Electron transfer from cytochrome c to O2.

Transient-state kinetic results on the reaction between oxygen and cytochrome oxidase alone and in its electrostatic complex with cytochrome c are reported. Time courses at 830 nm for the enzyme alone reacting with O2 reveal complex kinetics for the oxidation of CuA. The time course begins with a lag phase, which becomes progressively shorter as the O2 concentration is increased. When cytochrome c is included the lag phase is extended. Cytochrome c oxidation proceeds to a level of 80% in 5 milleseconds, and it is possible to resolve two rates over this time range. The dependence of these rates upon O2 concentration is reported here. The second, slower phase is rate-limited at a first-order value of 500 sec-1 at or above 200 microM O2. In contrast, the initial phase is proportional to O2 up to the highest O2 concentration used here (i.e. 340 microM) and reaches a rate of 6500 sec-1. In addition the time course of cytochrome c oxidation begins immediately (i.e., without a lag). It is proposed that the fast phase of cytochrome c oxidation is the result of electron transfer to O2, either via CuA or direct to the oxygen binding site. These pathways for electron transfer are not observed in the reductive half-reaction and may be the result of a conformational change in the oxidase that lowers a kinetic barrier to electron transfer present in the oxidized enzyme.