Effect of Ca2+ on the redox potential of heme a in cytochrome c oxidase

Subunit I of cytochrome c oxidase (CcO) from mitochondria and many bacteria contains a cation binding site (CBS) located at the outer positively charged aqueous phase not far from heme a. Binding of Ca2+ with the CBS in bovine CcO inhibits activity of the enzyme 2–3 -fold [Vygodina, T., Kirichenko, A. & Konstantinov A.A. (2013) Direct Regulation of Cytochrome c Oxidase by Calcium Ions, PLoS One.8 e74436]. Here we show that binding of Ca2+ at CBS of bovine CcO shifts Em of heme a to the positive by 15–20 mV. Na+ ions that bind to the same site and compete with Ca2+ do not affect Em of heme a and also prevent and reverse the effect of Ca2+. No effect of Ca2+ or EGTA is observed on Em of heme a with the wild type bacterial oxidases from R.sphaeroides or P.denitrificans that contain tightly-bound calcium at the site. In the D477A mutant CcO from P. denitrificans that binds Ca2+ reversibly like the mitochondrial CcO, calcium shifts redox titration curve of heme a to the positive by ∼35–50 mV that is in good agreement with the results of electrostatic calculations; however, as shown earlier, it does not inhibit CcO activity of the mutant enzyme. Therefore the data do not support the proposal that the inhibitory effect of Ca2+ on CcO activity may be explained by the Ca2+-induced shift of Em of heme a. Rather, Ca2+ retards electron transfer by inhibition of charge dislocation in the exit part of the proton channel H in mammalian CcO, that is absent in the bacterial oxidases. [Display omitted] •Ca binds with CBS of bovine CcO raising Em of heme a by 15–17 mV.•Na binding with CBS does not affect redox midpoint potential of heme a.•In D477A mutant CcO from P.denitrificans, Ca raises Em of heme a by 35–50 mV.•The Ca-induced shift in Em of heme a can be explained by electrostatic interactions.•Inhibition of CcO by Ca cannot be explained by shifted Em of heme a.