Gong Xiao-Min, Hochman Yehoshua, Lev Tal, Bunker Grant, Carmeli Chanoch
Department of Biochemistry, Tel Aviv University, Tel Aviv 69978, Israel.
Biochim Biophys Acta. 2009 Feb;1787(2):97-104. doi: 10.1016/j.bbabio.2008.11.006. Epub 2008 Nov 21.
Photosystem I (PS I) mediates light-induced electron transfer from P700 through a chlorophyll a, a quinone and a [4Fe-4S] iron-sulfur cluster F(X), located on the core subunits PsaA/B to iron-sulfur clusters F(A/B) on subunit PsaC. Structure function relations in the native and in the mutant (psaB-C565S/D566E) of the cysteine ligand of F(X) cluster were studied by X-ray absorption spectroscopy (EXAFS) and transient spectroscopy. The structure of F(X) was determined in PS I lacking clusters F(A/B) by interruption of the psaC2 gene of PS I in the cyanobacterium Synechocystis sp PCC 6803. PsaC-deficient mutant cells assembled the core subunits of PS I which mediated electron transfer mostly to the phylloquinone. EXAFS analysis of the iron resolved a [4Fe-4S] cluster in the native PsaC-deficient PS I. Each iron had 4 sulfur and 3 iron atoms in the first and second shells with average Fe-S and Fe-Fe distances of 2.27 A and 2.69 A, respectively. In the C565S/D566E serine mutant, one of the irons of the cluster was ligated to three oxygen atoms with Fe-O distance of 1.81 A. The possibility that the structural changes induced an increase in the reorganization energy that consequently decreased the rate of electron transfer from the phylloquinone to F(X) is discussed.
光系统I（PS I）介导光诱导的电子从P700通过叶绿素a、醌和位于核心亚基PsaA/B上的[4Fe-4S]铁硫簇F(X)转移至亚基PsaC上的铁硫簇F(A/B)。通过X射线吸收光谱（EXAFS）和瞬态光谱研究了F(X)簇半胱氨酸配体在天然及突变体（psaB-C565S/D566E）中的结构-功能关系。通过中断集胞藻PCC 6803中PS I的psaC2基因，在缺乏簇F(A/B)的PS I中确定了F(X)的结构。缺乏PsaC的突变细胞组装了PS I的核心亚基，其介导的电子转移主要至叶醌。对铁的EXAFS分析在天然缺乏PsaC的PS I中解析出一个[4Fe-4S]簇。每个铁在第一和第二壳层中分别有4个硫原子和3个铁原子，平均Fe-S和Fe-Fe距离分别为2.27 Å和2.69 Å。在C565S/D566E丝氨酸突变体中，簇中的一个铁与3个氧原子配位，Fe-O距离为1.81 Å。讨论了结构变化导致重组能增加从而降低电子从叶醌转移至F(X)速率的可能性。
