According to the chemiosmotic theory, the electron-transport system, powered by the transport of electrons, pumps protons (H+) from the mitochondrial matrix into the space between the inner and outer mitochondrial membranes. The electron-transport system is grouped into three complexes that serve as proton pumps (fig. 5.10). The first pump (the NADH-coenzyme Q reductase complex) transports four H+ from the matrix to the intermembrane space for every pair of electrons moved along the electron-transport system. The second pump (the cy-tochrome c reductase complex) also transports four protons into the intermembrane space, and the third pump (the cytochrome c oxidase complex) transports two protons into the intermembrane space. As a result, there is a higher concentration of H+ in the intermembrane space than in the matrix, favoring the diffusion of H+ back out into the matrix. The inner mitochondrial membrane, however, does not permit diffusion of H+, except through structures called respiratory assemblies.
The respiratory assemblies consist of a group of proteins that form a "stem" and a globular subunit. The stem contains a channel through the inner mitochondrial membrane that permits the passage of protons (H+). The globular subunit, which protrudes into the matrix, contains an ATP synthase enzyme that is capable of catalyzing the reaction ADP + Pi ^ ATP when it is activated by the diffusion of protons through the respiratory assemblies and into the matrix (fig. 5.10). In this way, phosphory-lation (the addition of phosphate to ADP) is coupled to oxidation (the transport of electrons) in oxidative phosphorylation.
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