Exposure of Bovine Cytochrome c Oxidase to High Triton X-100 or to Alkaline Conditions Causes a Dramatic Change in the Rate of Reduction of Compound F

The final step in the catalytic cycle of cytochrome oxidase, the reduction of oxyferryl heme a₃ in compound F, was investigated using a binuclear polypyridine ruthenium complex ([Ru(bipyridine)₂] ₂(1,4-bis[2-(4′-methyl-2, 2′ -bipyrid-4-yl)ethenyl]benzene) (PF₆)₄) as a photoactive reducing agent. In the untreated dimeric enzyme, the rate constant for reduction of compound F decreased from 700 s^-1 to 200 s^-1 as the pH was increased from 7.5 to 9.5. Incubation of dimeric enzyme at pH 10 led to an increase in the rate constant to 1650 s^-1, which was independent of pH between pH 7.4 and 10. This treatment resulted in a decrease in the sedimentation coefficient consistent with the irreversible conversion of the enzyme to a monomeric form. Similar results were obtained when the enzyme was incubated with Triton X-100 at pH 8.0. These treatments, which have traditionally been used to convert dimeric enzyme to monomeric form, have no effect on the steady-state activity. The data indicate that either the conversion of the bovine oxidase to a monomeric form or some structural change coincident with this conversion strongly influences the rate constant of this step in the catalytic cycle, perhaps by influencing the proton access to the heme-copper binuclear center.