Proton involvement in the transition from the "peroxy" to the ferryl intermediate of cytochrome c oxidase

In the absence of any external electron donor, the "peroxy" intermediate of cytochrome c oxidase (CcO-607) is converted to the ferryl form (CcO-580) a nd subsequently to oxidized enzyme. The rate of conversion of CcO-607 to th e CcO-580 form is pH dependent between pH 3.0 and pH 7.6. A plot of the log arithm of the rate constant for this conversion is a linear function of pH with a slope of -0.92, implying the involvement of a single proton in the t ransition. Upon rapidly lowering the pH from 8.1 to 5.8, the uptake of one proton was observed by direct pH measurement, and the kinetics of proton up take coincide with the spectral conversion of CcO-607 to CcO-580. We interp ret the slow endogenous decay of CcO-607 to CcO-580 to be the result of pro ton transfer to a deprotonated group generated in the binuclear cavity duri ng CcO-607 formation. This group is not freely accessible to protons from t he medium, and its pK(a) is probably higher than 9.0.