The ratio between the fast and slow forms of bovine cytochrome c oxidase is changed by cholate or nucleotides bound to the cholate-binding site closeto the cytochrome a(3)/Cu-B binuclear centre

We determined the fraction of 'slow' and 'fast' conformations of bovine cyt ochrome c oxidase, following the kinetics of cyanide binding to the oxidize d enzyme. We investigated whether treatment of heart mitochondrial particle s with different commercially available types of cholate (standard and ultr apure) can affect the fraction of cytochrome c oxidase in the two states. C ompared to standard cholate, the use of ultra-pure cholate for solubilizati on of heart mitochondrial particles significantly increased the Fraction of the fast enzyme. Complete homogeneity (similar to 100% fast) was observed when cytochrome c oxidase was solubilized with ultra-pure cholate from hear t mitochondrial particles pre-equilibrated with AMP; equilibration with ADP yielded a much smaller fraction of fast enzyme (similar to 35%). These obs ervations are discussed on the basis of the structural relationships betwee n the known cholate-binding site and the binuclear cytochrome a(3)-Cu-B sit e: variation in the occupancy of this binding site with cholate or nucleoti des may modify reactivity of the oxidized binuclear centre towards cyanide.