MODIFIED LIGANDS TO F-A AND F-B IN PHOTOSYSTEM-I .2. CHARACTERIZATIONOF A MIXED-LIGAND [4FE-4S] CLUSTER IN THE C51D MUTANT OF PSAC UPON REBINDING TO P700-F-X CORES

A Photosystem I (PS I) complex reconstituted with PsaC-C51D (aspartate in lieu of cysteine in position 51) shows light-induced EPR signals w ith g values, line widths, and photoreduction behavior characteristic of F-B. Contrary to an earlier report, a [3Fe-4S] cluster was not loca ted in the reconstituted PS I complex, Instead, a second set of resona nces 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 F-A' is likely to represent a [4Fe-4S] cluster in the mixed l igand (3Cys . 1Asp) site, Redox studies show that the E(m) of F-A' and F-B are -630 mV and -575 mV, respectively. Room temperature optical s tudies support the presence of two functioning electron accepters subs equent to F-X, and NADP(+) photoreduction rates mediated by ferredoxin and flavodoxin are nearly equivalent to the wild type, In addition to [3Fe-4S] clusters and S = 1/2 ground state [4Fe-4S] clusters, the fre e PsaC-C51D protein shows resonances near g = 5.5, which may represent a population of high spin (S = 3/2) [4Fe-4S] clusters in the mixed li gand F-A' site, Similar to the C14D-PS I mutant complex, it is propose d that the P700-F-X core selectively rebinds those free PsaC-C51D prot eins that contain two [4Fe-4S] clusters. These studies show that prima ry photochemistry and electron transfer rates in PS I are relatively u naffected by the presence of a highly reducing, mixed ligand cluster i n the F-A' site.