Frasch W D, Mei R
Department of Biological Sciences, University of Michigan, Ann Arbor 48109.
Biochemistry. 1987 Nov 17;26(23):7321-5. doi: 10.1021/bi00397a019.
The evolution of O2 from H2O2 catalyzed by the oxygen-evolving complex (OEC) in darkness was examined with photosystem II reaction center complex preparations from spinach. Flash illumination of dark-adapted reaction centers was used to make S0-enriched or S1-enriched complexes. The membranes catalyzed O2 evolution from H2O2 when preset to either the S0 or S1 state. However, only the S0-state reaction was inhibited by carbonyl cyanide m-chlorophenylhydrazone and dependent on chloride. These results indicate that (1) the S0-dependent and S1-dependent catalytic cycles can be separated by flash illumination, (2) the S0-dependent reaction involves the formation of the S2 state, and (3) the S1-dependent reaction does not involve the formation of the S2 or S3 states. A kinetic study of the S1-dependent reaction revealed a rapid equilibrium ordered mechanism in which (1) the binding of Ca(II) must precede the binding of H2O2 to the OEC and (2) the reaction of Ca(II) with the free enzyme is at thermodynamic equilibrium such that Ca(II) does not necessarily dissociate after each catalytic cycle.
利用菠菜的光系统II反应中心复合物制剂，研究了在黑暗中由放氧复合物（OEC）催化的H₂O₂产生O₂的过程。对暗适应的反应中心进行闪光照射，以制备富含S₀或富含S₁的复合物。当预先设定为S₀或S₁状态时，这些膜催化H₂O₂产生O₂。然而，只有S₀状态的反应受到羰基氰化物间氯苯腙的抑制且依赖于氯离子。这些结果表明：（1）通过闪光照射可以分离S₀依赖性和S₁依赖性催化循环；（2）S₀依赖性反应涉及S₂状态的形成；（3）S₁依赖性反应不涉及S₂或S₃状态的形成。对S₁依赖性反应的动力学研究揭示了一种快速平衡有序机制，其中：（1）Ca(II)的结合必须先于H₂O₂与OEC的结合；（2）Ca(II)与游离酶的反应处于热力学平衡，使得Ca(II)不一定在每个催化循环后解离。
