The fifth electron in the fully reduced caa(3) from Thermus thermophilus is competent in proton pumping.
Sergey A. Siletsky, Ilya Belevich, Tewfik Soulimane, Michael I. Verkhovsky and Mårten Wikström.

The time-resolved kinetics of membrane potential generation coupled to oxidation of the fully reduced (five-electron) caa(3) cytochrome oxidase from Thermus thermophilus by oxygen was studied in a single-turnover regime. In order to calibrate the number of charges that move across the vesicle membrane in the different reaction steps, the reverse electron transfer from heme a(3) to heme a and further to the cytochrome c/Cu(A) has been resolved upon photodissociation of CO from the mixed valence enzyme in the absence of oxygen. The reverse electron transfer from heme a(3) to heme a and further to the cytochrome c/Cu(A) pair is resolved as a single transition with tau~40µs. In the reaction of the fully reduced cytochrome caa(3) with oxygen, the first electrogenic phase (tau~30µs) is linked to OO bond cleavage and generation of the P(R) state. The next electrogenic component (tau~50µs) is associated with the P(R)->F transition and together with the previous reaction step it is coupled to translocation of about two charges across the membrane. The three subsequent electrogenic phases, with time constants of ~0.25ms, ~1.4ms and ~4ms, are linked to the conversion of the binuclear center through the F->O(H)->E(H) transitions, and result in additional transfer of four charges through the membrane dielectric. This indicates that the delivery of the fifth electron from heme c to the binuclear center is coupled to pumping of an additional proton across the membrane.

PMID: 23025918