Studies on Cytochrome Oxidase

The oxidation of the half-reduced cyanide complex of cytochrome oxidase (a2+·Cu1+·a33+CN) by ferricyanide showed a one to one correlation between the formation of the 830-nm absorption band and the generation of the EPR Cu(II) signal at g = 2. Both required 1.0 mole of ferricyanide per mole of cytochrome oxidase of 4 oxidation equivalents. On the other hand, the titration measured at 603 nm required 2.4 moles of oxidant; the fractional ratio apparently resulted from absorbance contribution by the Cu(II) chromophore. The titration curves measured at 830 nm and by EPR for the fully reduced cyanide complex (a2+·Cu1+·a32+CN) were sigmoidal. These and other observations suggest that significant heme-copper interaction exists between the EPR-sensitive copper chromophore and cytochrome a on one hand and cytochrome a3 on the other. Structurally and functionally the EPR-sensitive copper which shows the 830-nm absorption band is most likely to be sandwiched between cytochromes a and a3, whereas the EPR-insensitive copper, which does not contribute to the 830-nm band, may serve only as an electron sink. Some of the divergent results reported in the literature may be reconciled on the basis of these observations.