Saari H, Penttilä T, Wikström M
J Bioenerg Biomembr. 1980 Aug;12(3-4):325-38. doi: 10.1007/BF00744692.
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.
钙离子通过在线粒体内膜外侧发挥作用，改变还原型细胞色素a的吸收光谱。在分离的细胞色素氧化酶中，钙离子或氢离子均可诱导光谱位移，表观pK值在6.20至5.75之间变化，具体取决于细胞色素a3的状态。对索雷特带的研究表明，与线粒体或重构到脂质体中的分离氧化酶的情况相反，钙离子也会使分离的氧化酶中的亚铁细胞色素a3的光谱发生位移。对还原型双咪唑血红素A的模型研究揭示了由钙离子诱导的类似光谱位移。血红素A的丙酸羧基酯化消除了光谱位移，表明这是由于钙离子与这些基团的相互作用所致。结合完整线粒体的数据来看，这表明细胞色素a的丙酸侧链可从线粒体外膜外侧被钙离子和氢离子接触到。在可溶性酶中，血红素a和a3均可被接触到。因此，血红素a可能位于内膜外侧附近，而血红素a3所处的环境不同，不会发生钙离子诱导的光谱位移。
