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

Calcium-bound structure of bovine cytochrome c oxidase

The crystal structure of bovine cytochrome c oxidase (CcO) shows a sodium ion (Na⁺) bound to the surface of subunit I. Changes in the absorption spectrum of heme a caused by calcium ions (Ca²⁺) are detected as small red shifts, and inhibition of enzymatic activity under low turnover conditions is observed by addition of Ca²⁺ in a competitive manner with Na⁺. In this study, we determined the crystal structure of Ca²⁺-bound bovine CcO in the oxidized and reduced states at 1.7 Å resolution. Although Ca²⁺ and Na⁺ bound to the same site of oxidized and reduced CcO, they led to different coordination geometries. Replacement of Na⁺ with Ca²⁺ caused a small structural change in the loop segments near the heme a propionate and formyl groups, resulting in spectral changes in heme a. Redox-coupled structural changes observed in the Ca²⁺-bound form were the same as those previously observed in the Na⁺-bound form, suggesting that binding of Ca²⁺ does not severely affect enzymatic function, which depends on these structural changes. The relation between the Ca²⁺ binding and the inhibitory effect during slow turnover, as well as the possible role of bound Ca²⁺ are discussed.

Individual heme a and heme a3 contributions to the Soret absorption spectrum of the reduced bovine cytochrome c oxidase

Bovine cytochrome c oxidase (CcO) contains two hemes, a and a₃, chemically identical but differing in coordination and spin state. The Soret absorption band of reduced aa₃-type cytochrome c oxidase consists of overlapping bands of the hemes a²⁺ and a₃²⁺. It shows a peak at ∼444 nm and a distinct shoulder at ∼425 nm. However, attribution of individual spectral lineshapes to hemes a²⁺ and a₃²⁺ in the Soret is controversial. In the present work, we characterized spectral contributions of hemes a²⁺ and a₃²⁺ using two approaches. First, we reconstructed bovine CcO heme a²⁺ spectrum using a selective Ca²⁺-induced spectral shift of the heme a²⁺. Second, we investigated photobleaching of the reduced Thermus thermophilus ba₃- and bovine aa₃-oxidases in the Soret induced by femtosecond laser pulses in the Q-band. The resolved spectra show splitting of the electronic B_0x-, B_0y-transitions of both reduced hemes. The heme a²⁺ spectrum is shifted to the red relative to heme a₃²⁺ spectrum. The ∼425 nm shoulder is mostly attributed to heme a₃²⁺.

Interaction of Cytochrome C Oxidase with Steroid Hormones

Estradiol, testosterone and other steroid hormones inhibit cytochrome c oxidase (CcO) purified from bovine heart. The inhibition is strongly dependent on concentration of dodecyl-maltoside (DM) in the assay. The plots of K_i vs [DM] are linear for both estradiol and testosterone which may indicate an 1:1 stoichiometry competition between the hormones and the detergent. Binding of estradiol, but not of testosterone, brings about spectral shift of the oxidized CcO consistent with an effect on heme a₃³⁺. We presume that the hormones bind to CcO at the bile acid binding site described by Ferguson-Miller and collaborators. Estradiol is shown to inhibit intraprotein electron transfer between hemes a and a₃. Notably, neither estradiol nor testosterone suppresses the peroxidase activity of CcO. Such a specific mode of action indicates that inhibition of CcO activity by the hormones is associated with impairing proton transfer via the K-proton channel.

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 E_m 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 E_m of heme a and also prevent and reverse the effect of Ca²⁺. No effect of Ca²⁺ or EGTA is observed on E_m 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 E_m 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.