The effect of antibodies to subunit V of cytochrome oxidase on cyanide inhibition of electron transfer.

Binding of antibodies raised against subunit V of mammalian cytochrome oxidase to the intact membranous enzyme is redox-sensitive, suggesting the existence of 'open' and 'closed' protein conformers (Freedman, J.A., Cooper, C.E., Leece, B., Nicholls, P. and Chan, S.H.P. (1988) Biochem. Cell Biol. 66, 1210-1217). Similar open and closed states for the oxygen-reacting site have been proposed to explain cyanide binding kinetics (Jensen, P., Wilson, M.T., Aasa, R. and Malmström, B.G. (1984) Biochem. J. 224, 829-837). We therefore examined cyanide inhibition of oxidase activity polarographically and spectrophotometrically using soluble oxidase preincubated with and without anti-subunit V or non-immune rabbit gamma-globulin. The subunit-specific antibody decreased the cyanide 'on' rate and essentially eliminated the rapid phase of cyanide binding. We conclude that (i), bound antibody blocks HCN binding; (ii), antibody and HCN probably bind to the same conformation of the oxidase and (iii), the 'open'-'closed' conformation change that modulates binding of HCN may be similar to that which modulates antibody binding. The results are consistent with some reciprocating models of electron transfer and energy transduction by the oxidase (cf., Wikström, M.K.F., Krab, K. and Saraste, M. (1981) Cytochrome Oxidase: A Synthesis).