Boussac A, Rutherford A W
Departement de Biologie Cellulaire et Moleculaire, Centre d'Etudes Saclay, Gif sur Yvette, France.
J Biol Chem. 1994 Apr 29;269(17):12462-7.
Cl- and Ca2+ are obligatory cofactors in photosystem II (PS-II), the oxygen-evolving enzyme of plants. The electron transfer events in Cl(-)-depleted PS-II were investigated by using continuous wave (cw) and pulsed EPR, and the results were compared with those obtained in untreated and Ca(2+)-depleted PS-II. In Cl(-)-depleted PS-II, the S1 to S2 transition is not accompanied by the appearance of the S2 manganese signal in both cw and field-swept echo experiments. In the S3 state, the cw experiment reveals the presence of a radical signal, and the field-swept echo experiment reveals the presence of a manganese signal similar to that observed in the S2 state in Cl(-)-reconstituted PS-II. A relaxation enhancement study of the oxidized tyrosine D indicates that a relaxation enhancement occurs only in the S2 to S3 transition and not in the S1 to S2 transition. The results are interpreted by the following preferred model. In Cl(-)-depleted PS-II, the S1 to S2 transition corresponds to the oxidation of a component other than the manganese cluster, possibly an amino acid, and the S2 to S3 transition corresponds to the oxidation of the manganese complex. This oxidation sequence is the opposite of that observed in Ca(2+)-depleted PS-II and may be due to a modulation by chloride of the relative redox potentials of the manganese cluster and a nearby oxidizable amino acid. An alternative model involving manganese oxidation that is invisible in the S1 to S2 transition but that becomes visible on S3 formation cannot be ruled out, although it is considered less likely.
氯离子（Cl⁻）和钙离子（Ca²⁺）是植物光系统II（PS-II）中不可或缺的辅助因子，PS-II是植物的放氧酶。通过连续波（cw）和脉冲电子顺磁共振（EPR）研究了Cl⁻缺失的PS-II中的电子转移事件，并将结果与未处理的和Ca²⁺缺失的PS-II中获得的结果进行了比较。在Cl⁻缺失的PS-II中，在连续波和场扫回波实验中，从S1到S2的转变都不会伴随着S2锰信号的出现。在S3状态下，连续波实验揭示了自由基信号的存在，场扫回波实验揭示了存在一个类似于在Cl⁻重构的PS-II的S2状态中观察到的锰信号。对氧化态酪氨酸D的弛豫增强研究表明，弛豫增强仅发生在从S2到S3的转变中，而不是在从S1到S2的转变中。结果由以下首选模型解释。在Cl⁻缺失的PS-II中，从S1到S2的转变对应于锰簇以外的成分（可能是一种氨基酸）的氧化，而从S2到S3的转变对应于锰配合物的氧化。这种氧化顺序与在Ca²⁺缺失的PS-II中观察到的相反，可能是由于氯离子对锰簇和附近可氧化氨基酸的相对氧化还原电位的调节。尽管可能性较小，但不能排除另一种模型，即涉及在S1到S2转变中不可见但在S3形成时变得可见的锰氧化。
