title: Characterization of ferredoxin:thioredoxin reductase modified by site-directed mutagenesis.
abstract: Ferredoxin:thioredoxin reductase (FTR) is a key regulatory enzyme of oxygenic photosynthetic cells involved in the reductive regulation of important target enzymes. It catalyzes the two-electron reduction of the disulfide of thioredoxins with electrons from ferredoxin involving a 4Fe-4S cluster and an adjacent active-site disulfide. We replaced Cys-57, Cys-87, and His-86 in the active site of Synechocystis FTR by site-directed mutagenesis and studied the properties of the mutated proteins. Mutation of either of the active-site cysteines yields inactive enzymes, which have different spectral properties, indicating a reduced Fe-S cluster when the inaccessible Cys-87 is replaced and an oxidized cluster when the accessible Cys-57 is replaced. The oxidized cluster in the latter mutant can be reversibly reduced with dithionite showing that it is functional. The C57S mutant is a very stable protein, whereas the C87A mutant is more labile because of the missing interaction with the cluster. The replacement of His-86 greatly reduces its catalytic activity supporting the proposal that His-86 increases the nucleophilicity of the neighboring cysteine. Ferredoxin forms non-covalent complexes with wild type (WT) and mutant FTRs, which are stable except with the C87A mutant. WT and mutant FTRs form stable covalent heteroduplexes with active-site modified thioredoxins. In particular, heteroduplexes formed with WT FTR represent interesting one-electron-reduced reaction intermediates, which can be split by reduction of the Fe-S cluster. Heteroduplexes form non-covalent complexes with ferredoxin demonstrating the ability of FTR to simultaneously dock thioredoxin and ferredoxin, which is in accord with the proposed reaction mechanism and the structural analyses.
authors.affiliation: Laboratoire de Biochimie végétale, Université de Neuchâtel, CH-2007 Neuchâtel, Switzerland.
authors.forename: Dominique A
authors.initials: DA
authors.lastname: Glauser
authors.forename: Florence
authors.initials: F
authors.lastname: Bourquin
authors.forename: Wanda
authors.initials: W
authors.lastname: Manieri
authors.forename: Peter
authors.initials: P
authors.lastname: Schürmann
mesh_terms: D001426:Bacterial Proteins / Q000032:analysis* / Q000235:genetics / Q000378:metabolism; D000458:Cyanobacteria / Q000201:enzymology*; D007506:Iron-Sulfur Proteins; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010088:Oxidoreductases / Q000032:analysis* / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000032:analysis / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D007506:Iron-Sulfur Proteins; D011994:Recombinant Proteins; D010088:Oxidoreductases; C021588:ferredoxin-thioredoxin reductase