8.3: Energy-releasing Steps of Glycolysis
In the second half of glycolysis, the two molecules of glyceraldehyde-3-phosphate, G3P, are oxidized with the reaction catalyzed by the enzyme triose phosphate dehydrogenase, and a phosphate group is attached to the unstable sugar, forming 1,3-bis-phospho-glycerate. As a result, two high-energy electrons and two protons are released and picked up by the carrier NAD+, forming NADH and a hydrogen ion.
The 1,3-bis-phospho-glycerate then donates one of its phosphate groups to ADP, creating one molecule of ATP, and turns into 3-phospho-glycerate. The enzyme phosphor-glyceratemutase converts this molecule into its isomer 2-phosphoglycerate, allowing the enzyme enolase to form a double bond and release a water molecule. With the help of pyruvate kinase, the unstable result, phosphor-enol-pyruvate, or PEP, gives away its phosphate group to ADP—generating a second molecule of ATP along with the end product, pyruvate.
Thus, at the end of glycolysis, a net total of two ATPs are produced, along with two NADHs, and two pyruvate molecules. With oxygen present, pyruvate can be broken down further, while NADH can pass its electrons into the electron transport chain to regenerate NAD+.