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 a(3) in compound F, was investigated using a binuclear po lypyridine 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 a gent. In the untreated dimeric enzyme, the rate constant for reduction of c ompound F decreased from 700 s(-1) to 200 s(-1) as the pH was increased fro m 7.5 to 9.5. Incubation of dimeric enzyme at pH 10 led to an increase in t he 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 coeffici ent consistent with the irreversible conversion of the enzyme to a monomeri c form. Similar results were obtained when the enzyme was incubated with Tr iton 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 s tate activity. The data indicate that either the conversion of the bovine o xidase to a monomeric form or some structural change coincident with this c onversion strongly influences the rate constant of this step in the catalyt ic cycle, perhaps by influencing the proton access to the heme-copper binuc lear center.