Bickar D, Turrens J F, Lehninger A L
J Biol Chem. 1986 Nov 5;261(31):14461-6.
When cytochrome c oxidase is isolated from mitochondria, the purified enzyme requires both cytochrome c and O2 to achieve its maximum rate of internal electron transfer from cytochrome a to cytochrome a3. When reductants other than cytochrome c are used, the rate of internal electron transfer is very slow. In this paper we offer an explanation for the slow reduction of cytochrome a3 when reductants other than cytochrome c are used and for the apparent allosteric effects of cytochrome c and O2. Our model is based on the conventional understanding of cytochrome oxidase mechanism (i.e. electron transfer from cytochrome a/CuA to cytochrome a3/CuB), but assumes a relatively rapid two-electron transfer between cytochrome a/CuA and cytochrome a3/CuB and a thermodynamic equilibrium in the "resting" enzyme (the enzyme as isolated) which favors reduced cytochrome a and oxidized cytochrome a3. Using the kinetic constants that are known for this reaction, we find that the activating effects of O2 and cytochrome c on the rate of electron transfer from cytochrome a to cytochrome a3 conform to the predictions of the model and so provide no evidence of any allosteric effects or control of cytochrome c oxidase by O2 or cytochrome c.
当从线粒体中分离出细胞色素c氧化酶时，纯化后的酶需要细胞色素c和氧气才能实现从细胞色素a到细胞色素a3的最大内部电子传递速率。当使用细胞色素c以外的还原剂时，内部电子传递速率非常缓慢。在本文中，我们对使用细胞色素c以外的还原剂时细胞色素a3还原缓慢以及细胞色素c和氧气的明显变构效应给出了解释。我们的模型基于对细胞色素氧化酶机制的传统理解（即从细胞色素a/CuA到细胞色素a3/CuB的电子传递），但假设细胞色素a/CuA和细胞色素a3/CuB之间存在相对快速的双电子传递，并且在“静止”酶（分离得到的酶）中存在热力学平衡，该平衡有利于还原态的细胞色素a和氧化态的细胞色素a3。利用该反应已知的动力学常数，我们发现氧气和细胞色素c对从细胞色素a到细胞色素a3的电子传递速率的激活作用符合模型的预测，因此没有提供任何氧气或细胞色素c对细胞色素c氧化酶变构效应或调控的证据。
