Crystal Structure of Ferredoxin-NAD(P)⁺ Reductase from the Green Sulfur Bacterium Chlorobaculum Tepidum

Abstract

Green sulfur bacterium Chlorobaculum tepidum contains a novel type of ferredoxin-NAD(P)⁺ reductase (FNR) with high amino acid sequence homology to the NADPH-thioredoxin reductase (TdR) from prokaryotes. In this study, we determine the crystal structure of C. tepidum FNR by X-ray crystallography. C. tepidum FNR retains its structural topology with E. coli TdR but possesses several characteristic features that is absent in TdR. Each protomer is composed of two nucleotide binding domains, FAD-binding and NAD(P)⁺-binding. The two domains are connected by a hinge region. Homo-dimeric C. tepidum FNR shows an asymmetric domain orientation between two protomers. The observed C-terminal sub-domain covers the re-face of the isoalloxazine ring of FAD prosthetic group. The C-terminal sub-domain includes the stacking Phe337 on the reface of the isoalloxazine ring of the FAD. On the si-face, Tyr57 residue is stacked on. The two stacking ring systems are positioned almost parallel with respect to isoalloxazine ring at a distance of 3.5 Å. Such a configuration of stacking of two aromatic rings is absent in TdR but found in plastid-type FNRs, suggesting these structural characteristics are indispensable for the FNR reaction. To elucidate the function of these structural characteristics, mutational analysis was performed.