Copper in cytochrome c oxidase

Since only the cupric form is paramagnetic, ESR is an ideal method, determining and quantitating the valence state of copper in these proteins. Many are found to be a mixture of Cu and Cu, and only a few such as plastocyanin contain exclusively Cu. Studies of valence state changes of the copper in cytochrome c oxidase by ESR have demonstrated convincingly for the first time that copper plays a functional role. 

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