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 close to the cytochrome a3/Cu(B) binuclear centre

We determined the fraction of 'slow' and 'fast' conformations of bovine cytochrome c oxidase, following the kinetics of cyanide binding to the oxidized enzyme. We investigated whether treatment of heart mitochondrial particles with different commercially available types of cholate (standard and ultrapure) can affect the fraction of cytochrome c oxidase in the two states. Compared to standard cholate, the use of ultra-pure cholate for solubilization of heart mitochondrial particles significantly increased the fraction of the fast enzyme. Complete homogeneity (~ 100% fast) was observed when cytochrome c oxidase was solubilized with ultra-pure cholate from heart mitochondrial particles pre-equilibrated with AMP; equilibration with ADP yielded a much smaller fraction of fast enzyme (~ 35%). These observations are discussed on the basis of the structural relationships between the known cholate-binding site and the binuclear cytochrome a3-Cu(B) site: variation in the occupancy of this binding site with cholate or nucleotides may modify reactivity of the oxidized binuclear centre towards cyanide.