The structure of genetically modified iron–sulfur cluster Fx in photosystem I as determined by X-ray absorption spectroscopy

Photosystem I (PS I) mediates light-induced electron transfer from P700 through a chlorophyll a, a quinone and a [4Fe–4S] iron–sulfur cluster FX, located on the core subunits PsaA/B to iron–sulfur clusters FA/B on subunit PsaC. Structure function relations in the native and in the mutant (psaB-C565S/D566E) of the cysteine ligand of FX cluster were studied by X-ray absorption spectroscopy (EXAFS) and transient spectroscopy. The structure of FX was determined in PS I lacking clusters FA/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 Å and 2.69 Å, 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 Å. 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 FX is discussed.