Abstract
Ca2+ ions shift the absorption spectrum of reduced cytochromea 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 cytochromea 3. Studies of the Soret band show that Ca2+ also shifts the spectrum of ferrocytochromea 3 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 cytochromea are accessible to Ca2+ and H+ from the outside of the mitochondrial membrane. In the soluble enzyme both hemesa anda 3 are accessible. Thus hemea may be located near the outside of the inner membrane whereas hemea 3 experiences a different environment in which no Ca2+ shift occurs.
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Saari, H., Penttilä, T. & Wikström, M. Interaction of Ca2+ and H+ with heme A in cytochrome oxidase. J Bioenerg Biomembr 12, 325–338 (1980). https://doi.org/10.1007/BF00744692
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DOI: https://doi.org/10.1007/BF00744692