GLYCOLYSIS

Glycolysis is the breakdown or catabolism of a six carbon glucose molecule in a series of enzyme catalyzed reactions to give 2 molecules of pyruvate which is a 3 carbon compound. Glycolysis takes place in all the cells in the cytosol . During the sequential reactions of glycolysis some of the free energy is stored in the form of ATP and NADH. Glycolysis occurs in ten steps , and in two phases :- The first 5 steps constitute the PREPARATORY PHASE in which the energy of 2 ATP is invested, raising the free-energy content of the intermediates and the carbon chains of all the metabolized hexoses are converted into a common product glyceraldehyde-3-phosphate. The second phase is the PAY OFF PHASE where energy gain comes. There is net yield of 2 ATP molecules and also 2 molecules of NADH per molecule of glucose.

Importance of Phosphorylated Intermediates Because the plasma membrane generally lacks transporters for phosphorylated sugars, the phosphorylated intermediates cannot leave the cell and hence no energy is necessary to retain intermediates in the cell. Phosphoryl groups are essential components in the enzymatic conversion of metabolic energy. High energy phosphate compounds formed in glycolysis donate phosphoryl group to ADP to form ATP. Binding energy resulting from the binding of phosphate groups to the active sites of enzymes lowers the activation energy .

PREPARATORY PHASE OF GLYCOLYSIS

STEP 1 : Phosphorylation of Glucose

STEP 2 :Conversion of Glucose-6-phosphate to Fructose-6-phosphate

STEP 3: Phosphorylation of Fructose-6-phosphate to Fructose-1,6-bisphosphate

STEP 4: Cleavage of Fructose-1,6-bisphosphate

STEP 5: Interconversion of Triose Phosphates

PAY OFF PHASE OF GLYCOLYSIS

STEP 6: Oxidation of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate

STEP 7: Phosphoryl transfer from 1,3-bisphosphate to ADP

STEP 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate

STEP 9: Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate (PEP)

STEP 10: Transfer of the phosphoryl group from PEP to ADP

NET REACTION AND REGULATION OF GLYCOLYSIS Glycolysis is tightly regulated in coordination with other energy yielding pathways to assure a steady supply of ATP. Hexokinase, Phosphofructokinase-1 and pyruvate kinase are all subject to allosteric regulation that controls the flow of carbon through the pathway and maintains constant levels of metabolic intermediates.

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