Nitric oxide is now recognised as a powerful inhibitor of cytochrome c oxidase. It is potent at pathophysiological/physiological concentrations and in the presence of relatively high oxygen concentrations. The exact mechanism of inhibition is not known. Cytochrome c oxidase provides a complex ligand binding site comprising two metal centres, a haem iron and a copper atom. In addition, in turnover, this site may contain a variety of reactive oxygen intermediates. The interaction of NO with cytochrome c oxidase in turnover thus provides possibilities for a very rich chemistry. We have investestigated these interactions by optical spectroscopy, involving both static and rapid scan stopped-flow methods. Our results show that NO reacts rapidly with a number of species populated during turnover (eg P. and F.) and allow us to draw conclusions regarding the mode of inhibition.
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