Hill B C
Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada.
Ann N Y Acad Sci. 1988;550:98-104. doi: 10.1111/j.1749-6632.1988.tb35326.x.
Transient-state kinetic results on the reaction between oxygen and cytochrome oxidase alone and in its electrostatic complex with cytochrome c are reported. Time courses at 830 nm for the enzyme alone reacting with O2 reveal complex kinetics for the oxidation of CuA. The time course begins with a lag phase, which becomes progressively shorter as the O2 concentration is increased. When cytochrome c is included the lag phase is extended. Cytochrome c oxidation proceeds to a level of 80% in 5 milleseconds, and it is possible to resolve two rates over this time range. The dependence of these rates upon O2 concentration is reported here. The second, slower phase is rate-limited at a first-order value of 500 sec-1 at or above 200 microM O2. In contrast, the initial phase is proportional to O2 up to the highest O2 concentration used here (i.e. 340 microM) and reaches a rate of 6500 sec-1. In addition the time course of cytochrome c oxidation begins immediately (i.e., without a lag). It is proposed that the fast phase of cytochrome c oxidation is the result of electron transfer to O2, either via CuA or direct to the oxygen binding site. These pathways for electron transfer are not observed in the reductive half-reaction and may be the result of a conformational change in the oxidase that lowers a kinetic barrier to electron transfer present in the oxidized enzyme.
报道了氧气与单独的细胞色素氧化酶及其与细胞色素c的静电复合物之间反应的瞬态动力学结果。单独的酶与O₂在830nm处的时间进程揭示了CuA氧化的复杂动力学。时间进程开始有一个滞后阶段，随着O₂浓度增加，该滞后阶段逐渐变短。当加入细胞色素c时，滞后阶段延长。细胞色素c氧化在5毫秒内进行到80%的水平，并且在这个时间范围内可以分辨出两种速率。这里报道了这些速率对O₂浓度的依赖性。第二个较慢的阶段在200μM O₂及以上时，一级速率限制为500秒⁻¹。相比之下，初始阶段在本文使用的最高O₂浓度（即340μM）之前与O₂成正比，速率达到6500秒⁻¹。此外，细胞色素c氧化的时间进程立即开始（即没有滞后）。有人提出，细胞色素c氧化的快速阶段是电子通过CuA或直接转移到氧结合位点传递给O₂的结果。在还原半反应中未观察到这些电子转移途径，它们可能是氧化酶构象变化的结果，这种变化降低了氧化酶中存在的电子转移动力学障碍。
