Chromous ion reduction of mammalian cytochrome oxidase and some of its derivatives

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Abstract
The reduction of cytochrome c oxidase [EC 1.9.3.1] by Cr2+, followed by means of stopped-flow spectrophotometry, exhibits 2 phases: the faster Cr2+-concentration-dependent reaction has an initial rate constant of 1.1 .times. 104 M-1.cntdot.s-1, but reaches a rate limit at high concentration of reductant; the slower phase is concentration-independent with a rate of 0.3s-1. The activation energies of the fast and the slow processes are 35 and 71 kJ/mol, respectively. The reduction kinetics of the mixed-valence CO complex and the cyanide-inhibited enzyme were compared with those of the fully oxidized forms: both the liganded species have a fast phase identical with that found in the oxidized oxidase. A comparison of the kinetic difference spectra obtained for the fast phase of reduction of oxidized oxidase with those obtained on reduction of the liganded species suggests that the rapid phase arises from the reduction of heme a, and the slow phase from the reduction of heme a3.