Structural Basis of Redox Signaling in Photosynthesis: Structure and Function of Ferredoxin:thioredoxin Reductase and Target Enzymes.

The role of the ferredoxin:thioredoxin system in the reversible light activation of chloroplast enzymes by thiol-disulfide interchange with thioredoxins is now well established. Recent fruitful collaboration between biochemists and structural biologists, reflected by the shared authorship of the paper, allowed to solve the structures of all of the components of the system, including several target enzymes, thus providing a structural basis for the elucidation of the activation mechanism at a molecular level. In the present Review, these structural data are analyzed in conjunction with the information that was obtained previously through biochemical and site-directed mutagenesis approaches. The unique 4Fe-4S cluster enzyme ferredoxin:thioredoxin reductase (FTR) uses photosynthetically reduced ferredoxin as an electron donor to reduce the disulfide bridge of different thioredoxin isoforms. Thioredoxins in turn reduce regulatory disulfides of various target enzymes. This process triggers conformational changes on these enzymes, allowing them to reach optimal activity. No common activation mechanism can be put forward for these enzymes, as every thioredoxin-regulated protein undergoes specific structural modifications. It is thus important to solve the structures of the individual target enzymes in order to fully understand the molecular mechanism of the redox regulation of each of them.