The effect of sulphide on cytochrome aa3. Isosteric and allosteric shifts of the reduced alpha-peak.

1. Sulphide, like cyanide, is a slow-binding inhibitor of cytochrome aa3 with a high affinity (KD smaller than 0.1 MUM). 2. Unlike cyanide binding, the binding of sulphide is apparently independent of the redox state of components of the oxidase other than cytochrome a3 and shows no anomalous kinetics during complex formation. 3. Sulphide binding to cytochrome a3-3+ is accompanied by a blue-shift in the alpha-peak of the reduced enzyme (a-2+ a3-3+ H2S), similar to but smaller than that induced by azide. 4. The reduced sulphide-inhibited system shows a much higher Soret peak at 445 nm than the corresponding cyanide and azide complexes, suggesting that partial electron transfer from sulphide to haem may occur in the complex. No evidence was obtained for the formation of any sulfhaem derivatives of cytochrome a3. 5. The influence of energization on the spectrum of mitochondrial cytochrome oxidase, and the effects of calcium on the alpha-peak of isolated cytochrome aa3 (Wikström, M.K.F. (1974) Ann. N.Y. Acad. Sci. 227, 146-158) are distinct from the action of the cytochrome a3 ligands. 6. A classification of peak shifts in the alpha-region in terms of isosteric and allosteric ligands is proposed.