What is glycolysis | definition of glycolysis

Glycolysis-

- The oxidation of glucose to pyruvate and lactate is called glycolysis.Glycolysis is an anaerobic phase of carbohydrate metabolism.
- All the enzymes of glycolysis are found in the extramitochondrial soluble fraction of the cell.
- Glycolysis is derived from the Greek word "Glycose" means sweet or sugar and lysis means break down or dissolution.
- It is universal pathway in the living system.
- Glycolysis is defined as the sequence of reaction converting glucose or glycogen in to pyruvate or lactate with the production of ATP.
- This pathway also called as "EMP" pathway as it is proposed by "embden" "Mayerhoff" and "parns".

Reaction of glycolysis :

The reaction involved in glycolysis are as follows.

1.  Glucose to glucose 6 phosphate :

This reaction is catalyzed by the enzyme hexokinase.
-  ATP acts as a phosphate donor.
-  The presence of magnesium ion is necessary.
-  The reaction mainly occurs in liver and muscle. This reaction is irreversible.

2. Glucose 6 phosphate to fructose 6 phosphate :

This reaction is catalyzed by the enzyme phosphohexose isomerase.

3. Fructose 6 phosphate to fructose 1,6 diphosphate :

This reaction is catalyzed by the enzyme phospho fructokunase.
ATP acts as a phosphate donor.

4. Fructose 1,6 diphosphate to glyceraldehyde 3 phosphate and dihydroxy acetone phosphate :

This reaction is catalyzed by the enzyme aldolase. Both glyceraldehyde 3 phosphate and dihydroxy acetone are interconvertuble. This interconversion is catalyzed by the enzyme phospho triose isomerase. So two molecule of glyceraldehyde 3 phosphate are formed which are enter in to the next reaction.

5. Glyceraldehyde 3 phosphate to 1,3 diphosphoglycerate :

It is catalyzed by the enzyme glyceraldehyde 3phosphate dehydrogenase. This reaction requires the coenzyme NAD. 6 molecules of ATP are formed as a result of this reaction ( NADH formed in this reaction will be oxidised in electron transport chain to produce 3 molecules of ATP. For two molecules of 1,3 diphosphoglycerate a total of 6 molecules of ATP will be produced.

6. 1,3 diphosphoglycerate to 3 phosphoglycerate :

It is catalyzed by the enzyme phosphoglycerate kinase. In this reaction 2 molecules of ATP are formed for each molecules of glucose.

7.  3 phosphoglycerate to 2 phosphoglycerate :

It is catalyzed by the enzyme phosphoglycerate mutase.

8. 2 phosphoglycerate to phosphoenol pyruvate :

It is catalyzed by the enzyme enolase.

9. Phosphoenol pyruvate to enol pyruvate :

This reaction is catalyzed by the enzyme pyruvate kinase. 2 molecules of ATP are formed in this reaction for each molecules of glucose.

10. Enol pyruvate to keto pyruvate :

Keto pyruvate is the keto form of pyruvic acid. This is a spontaneous reaction.

11. Keto pyruvate to lactic acid :

This reaction occurs only when there is no oxygen. It does not occur under aerobic condition . This reaction is catalyzed by the enzyme lactate dehydrogenase. For the reduction of pyruvate to lactic acid the hydrogen atom is donated by NADH. 
Under aerobic condition lactic acid is not formed. In that case pyruvic acid is the end product of Glycolysis.
Glycolysis, Biochemistry Notes



Salient features-

- It takes place in all cell of the body.
- If glycolysis occurs in the presence of oxygen it is aerobic or if it occur in the absence of oxygen it is anaerobic.
- Lactate is the end product under anaerobic condition and pyruvate is the end product under aerobic condition.

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