Principles of Biochemistry/Glycolysis

Glycolysis is the first step of glucose catabolism. Glycolysis is divided into two categories: aerobic (chemical reactions that occur with the presence of oxygen) and anaerobic (chemical reactions that do not require oxygen). An example of anaerobic glycolysis is fermentation. Glucose is the reactant; while ATP and NADH are the products of the Glycolysis reaction. There are three stages in an aerobic glycolysis reaction: 1) decarboxylation of pyruvate 2) Citric Acid Cycle (also known as the Krebs Cycle) 3) Electron transport chain.

Glycolysis consists of a total of 10 chemical reactions that starts with the breakdown of glucose into pyruvate and NADH which takes place in the cytoplasm.

  • Step 1: Phosphorylation of Glucose
This reaction is irreversible under intracellular condition, transferase class
Catalyzed by hexokinase-soluble and cytosolic protein
Requires ATP, and Mg2+ as substrates, generates ADP
ΔG′°= -16.7 kJ/mol
  • Step 2: Conversion of Glucose 6-Phosphate to Fructose 6-Phosphate
This reaction is reversible, isomerase class
Catalyzed by phosphohexo isomerase
Requires Mg2+ as a substrate
ΔG′°= 1.7 kJ/mol
  • Step 3: Phosphorylation of Fructose 6-Phospate to Fructose 1,6-Bisphosphate
Irreversible reaction, transferase class
Catalyzed by phosphofructokinase-1 which is highly regulate allosteric enzyme
Requires ATP, and Mg2+ as substrates, generates ADP
ΔG′°= -14.2 kJ/mol
  • Step 4: Cleavage of Fructose 1,6-Bisphosphate
Reversible reaction, lyase class
Catalyzed by aldose
Yields 2 different triose phosphates: G3P (an aldose), and DHAP (a ketose)
ΔG′°= 23.8 kJ/mol
  • Step 5: Interconversion of the Triose Phosphate
Reversible reaction, isomerase class
Catalyzed by triose phosphate isomerase
ΔG′°= 7.5 kJ/mol
  • Step 6: Oxidation of Glyceraldehyde 3-Phosphate to 1,3-Bisphosphoglycerate
Reversible reaction, oxidoreductase class
Catalyzed by glyceraldehyde 3-phosphate dehydrogenase
Requires NAD+, yields NADH
ΔG′°= 6.3 kJ/mol
  • Step 7: Phosphoryl Transfer from 1,3-Bisphosphoglycerate to ADP
Reversible reaction, transferase class
Catalyzed by phosphoglycerate kinase
Requires ADP and Mg2+, generates ATP
ΔG′°= -18.5 kJ/mol
  • Step 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate
Reversible reaction, isomerase class
Catalyzed by phosphoglycerate mutase
Requires Mg2+
ΔG′°= 4.4 kJ/mol
  • Step 9: Dehydration of 2-phosphoglycerate to Phosphoenolpyruvate
Reversible reaction,, lyase class
Catalyzed by enolase
ΔG′°= 7.5 kJ/mol
  • Step 10: Transfer of the Phosphoryl Group from Phosphoenolpyruvate to ADP
Irreversible reaction, transferase class
Catalyzed by pyruvate kinase
Requires ADP, Mg2+, K+, generates ATP
ΔG′°= -31.4 kJ/mol
The product pyruvate first appears in enol form, then tautomerizes to keto form.

The net reaction for Glycolysis is :

Glucose+2ADP+2P1+2NAD+ --> 2 Pyruvate + 2ATP+2NADH+2H++2H2O


Fermentation Glycolysis in fermentation occurs under anaerobic condition, thus, NAD+ has to be regenerated. In order to do obtain NAD+, pyruvate is reduced into ethanol or lactic acid. During fermentation, only 2 ATP per glucose are produced; therefore, it is not too efficient. There are two types of fermentation: 1) Alcohol fermentation which occurs in yeast and some bacteria and 2) Lactic acid fermentation which occurs in some fungi and bacteria, and muscles cells.

Last modified on 12 November 2012, at 23:07