Glucose is the major form in which carbohydrate absorbed from the intestinal tract is presented to cells of the body. Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+ in the presence of oxygen.The free energy released in this process is used to form the high-energy compounds ATP (adenosine triphosphate) and NADH .Pyruvate can be further processed anaerobically to lactate. This pathway is called Embden Meyerhof pathway.
Reactions of Glycolysis
The breakdown of glucose (6-carbon compound) to two molecules of pyruvate (3-carbon compound) is brought about by sequential action of 10 enzymes which can be divided into two phases :
First Phase/Primary Phase/Preparatory Phase
The first five steps are regarded as the preparatory (or investment) phase. Since they consume energy to convert the glucose into two three-carbon sugar phosphates. In this phase two molecules of ATP are invested.
In the 1st step glucose is phosphorylated at C-6 by hexokinase to yield glucose 6-phosphate, with ATP as the phosphate donor. Hexokinase requires for activity. This reaction is irreversible under intracellular conditions.
2. Second Step
Glucose 6- Phosphate is then rearranged into fructose 6-phosphate (F6P) by phosphohexose isomerase. This reaction is freely reversible under normal cell conditions.
3. Third Step
Phosphofructokinase-1 catalyzes this step at ATP dependent phosphorylation of fructose 6- phosphate to give fructose 1,6- bisphosphate. The reaction is irreversible under intracellular conditions.
4. Fourth Step
Fructose 1,6 bisphosphate is cleaved by the enzyme aldolase to yield two different triose phosphates, gluceraldehyde 3-phosphate, an aldose and dihydroxyacetone phosphate, a ketose.
5. Fifth Step
In this step Dihydroxyacetone phosphate is isomerized to glyceraldehyde-3-phosphate by the enzyme phosphotriose isomerase .This reaction completes the preparatory phase of glycolysis.
Second Phase/Secondary Phase/Pay –off Phase
6. Sixth Step
The first step in the pay off phase is the oxidation of glyceraldehyde 3- phosphate to 1, 3 bisphosphoglycerate, catalyzed by glyceraldehyde 3- phosphate dehydrogenase.
7. Seventh Step
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.
8. Eight Step
In the eighth step the enzyme phosphoglycerate mutase isomerizes 3-Phosphoglycerate to 2-Phosphoglycerate.
9. Ninth Step
In this step the enzyme enolase promotes reversible removal of a molecule of water from 2- phosphoglycerate to yield phosphoenolpyruvate.
10. Tenth Step
The high energy phosphate of phosphoenol pyruvate is transferred to ADP by the enzyme pyruvate kinase to generate 2 molecules of ATP per molecule of glucose oxidized. Enol pyruvate formed in this reaction is converted spontaneously to the keto form of pyruvate.