Glycolysis or EMP (Embden-Meyerhof-Parnas) pathway

  • All the living organisms whether aerobes or anaerobes, initiate the mechanism of respiration by breaking down glucose (6 carbon compound) into two molecules of pyruvate (3 carbon compound).
  • This initial process that occurs in 10 steps, the first 5 of which constitute the preparatory phase and the last 5 constitute the payoff phase is called glycolysis.
  • The process of glycolysis was first described by Gustav Embden, Otto Meyerhof and Parnas and therefore also referred to as EMP pathway.




The Preparatory phase:

  1. Phosphorylation of glucose:
  • It is the first priming reaction in glycolysis.
  • Glucose molecule is phosphorylated in the presence of ATP to form glucose-6-phosphate. This reaction is catalyzed by the enzyme hexokinase which requires a divalent Mg++ as cofactor. ATP (phosphoryl donor) is converted to ADP in the reaction.


  1. Isomerization of glucose-6-phosphate to fructose-6-phosphate:
  • The enzyme phosphohexose isomerase (phosphoglucose isomerase) catalyzes the reversible isomerization of glucose-6-phosphate, an aldose, to fructose-6-phosphate, a ketose.
  • Fructose-6-phosphate may be formed directly from free fructose by its phosphorylation in the presence of an enzyme fructokinase, Mg++ and ATP.

  1. Phosphorylation of fructose-6-phosphate to fructose1, 6-biphosphate:
  • It is the second priming reaction in glycolysis.
  • Fructose-6-phosphate combines with another phosphoryl group from another ATP molecule, yielding fructose1,6-biphosphate catalyzed by phosphofructokinase-1 (PFK-1) in the presence of Mg++.

  1. Cleavage of frucose1,6-biphosphate:
  • The enzyme fructose 1, 6-biphosphate aldolase, often called simply aldolase, catalyzes a reversible aldol condensation.
  • Fructose 1,6- biphosphate is cleaved to yield two different 3-carbon (triose) phosphates, glyceraldehyde 3-phosphate (GAP), an aldose and dihydroxyacetone phosphate (DHAP), a ketose.

  1. Interconversion of two triose phosphates:
  • Only one of the two triose phosphates formed by aldolase, can be directly degraded in the subsequent steps of glycolysis.
  • DHAP is rapidly and reversibly converted into GAP with the aid of enzyme phosphotriose isomerase or triose phosphate isomerase.

The Payoff phase:

6. Oxidation of glyceraldehyde3-phosphate to 1,3-biphosphoglycerate:

  • Glyceraldehyde 3-phosphate combines with a phosphate group (derived from H3PO4 in the cytosol and not from ATP)) and is oxidized with the separation of two atoms of hydrogen from it to form 1,3-biphosphoglycerate which is catalyzed by glyceraldehyde3-phosphate dehydrogenase.
  • Of the two hydrogen separated, one complete hydrogen atom (proton and electron) and one additional electron are picked up by NAD+ which gets reduced to NADH. The remaining one hydrogen proton or ion (H+) remains free in the cytosol.

  1. Phosphoryl transfer from 1,3-biphospohglycerate to ADP:
  • High energy phosphate group on carbon 1 of 1,3-biphosphoglycerate is transferred to a molecule of ADP, converting it into an ATP molecule.
  • 1,3-biphosphoglycerate changes to 3-phosphoglycerate due to loss of a phosphate group.
  • This reaction is catalyzed by the enzyme phosphoglycerate kinase in the presence of Mg++.
  • Formation of ATP directly from metabolites (substrate) is known as substrate level phosphorylation.

  1. Conversion of 3-phosphoglycerate to 2-phosphoglycerate (isomerization):
  • Phosphate group on the third carbon of 3-phosphoglycerate shifts to the second carbon, producing 2-phosphoglycerate.
  • This change is aided by the enzyme phosphoglycerate mutase in the presence of Mg++.

  1. Dehydration of 2-phosphoglycerate to phosphoenol pyruvate:
  • 2-phosphoglycerate loses a water molecule (reversibly)in the presence of an enzyme, enolase and Mg++, and changes into phosphoenol pyruvate (PEP).

  1. Transfer of the phosphoryl group from phosphoenol pyruvate to ADP:
  • The last step in glycolysis is the transfer of the phosphoryl group from phosphoenol pyruvate to ADP, catalyzed by pyruvate kinase.
  • This process requires K+ and either Mg++ or Mn++.

Glycolysis or EMP (Embden-Meyerhof-Parnas) pathway