Structure and reactivity of multiple forms of cytochrome oxidase as evaluated by x-ray absorption spectroscopy and kinetics of cyanide binding

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Abstract
The extended X-ray absorption fine structure (EXAFS) data show differences between the active site structures of different [bovine heart] cytochrome oxidase preparations. In the resting (as isolated) state of the Yonetani preparation, the bridging atom between .**GRAPHIC**. and .**GRAPHIC**. is present, whereas in another preparation (e.g., Hartzell-Beinert) this atom seems to be bound only to .**GRAPHIC**. in a significant fraction of the molecules. Both preparations bind cyanide in a multiphasic fashion, suggesting that the resting cytochrome oxidase is not homogeneous but rather is a mixture of several forms. The proportion of these forms as detected by cyanide binding kinetics differs from different preparations. However, upon reduction and reoxidation (conversion to the oxygenated form) the cyanide binding kinetics become monophasic and all preparations of the oxygenated form bind cyanide at the same rate. Thus, a combination of structural and kinetic approaches seems necessary for evaluation of the nature of the active site of cytochrome oxidase in its various forms.

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