Subunit III of cytochrome c oxidase of Rhodobacter sphaeroides is required to maintain rapid proton uptake through the D pathway at physiologic pH.

The catalytic core of cytochrome c oxidase is composed of three subunits where subunits I and II contain all of the redox-active metal centers and subunit III is a seven transmembrane helix protein that binds to subunit I. The N-terminal region of subunit III is adjacent to D132 of subunit I, the initial proton acceptor of the D pathway that transfers protons from the protein surface to the buried active site ∼30 A distant. The absence of subunit III only slightly alters the initial steady-state activity of the oxidase at pH 6.5, but activity declines sharply with increasing pH, yielding an apparent pK a of 7.2 for steady-state O 2 reduction. When subunit III is present, cytochrome oxidase is more active at higher pH, and the apparent pK a of steady-state O 2 reduction is 8.5. Single-turnover experiments show that proton uptake through the D pathway at pH 8 slows from > 10000 s - 1 in the presence of subunit III to 350 s - 1 in its absence. At low pH (5.5) the D pathway of the oxidase lacking subunit III regains its capacity for rapid proton uptake. Analysis of the F → O transition indicates that the apparent pK a of the D pathway in the absence of subunit III is 6.8, similar to that of steady-state O 2 reduction (7.2). The pK a of D132 itself may decline in the absence of subunit III since its carboxylate group will be more exposed to solvent water. Alternatively, part of a proton antenna for the D pathway may be lost upon removal of subunit III. It is proposed that one role of subunit III in the normal oxidase is to maintain rapid proton uptake through the D pathway at physiologic pH.