Reactivity of photoreduced cytochrome aa3 complexes with molecular oxygen

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Abstract
Cytochrome c oxidase (ox heart cytochrome aa3) is reduced on illumination in the presence of a photocatalyst system containing deazaflavin and EDTA. The photoreduced enzyme reacts with oxygen at neutral pH to give a form of ferric enzyme, whereas a corresponding sample partially reduced by light in the absence of any photocatalysts reacts with oxygen to give an oxyferri species (oxygenated enzyme). Reduction by the photocatalyst system at an alkaline pH value (9.0) gives rise to fully reduced oxidase (both heme groups ferrous). At these pH values, the immediate product after oxygen addition is a species with a 605-606 nm absorption band, not identical with ferrous cytochrome a, but capable of oxidizing added cytochrome c. This intermediate, which is unstable at neutral pH, may be analogous to the compound B obtained by Chance and co-workers at low temperatures.

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