Conformational change of cytochrome a3induced 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 cgreatly 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+Bto 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+Bfor HCN. This could occur by virtue of loosening a bond between Cu2+Band one of its endogenous ligands facilitating displacement of the latter by HCN.