A novel family of cytochrome c quinol dehydrogenases, that play an important role in bacterial respiratory chains, was recognized in recent years. Here we describe the first structure of a member of this family, NrfH, from Desulfovibrio vulgaris, which forms a strong complex with its electron partner, the cytochrome c nitrite reductase NrfA.
The 2.3 Å crystal structure reveals that one NrfH molecule interacts with one NrfA dimer in an asymmetrical manner, forming a large membrane-bound complex with an overall a4b2 quaternary arrangement (Fig.1). NrfH is an a-helical protein with a novel protein fold (Fig.2a) which has a highly unusual heme coordination.
The menaquinol-interacting NrfH heme 1 is pentacoordinated, bound by a methionine from the CXXCHXM sequence (Met49), with an aspartate residue occupying the distal position (Fig.2b). The NrfH heme that transfers electrons to NrfA has a lysine residue from the closest NrfA molecule as distal ligand (Fig.2c). A likely menaquinol binding site, containing several conserved and essential residues, is identified.
References
Rodrigues, M.L., Oliveira, T., Pereira I.A.C., and Archer, M. (2006). The X-ray structure of the membrane-bound cytochrome c quinol dehydrogenase NrfH reveals novel heme coordination. EMBO J., in press.
Rodrigues, M.L., Oliveira, T., Matias, P.M., Martins, I.C., Valente, F.M.A., Pereira, I.A.C. and Archer, M. (2006). Crystallization and preliminary structure determination of the membrane-bound complex cytochrome c nitrite reductase from Desulfovibrio vulgaris Hildenborough. Acta Cryst. F62, 565-568.
Legends
Fig 1 (a) NrfHA viewed parallel to the membrane (gray rectangle) with hemes drawn as red sticks. Each NrfH subunit, shown in green and magenta, is tightly bound to one NrfA dimer shown in orange/yellow and light blue/dark blue. (b) Ensemble of 28 heme groups in the same orientation as in (a).
Fig 2 NrfH overall fold, where hemes are arranged in a di-heme parallel motif. The two longer a-helices are inserted in the membrane. (b) Menaquinol-interacting heme 1, with the proximal ligand, Met49, the His47 from the CXXCHXM motif and Asp89 displayed. (c) Heme 4, the gateway of electrons from NrfH to NrfA, has Lys331 from an internal NrfA subunit as distal heme ligand. The proximal ligand is His140 from NrfH.
