一些光合作用生物（例如松苗和其他裸子植物）能夠在黑暗中變綠，這與豌豆等被子植物幼苗變綠對光的嚴(yán)格要求形成對比,。這種“黑暗藝術(shù)”背后的酶是在黑暗中發(fā)揮作用的“原葉綠素酸脂（Pchlide）氧化還原酶”（DPOR）,，它催化Pchlide的C17-C18雙鍵的立體選擇性還原,，生成葉綠素酸酯-a（葉綠素-a的直接前體）。
現(xiàn)在,，來自紫色光養(yǎng)菌Rhodobacter capsulatus的DPOR的NB-蛋白部分的晶體結(jié)構(gòu)已被確定,。該結(jié)構(gòu)顯示了Pchlide 的C17-C18雙鍵還原反應(yīng)的一個可能的化學(xué)機(jī)制。有趣的是,，DPOR與眾所周知的固氮酶相似,，說明固氮的分子機(jī)制與在黑暗中產(chǎn)生葉綠素的分子機(jī)制之間存在一個密切的演化關(guān)系。（生物谷Bioon.com）
生物谷推薦原文出處：
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
