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 (glucokinase in liver)

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

• 
  Figure 1 briefly illustrates the steps in glycolysis Reference : en.wikipedia, original author- Tekks.

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.