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Structural basis for thioredoxin isoform‐based fine‐tuning of ferredoxin‐thioredoxin reductase activity

Photosynthetic electron transport occurs on the thylakoid membrane of chloroplasts. Ferredoxin (Fd), the final acceptor in the electron transport chain, distributes electrons to several Fd‐dependent enzymes including Fd‐thioredoxin reductase (FTR). A cascade from Fd to FTR further reduces Thioredoxin (Trx), which tunes the activity of target metabolic enzymes eventually in a light‐dependent manner. We previously reported that 10 Trx isoforms in Arabidopsis thaliana can be clustered into three classes based on the kinetics of the FTR‐dependent reduction (high‐, middle‐, and low‐efficiency classes). In this study, we determined the X‐ray structure of three electron transfer complexes of FTR and Trx isoform, Trx‐y1, Trx‐f2, and Trx‐m2, as representative examples of each class. Superposition of the FTR structure with/without Trx showed no main chain structural changes upon complex formation. There was no significant conformational change for single and complexed Trx‐m structures. Nonetheless, the interface of FTR:Trx complexes displayed significant variation. Comparative analysis of the three structures showed two types of intermolecular interactions; (i) common interactions shared by all three complexes and (ii) isoform‐specific interactions, which might be important for fine‐tuning FTR:Trx activity. Differential electrostatic potentials of Trx isoforms may be key to isoform‐specific interactions.