X-ray absorption spectroscopic investigations of cytochrome c oxidase structure and function.

Although the low sensitivity of the XAS technique imposes difficulties upon the study of an enzyme that is often heterogeneous, significant progress has been made in elucidation of the structures of the functional metal sites of cytochrome c oxidase. Figure 8 summarizes interpretations of the resting-state enzyme based on the XAS results obtained over the past decade by the two main groups involved. Aside from several persisting minor differences in distances and precise ligand compositions of the metal sites, the biggest difference between the two interpretations involves the binuclear O2 interaction site, especially the nature of the bridging ligand and its precise attachments to Fea3 and CuB. However, the structural models presented by the two groups have been converging recently, and there is hope that the next few years will see a resolution of the remaining differences. Other XAS approaches (e.g. studies on oriented multilayers) and other techniques will doubtless contribute to this resolution.