Glycolysis

CHAPTER: 11(B)

Glycolysis

Glycolysis:(Embden – Meyerhof – parnas, or EMP pathway)

•      Glycolysis is derived from the Greek word (glucose-sweet or sugar; lysis- dissolution).

•      It is a universal pathway in the living cells.

•      The complete pathway of glycolysis was elucidated in 1940.

•      The pathway is often referred to as Embden-Meyerhof pathway (EMP).

•      Glycolysis is defined as the sequence of reaction converting glucose to pyruvate or lactate , with the production of ATP.

•      Glycolysis takes place in all cells of the body.

•      The enzymes of this pathway are present in the cytosomal fraction of the cell.

•      Glycolysis occurs in the absence of oxygen (anaerobic) or in the presence of oxygen (aerobic).

•      Lactate is the end product under anaerobic condition.

•      In the aerobic condition, pyruvate is formed, which is then oxidized to carbondioxide and water.

Steps involved in Glycolysis:

•      The sequence of reaction of glycolysis can be divided into three distinct phases:

                        1. Energy investment phase

                        2. Splitting phase

                        3. Energy generation phase

A. Energy investment phase:

1. Glucose is phosphorylated to Glucose-6-P by hexokinase or  glucokinase  (both are isoenzymes). This is an irreversible rxn. dependent on ATP & Mg²⁺ .
2. Glucose-6-P undergoes isomerization to give Fructose-6-P in the presence of the enzyme phosphohexose isomerase & Mg²⁺ .
3. Fructose-6-P phosphorylated to Fructose-1,6-bis (P) by phosphofructokinase . This is an irreversible and a regulatory step in glycolysis.

2. Splitting phase:

4. The 6-C Fructose-1,6-bis (P) is split (hence the name glycolysis) to two 3-C compounds, Glyceraldehyde-3-(P)  &  Dihydroxyacetone phosphate  by the enzyme  aldolase (Fructose-1,6-bis (P)aldolase).
5. The enzyme phosphotriose isomerase catalyses the reversible interconversion of G-3-P & DHAP. Thus two molecules of G-3-P are obtained from one molecule of glucose.

3. Energy generation phase:

6. G-3-P dehydrogenase converts G-3-P to  1,3 –bisphosphoglycerate. This step is important as it is involved in the formation of  NADH + H⁺ & a high energy compound 1,3 –bisphosphoglycerate.
7. The enzyme phosphoglycerate kinase acts on 1,3-bisphosphoglycerate resulting in the synthesis of ATP & formation of 3- Phosphoglycerate.
8. 3- Phosphoglycerate  is converted to 2- Phosphoglycerate by phosphoglycero mutase.  This is an isomerization rxn.
9. The high energy compound phosphoenol pyruvate is generated from 2- Phosphoglycerate by the enzyme Enolase.This enzyme requires Mg²⁺  or Mn²⁺ .
10. The enzyme Pyruvate Kinase  catalyses the transfer of high energy phosphate from phosphoenol pyruvate to ADP , leading to the formation of ATP   &  Pyruvate.