The partially reduced species present in purified cytochrome oxidase from baker's yeast is cytochrome a.

Cytochrome oxidase purified from baker's yeast submitochondrial particles is found to exist in a partially reduced state in the resting enzyme. Studies utilizing optical and EPR spectroscopy indicate that the "inactive" fraction contains a reduced low-spin heme, cytochrome a possibly indicating a block of electron transfer from cytochrome a to cytochrome a 3. There is no apparent reduction of either the EPR-detectable copper or the species associated with the 830 nm band. Oxidative titrations of the resting-state yeast cytochrome oxidase indicate that the reduction potential of the species titrating is higher than that of ferricyanide. This "inactive" cytochrome oxidase is not the result of the isolation procedure, but seems to represent a species which is present in the intact yeast.