Crystal structures of BchU, a methyltransferase involved in bacteriochlorophyll c biosynthesis, and its complex with S-adenosylhomocysteine: implications for reaction mechanism.
Crystal structures of BchU, a methyltransferase involved in bacteriochlorophyll c biosynthesis, and its complex with S-adenosylhomocysteine: implications for reaction mechanism.
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DOI：
10.1016/j.jmb.2006.05.057
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发表时间：
2006-07
影响因子：
5.6
通讯作者：
K. Wada;H. Yamaguchi;J. Harada;Keiko Niimi;S. Osumi;Y. Saga;H. Oh-oka;H. Tamiaki;K. Fukuyama
中科院分区：
文献类型：
--
作者：
K. Wada;H. Yamaguchi;J. Harada;Keiko Niimi;S. Osumi;Y. Saga;H. Oh-oka;H. Tamiaki;K. Fukuyama
以S-腺苷蛋氨酸(SAM)为甲基来源，BchU通过催化环四吡咯氯的C-20位甲基化，在细菌-叶绿素c的生物合成中发挥作用。这种甲基化导致捕光色素的电子吸收光谱红移，使绿色光合作用细菌能够适应弱光环境。测定了BchU及其与S-腺苷同型半胱氨酸(SAH)的配合物的晶体结构。BchU形成一个二聚体，每个亚基由两个结构域组成，N-末端结构域和C-末端结构域。二聚是通过N-末端结构域之间的相互作用发生的，负责催化反应的残基在C-末端结构域中。SAH的结合位置位于两个结构域之间的一个大空腔中，在那里SAH被许多氢键和盐桥特异性识别。BchU与细菌叶绿素C类似物的电子密度图位于SAH结合部位附近的中心金属位置，但看不到四吡咯环，这表明该环与BchU的结合是松散的和/或环的占有率较低。His290很可能作为底物中心金属的配体。通过模拟预测了底物的取向，并提出了BchU定向甲基化的机制：严格保守的Tyr246残基通过类似SN2的机制催化甲基从SAM直接转移到底物。
BchU plays a role in bacteriochlorophyll c biosynthesis by catalyzing methylation at the C-20 position of cyclic tetrapyrrole chlorin using S-adenosylmethionine (SAM) as a methyl source. This methylation causes red-shifts of the electronic absorption spectrum of the light-harvesting pigment, allowing green photosynthetic bacteria to adapt to low-light environments. We have determined the crystal structures of BchU and its complex with S-adenosylhomocysteine (SAH). BchU forms a dimer and each subunit consists of two domains, an N-terminal domain and a C-terminal domain. Dimerization occurs through interactions between the N-terminal domains and the residues responsible for the catalytic reaction are in the C-terminal domain. The binding site of SAH is located in a large cavity between the two domains, where SAH is specifically recognized by many hydrogen bonds and a salt-bridge. The electron density map of BchU in complex with an analog of bacteriochlorophyll c located its central metal near the SAH-binding site, but the tetrapyrrole ring was invisible, suggesting that binding of the ring to BchU is loose and/or occupancy of the ring is low. It is likely that His290 acts as a ligand for the central metal of the substrate. The orientation of the substrate was predicted by simulation, and allows us to propose a mechanism for the BchU directed methylation: the strictly conserved Tyr246 residue acts catalytically in the direct transfer of the methyl group from SAM to the substrate through an SN2-like mechanism.