A cytochrome c oxidase proton pumping mechanism that excludes the O₂ reduction site

Abstract

A redox-coupled conformational change in Asp51 of subunit I and a hydrogen-bond network connecting Asp51 with the matrix surface have been deduced from X-ray structures of bovine heart cytochrome c oxidase. This has provided evidence that Asp51 may play a role in the proton pumping process. However, the lack of complete conservation of a residue analogous to Asp51, the inclusion of a peptide bond in the hydrogenbonding network and the lack of apparent involvement of the O 2 reduction site have been used as arguments against the involvement of Asp51 in the mechanism of proton pumping. This minireview re-examines these arguments.

Protonmotive cooperativity in cytochrome c oxidase

“…Or is it alternatively conceivable that the N98D mutation does impair proton/electron coupling at heme a? It should be noted that the Yoshikawa model [15,25] as well as our version [26] both would, in principle, imply that transfer of each electron from heme a to the binuclear site is coupled to pumping of c 1 H + from the N to the P phase.…”

“…The proton/electron coupling exhibited by heme a is likely to provide the basis for its obligatory role in the proton pump. Yoshikawa [15] and Tsukihara et al [25] have described in detail a proton pumping mechanism based on proton/electron coupling at heme a, which excludes the O 2 reduction site (Fig. 7).…”

“…The H channel is structurally well separated from the proton input channels D and K, which can be traced in the crystal structures of the oxidases described above [35 -38]. The proton exit from the environment of heme a is proposed by Yoshikawa et al [15,24,25] to be provided by a pathway involving R38, which is in fact conceived to represent an element of the gate of the pump, bound H 2 O in the environment of heme a, one propionate of heme a, the peptide bond between Y440 and S441 and D51 at the P surface (Fig. 7).…”

Direct measurement of proton release by cytochrome c oxidase in solution during the F→O transition

“…More accurately, the proton-binding sites on the input and output sides of the membrane are different for any consecutive uptake͞release sequence. The proton-conducting pathways of several known proton pumps, including bacteriorhodopsin (9) the F 1 F o ATP synthase (10,11), and CcO (12)(13)(14) contain protonatable amino acid side chains that have high pK a values during at least part of the catalytic cycle. To the extent that these groups are protonated, each of these proteins can be said to contain an internal proton pool.…”

“…It has been suggested, for example, that Asp-51 in the bovine oxidase (subunit I) is the proton release site (14), although it is only found in mammalian oxidases. Another possibility would be one of the heme propionates (17) or another residue, such as the Cu B ligand His-291 (31), that may be within the exit pathway.…”