A bacterial 2[4Fe-4S] ferredoxin as redox partner of the plastidic-type ferredoxin-NADP+ reductase from Leptospira interrogans

Background: Ferredoxins are small iron-sulfur proteins that participate as electron donors in various metabolic pathways. They are recognized substrates of ferredoxin-NADP reductases (FNR) in redox metabolisms in mitochondria, plastids, and bacteria. We previously found a plastidic-type FNR in Leptospira interrogans (LepFNR), a parasitic bacterium of animals and humans. Nevertheless, we did not identify plant-type ferredoxins or fiavodoxins, the common partners of this kind of FNR. Methods: Sequence alignment, phylogenetical analyses and structural modeling were performed for the identification of a 2[4Fe-45] ferredoxin (LepFd2) as a putative redox partner of LepFNR in L. interrogans. The gene encoding LepFd2 was cloned and the protein overexpressed and purified. The functional properties of LepFd2 and LepFNR-LepFd2 complex were analyzed by kinetic and mutagenesis studies. Results: We succeeded in expressing and purifying LepFd2 with its Fe-S cluster properly bound. We found that LepFd2 exchanges electrons with LepFNR. Moreover, a unique structural subdomain of LepFNR (loop P75-Y91), was shown to be involved in the recognition and binding of LepFd2. This structural subdomain is not found in other FNR homologs. Conclusions: We report for the first time a redox pair in L. interrogans in which a plastidic FNR exchanges electron with a bacterial 2[4Fe-4S] ferredoxin. We characterized this reaction and proposed a model for the productive LepFNR-LepFd2 complex. General significance: Our findings suggest that the interaction of LepFNR with the iron-sulfur protein would be different from the one previously described for the homolog enzymes. This knowledge would be useful for the design of specific LepFNR inhibitors.