Oxygen binding and activation: early steps in the reaction of oxygen with cytochrome c oxidase.

We have studied the flow-flash reaction of fully reduced cytochrome c oxidase with a high concentration of oxygen (1 mM), recording the first 200 microseconds of the reaction at a number of wavelengths between 400 and 455 nm. This approach has allowed us to observe kinetic phases with time constants of 8 and 32 microseconds and to separate their spectra. The spectrum of the first phase is comparable to that of oxygen binding to myoglobin, while the spectrum of the second phase appears to contain a contribution from a peroxy intermediate. The results are discussed in the context of a model in which the 8-microseconds phase reflects the establishment of an equilibrium of oxygen-bound states, while the 32-microseconds phase arises when this system is trapped as a peroxy intermediate, by inter-heme electron transfer.