Amplitude analysis of single-wavelength time-dependent absorption data does not support the conventional sequential mechanism for the reduction of dioxygen to water catalyzed by bovine heart cytochrome c oxidase.

The reactions of fully reduced and mixed-valence bovine heart cytochrome c oxidase with dioxygen have been reinvestigated in the absence and presence of metal ions (Zn(2+), Ni(2+), and Cd(2+)) by time-resolved optical absorption spectroscopy using the CO flow-flash technique. The time-resolved data were recorded on a microsecond to millisecond time scale at 442, 610, and 820 nm and subjected to quantitative amplitude analysis based on a conventional unidirectional sequential mechanism. The amplitudes of the sequential intermediates are derived from the absorbance changes associated with the different exponentials and from the kinetic equations of the sequential scheme. The general relationship between the pre-exponential factors and the absorbance of the successive intermediates in the sequential scheme is presented. A comparison of the experimental amplitudes of the individual intermediates with the model amplitudes at the three wavelengths indicates that the low spin heme a is incompletely oxidized during the formation of the sequential P(R) intermediate (P(R,s)). The conversion of the sequential F intermediate to the oxidized enzyme occurs on two millisecond time scales. The amplitude analysis of the single-wavelength data does not support the conventional sequential mechanism for the reduction of dioxygen to water catalyzed by cytochrome c oxidase.