西亚试剂：X-ray crystal structure of the light-independent protochlor

X-ray crystal structure of the light-independent protochlorophyllide reductase
Norifumi Muraki,Jiro Nomata,Kozue Ebata,Tadashi Mizoguchi,Tomoo Shiba,Hitoshi Tamiaki,Genji Kurisu& Yuichi Fujita

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 oxidoreductase5, 6, 7, 8, 9, and the second involves the light-independent (dark-operative) Pchlide oxidoreductase10 (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, respectively10, 11. Here we report the crystal structure of the NB-protein of DPOR from Rhodobacter capsulatus at a resolution of 2.3??. 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 α-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.

Department of Life Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
Norifumi Muraki & Tomoo Shiba
Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, Japan
Norifumi Muraki & Genji Kurisu
Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
Jiro Nomata, Kozue Ebata & Yuichi Fujita
Department of Bioscience and Biotechnology, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
Tadashi Mizoguchi & Hitoshi Tamiaki
Presto, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
Yuichi Fujita
Present address: Department of Biomedical Chemistry, Graduate School of Medicine, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Tomoo Shiba