The Crystal Structure of Fe4S4 Quinolinate Synthase Unravels an Enzymatic Dehydration Mechanism That Uses Tyrosine and a Hydrolase-Type Triad

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• 作者：Micka毛l V. Cherrier ; Alice Chan ; Claudine Darnault ; Debora Reichmann ; Patricia Amara ; Sandrine Ollagnier de Choudens ; Juan C. Fontecilla-Camps
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：April 9, 2014
• 年：2014
• 卷：136
• 期：14
• 页码：5253-5256
• 全文大小：284K
• 年卷期：v.136,no.14(April 9, 2014)
• ISSN：1520-5126

文摘

Quinolinate synthase (NadA) is a Fe₄S₄ cluster-containing dehydrating enzyme involved in the synthesis of quinolinic acid (QA), the universal precursor of the essential nicotinamide adenine dinucleotide (NAD) coenzyme. A previously determined apo NadA crystal structure revealed the binding of one substrate analog, providing partial mechanistic information. Here, we report on the holo X-ray structure of NadA. The presence of the Fe₄S₄ cluster generates an internal tunnel and a cavity in which we have docked the last precursor to be dehydrated to form QA. We find that the only suitably placed residue to initiate this process is the conserved Tyr21. Furthermore, Tyr21 is close to a conserved Thr-His-Glu triad reminiscent of those found in proteases and other hydrolases. Our mutagenesis data show that all of these residues are essential for activity and strongly suggest that Tyr21 deprotonation, to form the reactive nucleophilic phenoxide anion, is mediated by the triad. NadA displays a dehydration mechanism significantly different from the one found in archetypical dehydratases such as aconitase, which use a serine residue deprotonated by an oxyanion hole. The X-ray structure of NadA will help us unveil its catalytic mechanism, the last step in the understanding of NAD biosynthesis.