A re-examination of the reactions of cyanide with cytochrome c oxidase

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Abstract
Experiments were performed to examine the cyanide-binding properties of resting and pulsed cytochrome c oxidase in both their stable and transient turnover states. Inhibition of the oxidation of ferrocytochrome c was monitored as a function of cyanide concentration. Cyanide binding to partially reduced forms produced by mixing cytochrome c oxidase with sodium dithionite was also examined. A model is presented that accounts fully for cyanide inhibition of the enzyme, the essential feature of which is the rapid, tight, binding of cyanide to transient, partially reduced, forms of the enzyme populated during turnover. Computer fitting of the experimentally obtained data to the kinetic predictions given by this model indicate that the cyanide-sensitive form of the enzyme binds the ligand with combination constants in excess of 10(6) M-1 X s-1 and with KD values of 50 nM or less. Kinetic difference spectra indicate that cyanide binds to oxidized cytochrome a33+ and that this occurs rapidly only when cytochrome a and CuA are reduced.