Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, Russia.
Biochemistry (Mosc). 2010 Mar;75(3):342-52. doi: 10.1134/s0006297910030119.
Cytochrome c oxidase of the ba(3)-type from Thermus thermophilus does not interact with cyanide in the oxidized state and acquires the ability to bind heme iron ligands only upon reduction. Cyanide complexes of the reduced heme a(3) in cytochrome ba(3) and in mitochondrial aa(3)-type cytochrome oxidase are similar spectroscopically, but the a(3)(2+)-CN complex of cytochrome ba(3) is strikingly tight. Experiments have shown that the K(d) value of the cytochrome ba(3) complex with cyanide in the presence of reductants of the enzyme binuclear center does not exceed 10(-8) M, which is four to five orders of magnitude less than the K(d) of the cyanide complex of the reduced heme a(3) of mitochondrial cytochrome oxidase. The tightness of the cytochrome ba(3) complex with cyanide is mainly associated with an extremely slow rate of the ligand dissociation (k(off) < or = 10(-7) sec(-1)), while the rate of binding (k(on) ~ 10(2) M(-1).sec(-1)) is similar to the rate observed for the mitochondrial cytochrome oxidase. It is proposed that cyanide dissociation from the cytochrome ba(3) binuclear center might be hindered sterically by the presence of the second ligand molecule in the coordination sphere of Cu(B)(2+). The rate of cyanide binding with the reduced heme a(3) does not depend on pH in the neutral area, but it approaches linear dependence on H+ activity in the alkaline region. Cyanide binding appears to be controlled by protonation of an enzyme group with pK(a) = 8.75.
嗜热高温菌的 ba(3)-型细胞色素 c 氧化酶在氧化态下不与氰化物相互作用，只有在还原状态下才获得结合血红素铁配体的能力。还原的血红素 a(3)在细胞色素 ba(3)和线粒体 aa(3)-型细胞色素氧化酶中的氰化物复合物在光谱上相似，但细胞色素 ba(3)的 a(3)(2+)-CN 复合物非常紧密。实验表明，在酶双核中心的还原剂存在下，细胞色素 ba(3)与氰化物的复合物的 K(d)值不超过 10(-8) M，这比线粒体细胞色素氧化酶还原的血红素 a(3)的氰化物复合物的 K(d)值低四到五个数量级。细胞色素 ba(3)与氰化物的复合物的紧密性主要与配体解离的极慢速率（k(off) <或= 10(-7) sec(-1)）有关，而结合速率（k(on) ~ 10(2) M(-1).sec(-1)）与观察到的线粒体细胞色素氧化酶的速率相似。据推测，氰化物从细胞色素 ba(3)双核中心的解离可能受到配位球中第二个配体分子的空间位阻的阻碍。还原的血红素 a(3)与氰化物的结合速率在中性区域内不依赖于 pH，但在碱性区域内，它接近与 H+活性的线性关系。氰化物的结合似乎受到 pK(a) = 8.75 的酶基团的质子化控制。
