Structural snapshots along the reaction pathway of ferredoxin–thioredoxin reductase

Oxygen-evolving photosynthetic organisms regulate carbon metabolism through a light-dependent redox signalling pathway1. 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 flow2. 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 signal2. 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 enzyme3 and describe previously unknown interaction networks for both Fdx and Trx (refs 4–6).