Glycolysis ( Embden-Meyerhof Pathway)
In glycolysis pathway glucose is converted to pyruvate (aerobic condition) or lactate (anaerobic condition), along with production of a small quantity of energy.
Site of reaction: All the reaction steps take place in the cytoplasm.
Importance of the glycolysis pathway:
It is the only pathway that is taking place in all the cells of the body.
Glycolysis is the only source of energy in erythrocytes.
In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles.
The glycolytic pathway may be considered as the preliminary step before complete oxidation.
The glycolytic pathway provides carbon skeletons for synthesis of nonessential amino acids as well as glycerol part of fat.
Most of the reactions are reversible.
Steps of glycolytic pathway
1. Glucose is phosphorylated to glucose -6-phosphate. The enzyme is hexokinase, which splits ATP into ADP and the Pi is added on to the glucose. The energy released by hydrolysis of ATP is utilised for the forward reaction. Hexokinase is the key glycolytic enzyme and the reaction is irreversible.
2. Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase.
3. Fructose-6-phosphate is further phosphorylated to fructose-1,6-bisphosphate. The enzyme is phosphofructokinase, it is an important key enzyme and the reaction is irreversible.
4. Fructose-1, 6-bisphosphate is cleaved into two 3 carbon atoms; one glyceraldehyde-3-phosphate and another molecule of dihydroxyacetone phosphate. The enzyme is aldolase. Dihydroxyacetone phosphate is isomerised to glyceraldehyde-3-phosphate by the enzyme phophotriose isomerase.
5. Glyceraldehyde-3-phosphate is dehydrogenated and simultaneously phosphorylated to 1,3-bis-phosphoglycerate with the help of NAD+. The enzyme is glyceraldehyde-3-phosphate dehydrogenase.
6. 1, 3-bis-phosphoglycerate is converted to 3-phosphoglycerate by the enzyme 1, 3-bis-phosphoglycerate kinase. Here one molecule of ATP is formed and this reaction is an example for Substrate level phosphorylation.
7. 3-phosphoglycerate is isomerised to 2-phosphoglycerate by shifting the phosphate group from 3rd to 2nd carbon atom. The enzyme is phosphoglucomutase.
8. 2-phosphoglycerate is converted to phosphoenol pyruvate by the enzyme enolase. One water molecule is removed. A high energy phosphate bond is produced. This enzyme requires Mg++and inhibited by fluoride.
9. Phosphoenol pyruvate is dephosphorylated to pyruvate, by pyruvate kinase. One molecule of ATP is generated. This step is irreversible.
10. In anaerobic condition pyruvate is reduced to lactate by lactate dehydrogenase.In aerobic conditions pyruvate enters citric acid cycle for complete oxidation. The lactate from anaerobic cycle enters cori’s cycle.
Energy yield from glycolysis
Number of ATPs gained per glucose molecule is 8 ATPs
Number of ATPs gained per glucose molecule is 2 ATPs