Green sulfur bacterium Chlorobaculum tepidum contains a novel type of ferredoxin-NAD(P)+ reductase (FNR) with high amino acid sequence homology to the NADPHthioredoxin 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 re-face 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 A. 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.
Ferredoxin Flavor protein Electron transfer Green sulfur bacteria
Daisuke Seo Norifumi Muraki Tomoo Shiba Takeshi Sakurai Genji Kurisu
Division of Material Science, Graduate School of Science and Technology, Kanazawa University, Kakuma Department of Life Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan Institute Department of Life Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan Division of Material Science, Graduate School of Science and Technology, Kanazawa University, Kakuma Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, Japan Department of Macromo