Structural snapshots along the reaction pathway of ferredoxin-thioredoxin reductase

Oxygen-evolving photosynthetic organisms regulate carbon metabolism through a light-dependent redox signalling pathway(1). 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(2). 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(2). 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 atomin 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(3) and describe previously unknown interaction networks for both Fdx and Trx ( refs 4 - 6).