title: Purification and characterization of VanXY(C), a D,D-dipeptidase/D,D-carboxypeptidase in vancomycin-resistant Enterococcus gallinarum BM4174.
abstract: VanXY(C), a bifunctional enzyme from VanC-phenotype Enterococcus gallinarum BM4174 that catalyses D,D-peptidase and D,D-carboxypeptidase activities, was purified as the native protein, as a maltose-binding protein fusion and with an N-terminal tag containing six histidine residues. The kinetic parameters of His(6)-VanXY(C) were measured for a variety of precursors of peptidoglycan synthesis involved in resistance: for D-Ala-D-Ala, the K(m) was 3.6 mm and k(cat), 2.5 s(-1); for UDP-MurNAc-L-Ala-D-Glu-L-Lys-DAla-D-Ala (UDP-MurNAc-pentapeptide[Ala]), K(m) was 18.8 mm and k(cat) 6.2 s(-1); for D-Ala-D-Ser, K(m) was 15.5 mm and k(cat) 0.35 s(-1). His(6)-VanXYC was inactive against the peptidoglycan precursor UDP-MurNAc-L-Ala-D-Glu-L-Lys-D-Ala-D-Ser (UDP-MurNAc-pentapeptide[Ser]). The rate of hydrolysis of the terminal D-Ala of UDP-MurNAc-pentapeptide[Ala] was inhibited 30% by 2 mm D-Ala-D-Ser or UDP-MurNAc-pentapeptide[Ser]. Therefore preferential hydrolysis of substrates terminating in D-Ala would occur during peptidoglycan synthesis in E. gallinarum BM4174, leaving precursors ending in D-Ser with a lower affinity for glycopeptides to be incorporated into peptidoglycan. Mutation of an aspartate residue (Asp59) of His-tagged VanXY(C) corresponding to Asp68 in VanX to Ser or Ala, resulted in a 50% increase and 73% decrease, respectively, of the specificity constant (k(cat)/K(m)) for D-Ala-D-Ala. This situation is in contrast to VanX in which mutation of Asp68-->Ala produced a greater than 200,000-fold decrease in the substrate specificity constant. This suggests that Asp59, unlike Asp68 in VanX, does not have a pivotal role in catalysis.
authors.affiliation: Department of Biochemistry, University of Cambridge, UK. adrian_podmore@hotmail.com
authors.forename: Adrian H B
authors.initials: AH
authors.lastname: Podmore
authors.forename: Peter E
authors.initials: PE
authors.lastname: Reynolds
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002268:Carboxypeptidases; D004150:Dipeptidases / Q000302:isolation & purification / Q000378:metabolism*; D016983:Enterococcus / Q000502:physiology; D007700:Kinetics; D008565:Membrane Proteins; D002267:Muramoylpentapeptide Carboxypeptidase / Q000302:isolation & purification / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D046929:Serine-Type D-Ala-D-Ala Carboxypeptidase; D013379:Substrate Specificity; D020713:Vancomycin Resistance / Q000502:physiology*
chemical_list: D001426:Bacterial Proteins; D008565:Membrane Proteins; D011993:Recombinant Fusion Proteins; D002268:Carboxypeptidases; D004150:Dipeptidases; D046929:Serine-Type D-Ala-D-Ala Carboxypeptidase; C117912:VanY protein, Bacteria; C091956:VanX dipeptidase; D002267:Muramoylpentapeptide Carboxypeptidase