Modified Ligands to FA and FB in Photosystem I II. CHARACTERIZATION OF A MIXED LIGAND [4Fe-4S] CLUSTER IN THE C51D MUTANT OF PsaC UPON REBINDING TO P700-FX CORES

Abstract A Photosystem I (PS I) complex reconstituted with PsaC-C51D (aspartate in lieu of cysteine in position 51) shows light-induced EPR signals with g values, line widths, and photoreduction behavior characteristic of FB. Contrary to an earlier report, a [3Fe-4S] cluster was not located in the reconstituted PS I complex. Instead, a second set of resonances with g values of 2.044, 1.942, and 1.853 becomes EPR-visible when the C51D-PS I complex is measured at 4.2 K. This fast relaxing center, termed FA′ is likely to represent a [4Fe-4S] cluster in the mixed ligand (3Cys•1Asp) site. Redox studies show that the Em of FA′ and FB are −630 mV and −575 mV, respectively. Room temperature optical studies support the presence of two functioning electron acceptors subsequent to FX, and NADP+ photoreduction rates mediated by ferredoxin and flavodoxin are nearly equivalent to the wild type. In addition to [3Fe-4S] clusters and S = ground state [4Fe-4S] clusters, the free PsaC-C51D protein shows resonances near g = 5.5, which may represent a population of high spin (S =) [4Fe-4S] clusters in the mixed ligand FA′ site. Similar to the C14D-PS I mutant complex, it is proposed that the P700-FX core selectively rebinds those free PsaC-C51D proteins that contain two [4Fe-4S] clusters. These studies show that primary photochemistry and electron transfer rates in PS I are relatively unaffected by the presence of a highly reducing, mixed ligand cluster in the FA′ site.