Calcium ions inhibit reduction of heme a in bovine cytochrome c oxidase

The effect of Ca2+ on the rate of heme a reduction by dithionite and hexaammineruthenium (RuAm) was studied in the cyanide-complexed bovine cytochrome oxidase (CcO). The rate of heme a reduction is proportional to RuAm concentration below 300 μM with kv of 0.53 × 106 M−1s−1. Ca2+ inhibits the rate of heme a reduction by dithionite by ∼25%. As the reaction speeds up with increased concentrations of RuAm, the inhibition by Ca2+ disappears. The inhibition of heme a reduction may contribute to recently described partial inhibition of CcO by Ca2+ in the enzymatic assays. The inhibitory effect of Ca2+ on heme a reduction indicates that ET through heme a may be coupled to proton movement in the exit part of the proton channel H.