Cytochrome oxidase: an alternative model.

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Abstract
Oxidative titration of reduced [beef heart mitochondria] cytochrome oxidase (EC 1.9.3.1) in the presence of CO and sulfide, at potentials greater than +500 mV (vs. the neutral H electrode), failed to produce new Cu signals in the EPR spectrum of this enzyme. This observation implies that one of the Cu centers in cytochrome oxidase remains Cu(I) under strongly oxidizing conditions. The rationalization of this fact, and the possible explanation of a great accumulation of spectroscopic data, is that cytochrome a3 may be a 2 electron redox center, with stable Fe(IV), Fe(III) and Fe(II) states during its redox cycle. This oxidase model does not require an antiferromagnetic coupling scheme, in contrast to currently prevalent models.

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