title: Investigation of metal ion binding in phosphonoacetaldehyde hydrolase identifies sequence markers for metal-activated enzymes of the HAD enzyme superfamily.
abstract: The 2-haloalkanoic acid dehalogenase (HAD) family, which contains both carbon and phosphoryl transferases, is one of the largest known enzyme superfamilies. HAD members conserve an alpha,beta-core domain that frames the four-loop active-site platform. Each loop contributes one or more catalytic groups, which function in mediating the core chemistry (i.e., group transfer). In this paper, we provide evidence that the number of carboxylate residues on loop 4 and their positions (stations) on the loop are determinants, and therefore reliable sequence markers, for metal ion activation among HAD family members. Using this predictor, we conclude that the vast majority of the HAD members utilize a metal cofactor. Analysis of the minimum requirements for metal cofactor binding was carried out using Mg(II)-activated Bacillus cereus phosphonoacetaldehyde hydrolase (phosphonatase) as an experimental model for metal-activated HAD members. Mg(II) binding occurs via ligation to the loop 1 Asp12 carboxylate and Thr14 backbone carbonyl and to the loop 4 Asp186 carboxylate. The loop 4 Asp190 forms a hydrogen bond to the Mg(II) water ligand. X-ray structure determination of the D12A mutant in the presence of the substrate phosphonoacetaldehyde showed that replacement of the loop 1 Asp, common to all HAD family members, with Ala shifts the position of Mg(II), thereby allowing innersphere coordination to Asp190 and causing a shift in the position of the substrate. Kinetic analysis of the loop 4 mutants showed that Asp186 is essential to cofactor binding while Asp190 simply enhances it. Within the phosphonatase subfamily, Asp186 is stringently conserved, while either position 185 or position 190 is used to position the second loop 4 Asp residue. Retention of a high level of catalytic activity in the G185D/D190G phosphonatase mutant demonstrated the plasticity of the metal binding loop, reflected in the variety of combinations in positioning of two or three Asp residues along the seven-residue motif of the 2700 potential HAD sequences that were examined.
authors.affiliation: Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, USA.
authors.forename: Guofeng
authors.initials: G
authors.lastname: Zhang
authors.forename: Marc C
authors.initials: MC
authors.lastname: Morais
authors.forename: Jianying
authors.initials: J
authors.lastname: Dai
authors.forename: Wenhai
authors.initials: W
authors.lastname: Zhang
authors.forename: Debra
authors.initials: D
authors.lastname: Dunaway-Mariano
authors.forename: Karen N
authors.initials: KN
authors.lastname: Allen
mesh_terms: D000409:Alanine / Q000378:metabolism; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001224:Aspartic Acid / Q000378:metabolism; D001409:Bacillus cereus / Q000201:enzymology; D001665:Binding Sites; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004789:Enzyme Activation; D006860:Hydrogen Bonding; D006867:Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008274:Magnesium / Q000378:metabolism; D008670:Metals / Q000378:metabolism*; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D013379:Substrate Specificity; D014867:Water / Q000737:chemistry
chemical_list: D008024:Ligands; D008670:Metals; D014867:Water; D001224:Aspartic Acid; D006867:Hydrolases; C023920:phosphonoacetaldehyde hydrolase; D008274:Magnesium; D000409:Alanine