title: Structural snapshots along the reaction pathway of ferredoxin-thioredoxin reductase.
abstract: Oxygen-evolving photosynthetic organisms regulate carbon metabolism through a light-dependent redox signalling pathway. Electrons are shuttled from photosystem I by means of ferredoxin (Fdx) to ferredoxin-thioredoxin reductase (FTR), which catalyses the two-electron-reduction of chloroplast thioredoxins (Trxs). These modify target enzyme activities by reduction, regulating carbon flow. FTR is unique in its use of a [4Fe-4S] cluster and a proximal disulphide bridge in the conversion of a light signal into a thiol signal. We determined the structures of FTR in both its one- and its two-electron-reduced intermediate states and of four complexes in the pathway, including the ternary Fdx-FTR-Trx complex. Here we show that, in the first complex (Fdx-FTR) of the pathway, the Fdx [2Fe-2S] cluster is positioned suitably for electron transfer to the FTR [4Fe-4S] centre. After the transfer of one electron, an intermediate is formed in which one sulphur atom of the FTR active site is free to attack a disulphide bridge in Trx and the other sulphur atom forms a fifth ligand for an iron atom in the FTR [4Fe-4S] centre--a unique structure in biology. Fdx then delivers a second electron that cleaves the FTR-Trx heterodisulphide bond, which occurs in the Fdx-FTR-Trx complex. In this structure, the redox centres of the three proteins are aligned to maximize the efficiency of electron transfer from the Fdx [2Fe-2S] cluster to the active-site disulphide of Trxs. These results provide a structural framework for understanding the mechanism of disulphide reduction by an iron-sulphur enzyme and describe previously unknown interaction networks for both Fdx and Trx (refs 4-6).
authors.affiliation: Howard Hughes Medical Institute, Integrated Department of Immunology, National Jewish Medical and Research Center & University of Colorado Health Sciences Center, 1400 Jackson Street, Denver, Colorado 80206, USA. dais@njc.org
authors.forename: Shaodong
authors.initials: S
authors.lastname: Dai
authors.forename: Rosmarie
authors.initials: R
authors.lastname: Friemann
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
authors.forename: Hans
authors.initials: H
authors.lastname: Eklund
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D004579:Electron Transport; D005288:Ferredoxins / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins; D018724:Spinacia oleracea; D013329:Structure-Activity Relationship; D046939:Synechocystis; D013879:Thioredoxins / Q000737:chemistry / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D005288:Ferredoxins; D007506:Iron-Sulfur Proteins; D010940:Plant Proteins; D011994:Recombinant Proteins; D013879:Thioredoxins; D007501:Iron; D010088:Oxidoreductases; C021588:ferredoxin-thioredoxin reductase