title: 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 = 1/2 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 = 3/2) [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.
mesh_terms: D001224:Aspartic Acid; D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism*; D003001:Cloning, Molecular; D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism; D003545:Cysteine; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D004926:Escherichia coli; D005615:Freezing; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D046911:Macromolecular Substances; D008565:Membrane Proteins; D016297:Mutagenesis, Site-Directed; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D010777:Photochemistry; D045331:Photosystem I Protein Complex; D017354:Point Mutation; D011506:Proteins / Q000737:chemistry* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013053:Spectrophotometry; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D007506:Iron-Sulfur Proteins; D008024:Ligands; D046911:Macromolecular Substances; D008565:Membrane Proteins; D045331:Photosystem I Protein Complex; D011506:Proteins; D011994:Recombinant Proteins; C089909:photosystem I, psaB subunit; D002734:Chlorophyll; D001224:Aspartic Acid; D009249:NADP; C001785:chlorophyll P 700; D003545:Cysteine