Because NADH enters the oxidative phosphorylation at Complex I, three steps of proton translocation result from electron transport, leading to three equivalents of ATP made by ATP synthase. Substrates oxidized by NAD are said to have a P/O ratio (phosphates fixed per oxygen atom reduced) of three. Substrates oxidized by FAD, primarily succinate, have a P/O ratio of two. This consideration allows a calculation of the energy fixed from the complete oxidation of a mole of glucose:
- Two ATPs result from glycolysis, with 2 NADH produced by glyceraldehyde −3 − phosphate dehydrogenase.
- Two turns of the TCA cycle, with NADH produced at the pyruvate dehydrogenase, isocitrate dehydrogenase, alpha‐ketoglutarate, and malate dehydrogenase steps and FADH 2 produced at the succinate dehydrogenase step and GTP (equivalent to ATP) produced at the succinyl‐coenzyme A synthetase step.
Comparison of this energy yield with the two ATPs produced by glycolysis alone indicates the enormous competitive advantage allowed by aerobic metabolism in oxygen‐rich environments.