一些光合作用生物（例如松苗和其他裸子植物）能够在黑暗中变绿，这与豌豆等被子植物幼苗变绿对光的严格要求形成对比。这种“黑暗艺术”背后的酶是在黑暗中发挥作用的“原叶绿素酸脂（Pchlide）氧化还原酶”（DPOR），它催化Pchlide的C17-C18双键的立体选择性还原，生成叶绿素酸酯-a（叶绿素-a的直接前体）。
现在，来自紫色光养菌Rhodobacter capsulatus的DPOR的NB-蛋白部分的晶体结构已被确定。该结构显示了Pchlide 的C17-C18双键还原反应的一个可能的化学机制。有趣的是，DPOR与众所周知的固氮酶相似，说明固氮的分子机制与在黑暗中产生叶绿素的分子机制之间存在一个密切的演化关系。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature08950
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
