Rates of cyanide binding to the catalytic intermediates of mammalian cytochrome c oxidase, and the effects of cytochrome c and poly(L-lysine).
Rate constants of cyanide binding to 'fast' oxidase have been measured in the fully-oxidised (O), peroxy (P) and ferryl (F) states at pH 8.0. Values of 2.2, 8 and 10 M-1 s-1, respectively, were obtained. Thus, none of these states appears to exhibit a rate that would identify it as the species responsible for the extremely rapid cyanide binding observed during turnover. On the other hand, with 'oxidised' enzyme as prepared, containing a very small fraction of one-electron-reduced (E state) oxidase, a corresponding fraction of enzyme exhibited spectral changes consistent with cyanide binding with a rate constant in excess of 10(4) M-1 s-1. Evidence is presented suggesting that mediation of electron transfer from one-electron-reduced, cyanide-liganded enzyme to free, ferric oxidase, rather than a global protein conformational change of the enzyme, is responsible for the greatly enhanced cyanide binding rates seen in the presence of cytochrome c or poly(L-lysine). Inter-oxidase electron exchange in 'oxidised' enzyme can result in a complicated dependence of the binding rate on cyanide concentration. We have demonstrated that this may give rise to a saturation of the rate of cyanide binding.
Mitchell
R
R
Glynn Research Institute, Bodmin, UK.
Brown
S
S
Mitchell
P
P
Rich
P R
PR
D000818:Animals; D002384:Catalysis; D002417:Cattle; D003486:Cyanides / Q000378:metabolism*; D003574:Cytochrome c Group / Q000378:metabolism*; D003576:Electron Transport Complex IV / Q000378:metabolism*; D005292:Ferricyanides / Q000494:pharmacology; D005295:Ferrocyanides / Q000494:pharmacology; D007700:Kinetics; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D011107:Polylysine / Q000378:metabolism*; D013057:Spectrum Analysis
D003486:Cyanides; D003574:Cytochrome c Group; D005292:Ferricyanides; D005295:Ferrocyanides; C007931:hexacyanoferrate III; D011107:Polylysine; D003576:Electron Transport Complex IV; C020354:hexacyanoferrate II