Oxidized cytochrome c oxidase is shown to exist in three conformations in addition to the transient “g5” conformation previously reported [Shaw, R. W., Hansen, R. E., & Beinert, H. (1978) J. Biol. Chem. 253, 6637-6640]. The “resting” and “gl2” conformations are distinguished by an NO-induced cytochrome cz3 electron paramagnetic resonance (EPR) signal and an EPR signal at g' = 12, respectively. The “oxygenated” conformation exhibits an unusual EPR signal in the presence of fluoride and is identical with the “oxygenated” state first discovered by Okunuki et al. [Okunuki, K., Hagihora, B., Sekuzu, L, & Horio, T. (1958) Proc. Int. Symp. Enzyme Chem., Tokyo, Kyoto, 264]. It is proposed that when the reduced enzyme is reoxidized by dioxygen, the oxidized enzyme first relaxes from the “g5” into the “oxygenated” conformation after which a percentage of the molecules slowly relax into the “g12” conformation. The “resting” conformation is not formed when the enzyme is reoxidized. On the basis of the EPR observations, it is proposed that these various conformations of the oxidized enzyme differ in the structure of the cytochrome a3-Cuaj site. Structures for the cytochrome a3-Cua3 site are proposed for each conformation, and a mechanism by which these conformations undergo interconversion among themselves is described.
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