Cavitation as a Mechanism of Substrate Discrimination by Adenylosuccinate Synthetases

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• 作者：Cristina V. Iancu ; Yang Zhou ; Tudor Borza ; Herbert J. Fromm ; Richard B. Honzatko
• 刊名：Biochemistry
• 出版年：2006
• 出版时间：September 26, 2006
• 年：2006
• 卷：45
• 期：38
• 页码：11703 - 11711
• 全文大小：431K
• 年卷期：v.45,no.38(September 26, 2006)
• ISSN：1520-4995

文摘

Adenylosuccinate synthetase catalyzes the first committed step in the de novo biosynthesis ofAMP, coupling L-aspartate and IMP to form adenylosuccinate. K_m values of IMP and 2'-deoxy-IMP arenearly identical with each substrate supporting comparable maximal velocities. Nonetheless, the K_m valuefor L-aspartate and the K_i value for hadacidin (a competitive inhibitor with respect to L-aspartate) are29-57-fold lower in the presence of IMP than in the presence of 2'-deoxy-IMP. Crystal structures of thesynthetase ligated with hadacidin, GDP, and either 6-phosphoryl-IMP or 2'-deoxy-6-phosphoryl-IMP areidentical except for the presence of a cavity normally occupied by the 2'-hydroxyl group of IMP. In thepresence of 6-phosphoryl-IMP and GDP (hadacidin absent), the L-aspartate pocket can retain its fullyligated conformation, forming hydrogen bonds between the 2'-hydroxyl group of IMP and sequence-invariant residues. In the presence of 2'-deoxy-6-phosphoryl-IMP and GDP, however, the L-aspartatepocket is poorly ordered. The absence of the 2'-hydroxyl group of the deoxyribonucleotide may destabilizebinding of the ligand to the L-aspartate pocket by disrupting hydrogen bonds that maintain a favorableprotein conformation and by the introduction of a cavity into the fully ligated active site. At an approximateenergy cost of 2.2 kcal/mol, the unfavorable thermodynamics of cavity formation may be the major factorin destabilizing ligands at the L-aspartate pocket.