Quinone biogenesis: Structure and mechanism of PqqC, the final catalyst in the production of pyrroloquinoline quinone
Quinone biogenesis: Structure and mechanism of PqqC, the final catalyst in the production of pyrroloquinoline quinone
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DOI：
10.1073/pnas.0402640101
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发表时间：
2004-05-25
影响因子：
11.1
通讯作者：
Schwarzenbacher, R
中科院分区：
文献类型：
--
作者：
Magnusson, OT;Toyama, H;Schwarzenbacher, R
吡咯喹啉醌(PQQ)是藜麦蛋白脱氢酶的一种维生素和氧化还原辅因子，它的生物合成是由一条未知的途径促进的，该途径需要pqA到-F六个基因的表达。PqqC是由pqqC编码的蛋白质，催化该途径的最后一步反应，该反应涉及3a-(2-amino-2-carboxyethyl)-4，5-dioxo-4，5，6，7，8，9-hexahydroquinoline-7，9-dicarboxylic-acid的环化和八电子氧化为PQQ。在这里，我们描述了PqqC及其与PQQ的络合物的晶体结构，并测定了反应过程中H_2O_2生成和O-2吸收的化学计量比。PqqC结构(S)显示了一个紧凑的七螺旋束，为正电荷的活性部位腔提供了支架。产物结合引起较大的构象变化，导致氨基酸侧链的活性位点募集，这被认为在催化机制中起关键作用。PqqC的不同寻常之处在于，它将氧化还原当量转移到分子氧，而不需要氧化还原活性金属或辅因子的辅助。酶-产物复合体的结构在PQQ的R179和C5旁边显示出额外的电子密度，可以模拟为O-2或H_2O_2，表明有一个氧结合位置。我们提出了一个反应序列，包括碱催化的环化反应和一系列的醌-喹酚互变异构化，然后是O-2/H_2O_2介导的氧化循环。
The biosynthesis of pyrroloquinoline quinone (PQQ), a vitamin and redox cofactor of quinoprotein dehydrogenases, is facilitated by an unknown pathway that requires the expression of six genes, pqqA to -F. PqqC, the protein encoded by pqqC, catalyzes the final step in the pathway in a reaction that involves ring cyclization and eight-electron oxidation of 3a-(2-amino-2-carboxyethyl)-4,5-dioxo-4,5,6,7,8,9-hexahydroquinoline-7,9-dicarboxylic-acid to PQQ. Herein, we describe the crystal structures of PqqC and its complex with PQQ and determine the stoichiometry of H2O2 formation and O-2 uptake during the reaction. The PqqC structure(s) reveals a compact seven-helix bundle that provides the scaffold for a positively charged active site cavity. Product binding induces a large conformational change, which results in the active site recruitment of amino acid side chains proposed to play key roles in the catalytic mechanism. PqqC is unusual in that it transfers redox equivalents to molecular oxygen without the assistance of a redox active metal or cofactor. The structure of the enzyme-product complex shows additional electron density next to R179 and C5 of PQQ, which can be modeled as O-2 or H2O2, indicating a site for oxygen binding. We propose a reaction sequence that involves base-catalyzed cyclization and a series of quinone-quinol tautomerizations that are followed by cycles of O-2/H2O2-mediated oxidations.