Glycolysis (Greek: glycos meaning sugar, and lysis meaning splitting) literally means the splitting up of sugar. It represents reactions in which glucose (and also fructose) molecule is partially oxidi/cd into two molecules of pyruvic acid. The reactions of glycolysis occur in living organisms –

Both prokaryotes and eukaryotes these reactions arc localized in the cytoplasm of cell and do not require oxygen. In plants, partial glycolytic pathway occurs in the plastids (chloroplast and amyloplast).

Glycolysis is conveniently considered in two major groups of reactions. The first group is a set of reactions by which glucose and fructosc derived from storage carbohydrates are converted to trioscphosphatcs via fructose-1, 6-bisphosphate. This first group of reactions is regarded as the preparatory (or investment) phase sincc energy in the form of ATP is consumed during th’ phase.

In the second set of reactions triosephosphates are converted to pyruvate, which is the end product of glycolysis. This second set of reactions is known as pay-off (or energy-conserving phase, which is characterized by net gain of energy-rich molecules, such as ATP and NADU. The reactions of glycolysis are shown in.

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The preparatory phase of glycolysis begins with the conversion of glucose to glucose 6- phosphate by the activity of the enzyme hexokinase (glucokinase). This phosphorylation reaction catalyzed by hexokinase requires ATP and MgT\ Glucose 6-phosphate then isomerizes to form fructose 6-phosphate being catalyzed by the enzyme hexosephosphate isomerase.

When fructose is the substrate, it is phosphorylated to fructose 6-phosphate by the enzyme hexokinase (fructokinase) which requires ATP and Mg” for its activity. The enzyme phosphofructokinase, which also requires ATP and Mg+T for its activity, catalyzes the phosphorylation of fructose 6- phosphate to form fructose-i, 6-bisphosphate.

This reaction is essentially irreversible. In plants, an additional cytosolic enzyme, pyrophosphate (PPi)-dependent phosphofructokinase is present which catalyzes the reversible inter conversion of fructose 6-phosphate and fructose-1, 6- bisphosphate. In the next step, fructose-i, 6-bisphosphate aldolase catalyzes the conversion of fructose-i, 6-bisphosphate into two triosephosphate molecules. One is glyceraldehyde 3-phosphate, which will proceed further through glycolysis.

Another is dihydroxyacetone phosphate, which unless otherwise used up, is converted to glyceraldehydes 3-phosphate to proceed through glycolysis. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-convertible, an equilibrium reaction catalyzed by the enzyme triosephosphate isomerase.

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In the pay-off phase of glycolysis, glyceraldehyde 3-phosphate dehydrogenase catalyzes the oxidation of glyceraldehyde 3-phosphate to produce 1, 3-bisphosphoglycerate with the integration of one inorganic phosphate molecule. In this reaction XAD* removes two HT from glyceraldehydes 3-phosphate forming XADH + IT and sufficient free energy is released to allow the phosphorylation (using inorganic phosphate) of glyceraldehydes 3-phosphate to produce 1, 3-bisphosphoglycerate. In the next step, 1, 3-bisphosphoglycerate transfers the phosphate on its carbon 1 to a molecule of ADP, yielding ATP and 3-phosphoglycerate.

This reaction is catalyzed by the enzyme phosphoglycerate kinase. This type of ATP synthesis, involving the direct transfer of phosphate group from a substrate molecule to ADP to form ATP, is traditionally referred to as substrate-level phosphorylation. The enzyme phosphoglyceratemutase catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate and then the enzyme enolase catalyzes the removal of water from 2-phosphoglycerate forming phosphoenolpyruvate.

The enolphosphate in phosphoenolpyruvate has the properties of a high-energy bond. Pyruvate kinase irreversibly catalyzes the transfer of phosphate group from phosphoenolpyruvate to ADP forming ATP and pyruvate.

Each glyccraldchydc 3-phosphalc molecule, on being converted to pyruvate by a series of reactions during the pay- iff phase, produces 2 molecules of ATP and 1 molecule of XADII + 11 . So one glucose molecule when converted to pyruvate by glycolysis can produce 4 molecules of ATP and 2 molecules of XADII + II during the preparatory phase, activation of glucose (or fructose) to form fructose-i, 6-bisphosphate uses 2 molecules of ATP hence, there is a net gain of 2 molecules of ATP and 2 molecules of NADU + II’ when 1 molecule of glucose (or fructose) is converted to 2 molecules of pyruvate during glycolysis.