title: X-ray crystal structure of the light-independent protochlorophyllide reductase.
abstract: Photosynthetic organisms adopt two different strategies for the reduction of the C17 = C18 double bond of protochlorophyllide (Pchlide) to form chlorophyllide a, the direct precursor of chlorophyll a (refs 1-4). The first involves the activity of the light-dependent Pchlide oxidoreductase, and the second involves the light-independent (dark-operative) Pchlide oxidoreductase (DPOR). DPOR is a nitrogenase-like enzyme consisting of two components, L-protein (a BchL dimer) and NB-protein (a BchN-BchB heterotetramer), which are structurally related to nitrogenase Fe protein and MoFe protein, respectively. Here we report the crystal structure of the NB-protein of DPOR from Rhodobacter capsulatus at a resolution of 2.3A. As expected, the overall structure is similar to that of nitrogenase MoFe protein: each catalytic BchN-BchB unit contains one Pchlide and one iron-sulphur cluster (NB-cluster) coordinated uniquely by one aspartate and three cysteines. Unique aspartate ligation is not necessarily needed for the cluster assembly but is essential for the catalytic activity. Specific Pchlide-binding accompanies the partial unwinding of an alpha-helix that belongs to the next catalytic BchN-BchB unit. We propose a unique trans-specific reduction mechanism in which the distorted C17-propionate of Pchlide and an aspartate from BchB serve as proton donors for C18 and C17 of Pchlide, respectively. Intriguingly, the spatial arrangement of the NB-cluster and Pchlide is almost identical to that of the P-cluster and FeMo-cofactor in nitrogenase MoFe-protein, illustrating that a common architecture exists to reduce chemically stable multibonds of porphyrin and dinitrogen.
authors.affiliation: Department of Life Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan.
authors.forename: Norifumi
authors.initials: N
authors.lastname: Muraki
authors.forename: Jiro
authors.initials: J
authors.lastname: Nomata
authors.forename: Kozue
authors.initials: K
authors.lastname: Ebata
authors.forename: Tadashi
authors.initials: T
authors.lastname: Mizoguchi
authors.forename: Tomoo
authors.initials: T
authors.lastname: Shiba
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Tamiaki
authors.forename: Genji
authors.initials: G
authors.lastname: Kurisu
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Fujita
mesh_terms: D018360:Crystallography, X-Ray; D008958:Models, Molecular; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry* / Q000378:metabolism; D017434:Protein Structure, Tertiary; D016354:Rhodobacter capsulatus / Q000201:enzymology*
chemical_list: D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase
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