EPR evidence of cyanide binding to the Mn(Mg) center of cytochrome c oxidase: support for [Cu.sub.A]-Mg involvement in proton pumping

The anion binding behavior of the Mg(Mn) site in cytochrome c oxidase from Rhodobacter sphaeroide is examined to understand its role in proton pumping at Mg(Mn) center when [Cu.sub.A] is oxidized. The results for the aqueous ligands of Mn(II) probed with deuterium, cyanide, and azide under reduced and oxidized conditions showed that cyanide binds to the Mn(II) ion of the [Cu.sub.A]-Mg(Mn) site when [Cu.sub.A] is reduced but not when [Cu.sub.A] is oxidized.