into two phosphotriose molecules;
isomerization of triosephosphates with transition of dihydroxyacetone phosphate into glyceraldehyde-3-phosphate.
The second stage includes these processes:
dehydration of the obtained aldehyde with the formation of 1,3-diphosphoglycerate;
first substrate phosphorylation of 1,3-diphosphoglycerate with formation of ATP molecule and 3-phosphoglycerate;
migration of phosphate group from the 3ʳᵈ carbon atom to the 2ⁿᵈ in 3-phosphoglycerate with formation of phosphoenolpyruvate;
second substrate phosphorylation of obtained phosphoenolpyruvate with formation of ATP molecule;
formation of pyruvate from the pyruvate-kinase enzyme;
reduction of pyruvate to lactic acid with use of NADH.
4 ATP molecules are formed, and not 2 (as we can see according to the oxidation diagram), as this process does not take place for one molecule, but for two, as in the first stage 2 aldehyde molecules were formed, each of which in subsequent oxidation gives 2 ATP molecules.
Further course of reaction
Lactic acid is not always the concluding stage of the reaction – for example, if after anaerobic oxidation of glucose, lactic acid enters aerobic conditions with excess of oxygen, it may transform into pyruvic acid and through the coenzyme acetyl-CoA, which is important for biochemical processes, enter into the metabolism of the organism, and namely the Krebs cycle.
More experiments with sugars you can find here.
In some cases, the Cori cycle may also take place – if lactic acid formed after anaerobic oxidation of glucose enters the liver from muscle tissues, the formation of glucose will take place with its participation.
Although anaerobic oxidation of glucose is a process that is rather inefficient for the organism energy-wise, it is necessary, as it the only path of anaerobic oxidation of glucose in living organisms. Anaerobic glycolysis is often active in conditions of a deficit of oxygen.