Crystal structure of the catalytic subunit of magnesium chelatase
Crystal structure of the catalytic subunit of magnesium chelatase
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DOI：
10.1038/nplants.2015.125
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发表时间：
2015-08-24
期刊：
影响因子：
18
通讯作者：
Liu, Lin
中科院分区：
文献类型：
--
作者：
Chen, Xuemin;Pu, Hua;Liu, Lin
四吡啶，包括血红素和叶绿素，在各种生物过程中起着至关重要的作用，如呼吸和光合作用，它们的生物合成对几乎所有生物都至关重要。在光合生物中，镁螯合酶（MgCh）催化镁插入原卟啉IX的中心，原卟啉IX是血红素和叶绿素的分支点前体，导致四吡啶生物合成进入镁分支1,2。这个反应需要MgCh的三个亚基：催化亚基ChlH和两个AAA（+）亚基ChlI和ChlD的协同作用（参考文献3-5）。迄今为止，由于缺乏高分辨率的结构，特别是类似于150 kDa的催化亚基，MgCh的机制有待进一步阐明。本文报道了光合蓝藻聚胞菌PCC 6803中ChlH的晶体结构，其分辨率为2.5埃。活性位点深埋在蛋白质内部，周围的残基在整个进化过程中是保守的。这种结构有助于解释ChlH亚基cch和gun5突变的功能丧失，并为镁螯合过程中底物通道提供分子基础。该结构促进了我们对MgCh全酶的理解，MgCh是一种金属螯合酶，而不是铁螯合酶。
Tetrapyrroles, including haem and chlorophyll, play vital roles for various biological processes, such as respiration and photosynthesis, and their biosynthesis is critical for virtually all organisms. In photosynthetic organisms, magnesium chelatase (MgCh) catalyses insertion of magnesium into the centre of protoporphyrin IX, the branch-point precursor for both haem and chlorophyll, leading tetrapyrrole biosynthesis into the magnesium branch1,2. This reaction needs a cooperated action of the three subunits of MgCh: the catalytic subunit ChlH and two AAA(+) subunits, ChlI and ChlD ( refs 3-5). To date, the mechanism of MgCh awaits further elucidation due to a lack of high-resolution structures, especially for the similar to 150 kDa catalytic subunit. Here we report the crystal structure of ChlH from the photosynthetic cyanobacterium Synechocystis PCC 6803, solved at 2.5 angstrom resolution. The active site is buried deeply inside the protein interior, and the surrounding residues are conserved throughout evolution. This structure helps to explain the loss of function reported for the cch and gun5 mutations of the ChlH subunit, and to provide the molecular basis of substrate channelling during the magnesium-chelating process. The structure advances our understanding of the holoenzyme of MgCh, a metal chelating enzyme other than ferrochelatase.