8.2Outline of the Respiratory Chain and Oxidative Phosphorylation
As outlined in Chapter 7, glucose is fully broken down into carbon dioxide and hydrogen (NADH and FADH2), and only four ATP molecules per glucose are synthesized during the process. However, coupling with a series of reactions through which NADH and FADH2 (generated during the process) are fully oxidized by oxygen to become water molecules generates 38 molecules of ATP*4. The series of oxidation-reduction reactions beginning from NADH and FADH2 are known as electron transport reactions or the respiration chain, and the mechanism of synthesizing ATP coupling with the reactions is called oxidative phosphorylation.
In the 1950s, researchers initially looked for metabolic intermediates with high-energy phosphate bonds on the assumption that, as in glycolysis, kinases (enzymes that transfer the phosphate group to ATP) were also involved in reactions that synthesize large amounts of ATP, but these could not be found. Soon thereafter, the mystery surrounding ATP synthesis was solved by the chemiosmotic theory proposed in 1961 by P. Mitchell, who was involved in the investigation of active transport. This is the process that transports H+ and other substances across the membrane against the concentration gradient using ATP energy. In Mitchell’s hypothesis, ATP is synthesized using the concentration gradient of transported H+ coupled with electron transport reactions. In other words, the concentration gradient of H+ is a high-energy state that is interconvertible with ATP, and the conversion is catalyzed by F-ATP synthase (discussed later).