对氢键网络的干扰降低细胞色素b_6f复合体中Chla的光稳定性

细胞色素b6f蛋白复合体(Cyt b6f)参与光合膜上电子传递和跨膜质子转移,在体内以二体形式存在,每个单体只结合1分子叶绿素a(Chla).该Chla性质独特,光照条件下十分稳定,是甲醇中游离Chla的120~130倍,然而其光稳定性的机制仍未彻底阐明.Cyt b6f 2.7的晶体结构显示,Chla中心的Mg离子和H2O分子配位,并且该H2O分子通过氢键与复合体亚基Ⅳ的氨基酸G136和T137相互作用.本研究基于这一结构特点,对上述2个氨基酸进行了定点突变,以干扰、破坏氢键网络.结果发现,突变不仅导致蛋白与Chla结合能力下降,而且显著降低了Chla的光稳定性,这一结果表明,Cyt b6f中Chla相关的氢键网络对其稳定性具有重要的作用.

Disturbing the Hydrogen Bond Network Decreases the Photostability of the Chlorophyll a in Cytochrome b6f from Synechocystis sp. PCC 6803

Cytochrome b₆f complex (Cyt b₆f) is one of the three thylaknoid membrane complexes which compose the photosynthetic electron transport chain and it contains only one chlorophyll a (Chla) per monomer. The Chla in the complex is much stable (about 120~130 times) than the free Chla in organic solvent under light. However, the mechanism of the high stability remains unclear. The crystal structure of Cyt b₆f at a 2.7 Å resolution shows that the magnesium ion of the Chla is coordinated by a water molecular which forms hydrogen bonds with residues G136 and T137 of subunit IV. In the present study, the stability mechanism was studied through disturbing the hydrogen bond network via site-directed mutagenesis of G136 and Thr137 of subunit IV. The results showed that disturbing the hydrogen bond network decreased not only the affinity between the protein and Chla, but also the stability of Chla against photodamage, which strongly suggested the hydrogen bond network involved in the Chla was important to maintain the high photostability of the Chla in the complex.