Manna P, LoBrutto R, Eijckelhoff C, Dekker J P, Vermaas W
Department of Plant Biology, Arizona State University, Tempe 85287-1601, USA.
Eur J Biochem. 1998 Jan 15;251(1-2):142-54. doi: 10.1046/j.1432-1327.1998.2510142.x.
On the basis of sequence comparison with the M subunit of the reaction center of purple bacteria, no residues in photosystem II can be clearly identified that may be predicted to correspond to the His residue that binds one of the accessory bacteriochlorophylls in the purple bacterial reaction center. However, the Arg180 residue of the D2 protein is close to where this residue is predicted to be and could conceivably serve as a chlorophyll ligand. To analyze the function of Arg180, it was changed to nine different amino acids in the cyanobacterium Synechocystis sp. PCC 6803. Except for the Arg180-->Gln (R180Q) mutant, the resulting strains were no longer photoautotrophic. The properties of photosystem II upon mutation of Arg180 were probed in strains from which photosystem I had been deleted genetically. Mutations at the Arg180 residue affected oxygen evolution capacity and the amount of photosystem II that was present in thylakoids. Surprisingly, in the Arg180 mutants, EPR signals that may originate from the oxidized redoxactive Tyr160 of the D2 protein (Y(D)ox) were small and generally did not resemble the usual signal IIs, signifying an effect of the Arg180 mutations on the environment surrounding Tyr160. In addition, in most mutants, the charge recombination kinetics between the primary electron-accepting quinone in photosystem II (Q(A)-) and oxidized species on the donor side were faster upon introducing mutations at Arg180 suggesting an increased steady-state concentration of P680+ in the mutants. However, Arg180 mutations also affected Q(A)- oxidation by the secondary electron-accepting quinone (Q(B)). HPLC analysis showed that, in the Arg180 mutants that were assayed, the pheophytin/chlorophyll ratio of photosystem II had not changed, indicating that the mutations did not lead to a pheophytinization of one of the chlorophyll molecules. Even though the results presented do not provide positive evidence that Arg180 of the D2 protein corresponds in function to the ligand to the central Mg in an accessory bacteriochlorophyll in reaction centers of purple bacteria, it is clear that changes in Arg180 greatly affect Tyr160 and P680. Various scenarios are discussed that are compatible with the data presented, and include an apparently close interaction between Arg180, His189, and Tyr160, and the possibility of the involvement of multiple chlorophylls to together form P680.
基于与紫色细菌反应中心M亚基的序列比较，在光系统II中无法明确鉴定出可能对应于紫色细菌反应中心中结合一种辅助细菌叶绿素的组氨酸残基的残基。然而，D2蛋白的Arg180残基靠近预计该残基所在的位置，并且可以想象它可作为叶绿素配体。为了分析Arg180的功能，在集胞藻属蓝细菌PCC 6803中将其替换为九种不同的氨基酸。除了Arg180→Gln（R180Q）突变体之外，所得菌株不再是光合自养型。在通过基因手段删除了光系统I的菌株中，研究了Arg180突变时光系统II的特性。Arg180残基处的突变影响了放氧能力以及类囊体中存在的光系统II的量。令人惊讶的是，在Arg180突变体中，可能源自D2蛋白氧化态的氧化还原活性Tyr160（Y（D）ox）的电子顺磁共振（EPR）信号很小，并且通常与常见的信号II不同，这表明Arg180突变对Tyr160周围的环境有影响。此外，在大多数突变体中，光系统II中初级电子受体醌（Q（A）-）与供体侧氧化物种之间的电荷复合动力学在Arg180处引入突变后更快，这表明突变体中P680+的稳态浓度增加。然而，Arg180突变也影响了次级电子受体醌（Q（B））对Q（A）-的氧化。高效液相色谱（HPLC）分析表明，在所检测的Arg180突变体中，光系统II的脱镁叶绿素/叶绿素比率没有变化，这表明这些突变没有导致其中一个叶绿素分子脱镁叶绿素化。尽管所呈现的结果没有提供确凿证据表明D2蛋白的Arg180在功能上对应于紫色细菌反应中心中辅助细菌叶绿素中心镁的配体，但很明显Arg180的变化极大地影响了Tyr160和P680。讨论了与所呈现数据相符的各种情况，包括Arg180、His189和Tyr160之间明显紧密的相互作用，以及多个叶绿素共同形成P680的可能性。
