Conformational change of cytochrome a₃ induced by oxidized cytochrome c

Cyanide binding with the oxidized resting Yonetani-type cytochrome c-oxidase followed spectrophotometrically reveals a relatively rapid initial phase the rate of which shows saturation behaviour with respect to [HCN] and secondary slower absorption changes to a first approximation independent of the ligand concentration. Oxidized cytochrome c greatly accelerates the initial phase of cyanide binding but does not affect significantly contribution or rate constant of the slow phase. The same effect is exerted by poly-L-lysine. Within a framework of a reaction mechanism assuming Cu²⁺_B to be the initial HCN-binding site, cytochrome c³⁺ and other polycations are likely to bring about a conformational change of cytochrome oxidase resulting in an increased affinity of Cu²⁺_B for HCN. This could occur by virtue of loosening a bond between Cu²⁺_B and one of its endogenous ligands facilitating displacement of the latter by HCN.