Cyanide binding to different redox states of the cytochrome caa(3) complex from Bacillus subtilis; A member of the cytochrome oxidase super-family of enzymes

Cytochrome caa(3) from Bacillus subtilis is a member of the cytochrome oxidase family. The enzyme has a cytochrome c domain fused as a c-terminal extension into the structure of the oxidase subunit II. Analysis of the visible spectrum of cytochrome caa(3) by pyridine hemochromogen assay reveals an unusually intense alpha-band extinction for the cytochrome c domain relative to mitochondrial cytochrome c. When cyanide is added to oxidized cytochrome caa(3) it binds to cytochromes a(3) and c, and induces autoreduction of cytochrome a. Binding of cyanide to cytochrome c and autoreduction of cytochrome a are prevented if the reaction is done at pH 6.4 in the presence of ferricyanide. The reaction is then composed only of cyanide binding to cytochrome a(3), but is still kinetically complex with three cyanide concentration dependent rates. The resting bacterial oxidase cannot be converted fully into 'fast' or 'pulsed' forms as can the mitochondrial enzyme. It is concluded that the cyanide bound states of the caa(3) oxidase are similar to the mammalian oxidase. However, the kinetics of their formation, particularly with reference to the oxidized, resting caa(3), is distinct from the mitochondrial oxidase. This difference is ascribed to structural differences in the region of the binuclear centre of caa(3).