Interactions of Ca2+ and H+ with heme A in cytochrome oxidase

Ca2+ ions shift the absorption spectrum of reduced cytochrome a in mitochondria by acting from the outside of the membrane. In isolated cytochrome oxidase the shift may be induced by either Ca2+ or H+, the apparent pK varying between 6.20 and 5.75 depending on the state of cytochrome a3. Studies of the Soret band show that Ca2+ also shifts the spectrum of ferrocytochrome a3 in isolated oxidase in contrast to the situation in mitochondria or isolated oxidase reconstituted into liposomes. Model studies with reduced bis-imidazole heme A reveals an analogous spectral shift induced by Ca2+. Esterification of the propionate carboxyls of heme A abolishes the spectral shift, suggesting that it is due to interaction of Ca2+ with these groups. When taken together with the data with intact mitochondria, this suggests that the propionate side chains of cytochrome a are accessible to Ca2+ and H+ from the outside of the mitochondrial membrane. In the soluble enzyme both hemes a and a3 are accessible. Thus heme a may be located near the outside of the inner membrane whereas heme a3 experiences a different environment in which no Ca2+ shift occurs.