If a person is in a catabolic state or needs energy, how pyruvate will be used depends on whether adequate oxygen levels are present. If there are adequate oxygen levels (aerobic conditions), pyruvate moves from the cytoplasm, into the mitochondria, and then undergoes the transition reaction. If there are not adequate oxygen levels (anaerobic conditions), pyruvate will instead be used to produce lactate in the cytoplasm. We are going to focus on the aerobic pathway to begin with, then we will address what happens under anaerobic conditions in the anaerobic respiration section.
Figure 6.241 Pyruvate fork in the road. What happens depends on whether it is aerobic or anaerobic respiration1
The transition reaction is the transition between glycolysis and the citric acid cycle. The transition reaction converts pyruvate (3 carbons) to acetyl CoA (2 carbons), producing carbon dioxide (CO2) and a NADH as shown below. The figure below shows the transition reaction with CoA and NAD entering, and acetyl-CoA, CO2, and NADH being produced.
Figure 6.242 Transition reaction2
The acetyl is combined with coenzyme A (CoA) to form acetyl-CoA. The structure of CoA is shown below.
Figure 6.243 Structure of coenzyme A3
Thus, for one molecule of glucose, the transition reaction produces 2 acetyl-CoAs, 2 molecules of CO2, and 2 NADHs.