The following types of reaction occur in glycolysis, except
Step 1: Uptake and Phosphorylation of Glucose Glucose is phosphorylated to form glucose-6-phosphate. The reaction is catalysed by the specific enzyme glucokinase in liver cells and by non-specific enzyme hexokinase in liver and extrahepatic tissue. The enzyme splits the ATP into ADP, and the Pi is added to the glucose. Hexokinase is a key glycolytic enzyme. Hexokinase catalyses a regulatory step in glycolysis that is irreversible. Hexokinase, like many other kinases, requires Mg2+ for its activity. Step 2: Isomerization of Glucose-6-Phosphate to Fructose-6-Phosphate Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase. This reaction involves an aldose-ketose isomerisastion catalysed by phosphohexose isomerase. There is opening of the glucopyranose ring of glucose-6-phosphate to a linear structure which then changes to the furanose ring structure of fructose-6-phosphate. Step 3: Phosphorylation of F-6-P to Fructose 1,6-Biphosphate Fructose-6-phosphate is fuher phosphorylated to fructose 1,6-bisphosphate. The enzyme is phosphofructokinase-1. It catalyses the transfer of a phosphate group from ATP to fructose-6-phosphate. The reaction is irreversible. One ATP is utilised for phosphorylation. Phosphofructokinase-1 is the key enzyme in glycolysis which regulates breakdown of glucose. Step 4: Cleavage of Fructose 1,6-Biphosphate The 6 carbon fructose-1,6-bisphosphate is cleaved into two 3 carbon units; one glyceraldehyde-3-phosphate (GAP) and another molecule of dihydroxyacetone phosphate (DHAP). The enzyme which catalyses the reaction is aldolase. Since the backward reaction is an aldol condensation, the enzyme is called aldolase. The reaction is reversible. Step 5: Interconversion of the Triose Phosphates GAP is on the direct pathway of glycolysis, whereas DHAP is not. Hence Triose-phosphate isomerase conves DHAP into GAP useful for generating ATP. Thus net result Is that glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate. This reaction is rapid and reversible. Step 6: Oxidative phosphorylation of GAP to 1,3-Bisphosphoglycerate The first step in the payoff phase is the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. This reaction is catalyzed by glyceraldehyde 3-phosphate dehydrogenase. It is the energy-yielding reaction. Reactions of this type in which an aldehyde group is oxidised to an acid are accompanied by the liberation of large amounts of potentially useful energy. During this reaction, NAD+ is reduced to NADH. This is a reversible reaction. Step 7: Conversion of 1,3-Biphosphoglycerate to 3-Phosphoglycerate The enzyme phosphoglycerate kinase transfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. This is a unique example where ATP can be produced at substrate level without paicipating in electron transpo chain. This type of reaction where ATP is formed at substrate level is called as Substrate level phosphorylation. Step 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate 3-phosphoglycerate is isomerized to 2-phosphoglycerate by shifting the phosphate group from 3rd to 2nd carbon atom. The enzyme is phosphoglucomutase. This is a readily reversible reaction. Mg2+ is essential for this reaction. Step 9: Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate 2-phosphoglycerate is conveed to phosphoenolpyruvate by the enzyme enolase. One water molecule is removed. A high energy phosphate bond is produced. The reaction is reversible. Enolase requires Mg++. Step 10: Conversion of Phosphoenol Pyruvate to Pyruvate Phosphoenolpyruvate (PEP) is dephosphorylated to pyruvate, by pyruvate kinase. First PEP is made into a transient intermediary of enol pyruvate; which is spontaneously isomerized into keto pyruvate, the stable form of pyruvate. One mole of ATP is generated during this reaction. This is again an example of substrate level phosphorylation. The pyruvate kinase is a key glycolytic enzyme. This step is irreversible. REF:DM VASUDEVAN TEXTBOOK OF BIOCHEMISTRY,EIGHTH EDITION,PG.NO.,129.
Hydration
Isomerisations
Phosphoryl transfer
Aldol cleavage
Metabolism of carbohydrate