title: The hydratase activity of malonate semialdehyde decarboxylase: mechanistic and evolutionary implications.
abstract: Malonate semialdehyde decarboxylase (MSAD) is a member of the tautomerase superfamily, a group of structurally homologous proteins that have a characteristic beta-alpha-beta-fold and a catalytic amino-terminal proline. In addition to its physiological decarboxylase activity, the conversion of malonate semialdehyde to acetaldehyde and carbon dioxide, the enzyme has now been found to display a promiscuous hydratase activity, converting 2-oxo-3-pentynoate to acetopyruvate, with a kcat/Km value of 6.0 x 102 M-1 s-1. Pro-1 and Arg-75 are critical for both activities, and the pKa of Pro-1 was determined to be approximately 9.2 by a direct 15N NMR titration. These observations implicate a decarboxylation mechanism in which Pro-1 polarizes the carbonyl oxygen of substrate by hydrogen bonding and/or an electrostatic interaction. Arg-75 may position the carboxylate group into a favorable orientation for decarboxylation. Both the hydratase activity and the pKa value of Pro-1 are shared with trans-3-chloroacrylic acid dehalogenase, another tautomerase superfamily member that precedes MSAD in a bacterial degradation pathway for trans-1,3-dichloropropene. Hence, MSAD and CaaD could have evolved by divergent evolution from a common ancestral protein, retaining the necessary catalytic components for the conjugate addition of water.
authors.affiliation: Division of Medicinal Chemistry, College of Pharmacy, and Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, The University of Texas, Austin, Texas 78712, USA.
authors.forename: Gerrit J
authors.initials: GJ
authors.lastname: Poelarends
authors.forename: Hector
authors.initials: H
authors.lastname: Serrano
authors.forename: William H
authors.initials: WH
authors.lastname: Johnson
authors.forename: David W
authors.initials: DW
authors.lastname: Hoffman
authors.forename: Christian P
authors.initials: CP
authors.lastname: Whitman
mesh_terms: D002262:Carboxy-Lyases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019143:Evolution, Molecular; D005231:Fatty Acids, Unsaturated / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006867:Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D019906:Nuclear Magnetic Resonance, Biomolecular; D017510:Protein Folding; D011549:Pseudomonas / Q000201:enzymology; D017186:Titrimetry
chemical_list: C109822:2-oxo-3-pentynoic acid; D005231:Fatty Acids, Unsaturated; D006867:Hydrolases; C077721:trans-3-chloroacrylic acid dehalogenase; D002262:Carboxy-Lyases; C071850:malonate semialdehyde decarboxylase