Glycolysis: The Energy Payoff Phase

Glycolysis can be divided into two phases: the energy investment phase and the energy payoff phase. The energy investment phase requires an input of two molecules of ATP. In the energy payoff phase (a continuation of the energy investment phase), four molecules of ATP are produced for a net total of two ATP during glycolysis.

The steps of the energy payoff phase are explained below. Notice that all of the steps are catalyzed by enzymes.

The enzyme triose phosphate dehydrogenase catalyzes the oxidation of G3P. Electrons are transferred from G3P to NAD+, forming NADH. Using the energy from this exergonic redox reaction, a phosphate group is attached to the oxidized product, forming 1, 3-biphosphoglycerate.
The enzyme phosphoglycerokinase catalyzes the transfer of a phosphate group from 1, 3-biphosphoglycerate to ADP, forming ATP. This process is known as substrate-level phosphorylation.
Phosphoglyceromutase (an enzyme) relocates the remaining phosphate group, forming 2-phosphoglycerate.
Enolase (enzyme) forms a double bond by removing a water molecule. This forms phosphoenolpyruvate (PEP).
Pyruvate kinase catalyzes the transfer of a phosphate group from PEP, forming ATP and pyruvate. Pyruvate is the final product of glycolysis.

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