Coenzyme binding in a highly specific isocitrate dehydrogenase

Sessions C775Ferredoxin-NADP + reductase (FNR) catalyzes the redox reaction between NAD(P) + and NAD(P)H with the electron carrier protein, Ferredoxin (Fd).Recently, new FNR subfamily that shares a structural homology to NADPH-dependent thioredoxin reductase (TrxR) was identified.We have solved the first crystal structure of the TrxR-like FNR from the green sulfur bacterium Chlorobaculum tedium and reported the several unique structural features of TrxR-like FNR [1].The additional C-terminal sub-domain, that covers the re-face of the isoalloxazine ring of FAD, was newly found in C. tedium FNR.The unique asymmetric domain arrangement suggests the bending motion of the hinge region between the FAD and NADPH binding domains.Then, the crystal structure of Bacillus subtilis FNR, classified into TrxR-like FNR, was reported as a complex with NADP + [2].On the basis of amino acid sequence analysis, it has been recently reported that photosynthetic purple bacterium Rhodopseudomonas palustris also has a TrxR-like FNR.In this study, we have examined the crystal structure of Rps.palustris FNR by X-ray crystallography in order to confirm the reported structural features of TrxR-like FNR.The crystal structure of Rps.palustris FNR was determined by the molecular replacement method at 2.4 Å resolution.The C-terminal sub-domain containing the FAD stacking Tyr residue was confirmed in the Rps.palustris FNR structure.Rps.palustris FNR exists as a homodimer in the crystallographic asymmetric unit.When the FAD domain of one protomer is superimposed on that of the other, one NAD(P)H domain is rotated by 16.5° with respect to the other.The domain arrangements of Rps.palustris FNR is more open, when compared to those of C. tepidum FNR and B. subtilis FNR.Sequential comparison of the all NADPH domains of TrxR-like FNRs and TrxRs proposes the unique trajectory of the domain, which might be closely related to the replacement of the structurally conserved C-terminal subdomain during the catalytic cycle.