ABSTRACT
Oxidative phosphorylation is responsible for the vast majority of ATP found within nonphotosynthetic eukaryotes. In the mitochondria, electron transfer from NADH and succinate to molecular oxygen is utilized to generate a proton gradient across the inner membrane. The potential energy thus generated is dissipated by ATP synthase, which catalyzes the conversion of ADP and inorganic phosphate to ATP. Electron transfer occurs through a series of membrane-bound protein complexes. In the first step, NADH and succinate are oxidized by their respective dehydrogenases (Complex I or Complex II) and, in a two-electron reduction, ubiquinone (UQ) is reduced to ubiquinol (UQH2). In the second step, the reducing equivalents on UQH2 are delivered to the cytochrome be1 complex (Complex III), which reduces cytochrome e and regenerates oxidized ubiquinone. Cytochrome e donates a single electron at a time to cytochrome oxidase (Complex IV) until four electrons have been delivered, upon which oxygen is reduced to water.
