Structural Insights into the Human and Avian IMP Cyclohydrolase Mechanism via Crystal Structures with the Bound XMP Inhibitor

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• 作者：Dennis W. Wolan ; Cheom-Gil Cheong ; Samantha E. Greasley ; and Ian A. Wilson
• 刊名：Biochemistry
• 出版年：2004
• 出版时间：February 10, 2004
• 年：2004
• 卷：43
• 期：5
• 页码：1171 - 1183
• 全文大小：827K
• 年卷期：v.43,no.5(February 10, 2004)
• ISSN：1520-4995

文摘

Within de novo purine biosynthesis, the AICAR transformylase and IMP cyclohydrolaseactivities of the bifunctional enzyme ATIC convert the intermediate AICAR to the final product of thepathway, IMP. Identification of the AICAR transformylase active site and a proposed formyl transfermechanism have already resulted from analysis of crystal structures of avian ATIC in complex with substrateand/or inhibitors. Herein, we focus on the IMPCH active site and the cyclohydrolase mechanism throughcomparison of crystal structures of XMP inhibitor complexes of human ATIC at 1.9 Å resolution withthe previously determined avian enzyme. This first human ATIC structure was also determined to ascertainwhether any subtle structural differences, compared to the homologous avian enzyme, should be takeninto account for structure-based inhibitor design. These structural comparisons, as well as comparativeanalyses with other IMP and XMP binding proteins, have enabled a catalytic mechanism to be formulated.The primary role of the IMPCH active site appears to be to induce a reconfiguration of the substrateFAICAR to a less energetically favorable, but more reactive, conformer. Backbone (Arg⁶⁴ and Lys⁶⁶) andside chain interactions (Thr⁶⁷) in the IMPCH active site reorient the 4-carboxamide from the preferredconformer that binds to the AICAR Tfase active site to one that promotes intramolecular cyclization.Other backbone amides (Ile¹²⁶ and Gly¹²⁷) create an oxyanion hole that helps orient the formyl group fornucleophilic attack by the 4-carboxamide amine and then stabilize the anionic intermediate. Several otherresidues, including Lys⁶⁶, Tyr¹⁰⁴, Asp¹²⁵, and Lys^137', provide substrate specificity and likely enhance thecatalytic rate through contributions to acid-base catalysis.