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 approximately 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.