title: Analysis of UDP-D-apiose/UDP-D-xylose synthase-catalyzed conversion of UDP-D-apiose phosphonate to UDP-D-xylose phosphonate: implications for a retroaldol-aldol mechanism.
abstract: UDP-D-apiose/UDP-D-xylose synthase (AXS) catalyzes the conversion of UDP-D-glucuronic acid to UDP-D-apiose and UDP-D-xylose. An acetyl-protected phosphonate analogue of UDP-D-apiose was synthesized and used in an in situ HPLC assay to demonstrate for the first time the ability of AXS to interconvert the two reaction products. Density functional theory calculations provided insight into the energetics of this process and the apparent inability of AXS to catalyze the conversion of UDP-D-xylose to UDP-D-apiose. The data suggest that this observation is unlikely to be due to an unfavorable equilibrium but rather results from substrate inhibition by the most stable chair conformation of UDP-D-xylose. The detection of xylose cyclic phosphonate as the turnover product reveals significant new details about the AXS-catalyzed reaction and supports the proposed retroaldol-aldol mechanism of catalysis.
authors: Division of Medicinal Chemistry, College of Pharmacy, and Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712, USA.
authors: Sei-hyun
authors: SH
authors: Choi
authors: Steven O
authors: SO
authors: Mansoorabadi
authors: Yung-nan
authors: YN
authors: Liu
authors: Tun-Cheng
authors: TC
authors: Chien
authors: Hung-wen
authors: HW
authors: Liu
mesh_terms: D002262:Carboxy-Lyases / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D063065:Organophosphonates / Q000737:chemistry / Q000378:metabolism*; D011789:Quantum Theory; D014539:Uridine Diphosphate Sugars / Q000737:chemistry / Q000378:metabolism*; D014540:Uridine Diphosphate Xylose / Q000737:chemistry / Q000378:metabolism*
chemical_list: D029968:Escherichia coli Proteins; D063065:Organophosphonates; C511616:UDP-apiose; D014539:Uridine Diphosphate Sugars; D014540:Uridine Diphosphate Xylose; D002262:Carboxy-Lyases; C023257:UDPglucuronate decarboxylase