title: Molecular mechanism of the enterococcal aminoglycoside 6'-N-acetyltransferase': role of GNAT-conserved residues in the chemistry of antibiotic inactivation.
abstract: The Gram-positive pathogen Enterococcus faecium is intrinsically resistant to aminoglycoside antibiotics due to the presence of a chromosomally encoded aminoglycoside 6'-N-acetyltransferase [AAC(6')-Ii]. This enzyme is a member of the GCN5-related N-acetyltransferase (GNAT) superfamily and is therefore structurally homologous to proteins that catalyze acetyl transfer to diverse acyl-accepting substrates. This study reports the investigation of several potential catalytic residues that are present in the AAC(6')-Ii active site and also conserved in many GNAT enzymes. Site-directed mutagenesis of Glu72, His74, Leu76, and Tyr147 with characterization of the purified site mutants gave valuable information about the roles of these amino acids in acetyl transfer chemistry. More specifically, steady-state kinetic analysis of protein activity, solvent viscosity effects, pH studies, and antibiotic resistance profiles were all used to assess the roles of Glu72 and His74 as potential active site bases, Tyr147 as a general acid, and the importance of the amide NH group of Leu76 in transition-state stabilization. Taken together, our results indicate that Glu72 is not involved in general base catalysis, but is instead critical for the proper positioning and orientation of aminoglycoside substrates in the active site. Similarly, His74 is also not acting as the active site base, with pH studies revealing that this residue must be protonated for optimal AAC(6')-Ii activity. Mutation of Tyr147 was found not to affect the chemical step of catalysis, and our results were not consistent with this residue acting as a general acid. Last, the amide NH group of Leu76 is implicated in important interactions with acetyl-CoA and transition-state stabilization. In summary, the work described here provides important information regarding the molecular mechanism of AAC(6')-Ii catalysis that allows us to contrast our findings with those of other GNAT proteins and to demonstrate that these enzymes use a variety of chemical mechanisms to accelerate acyl transfer.
authors.affiliation: Antimicrobial Research Centre, Department of Biochemistry, McMaster University, 1200 Main Street West, Hamilton, Ontario L8N 3Z5, Canada.
authors.forename: Kari-ann
authors.initials: KA
authors.lastname: Draker
authors.forename: Gerard D
authors.initials: GD
authors.lastname: Wright
mesh_terms: D000123:Acetyltransferases / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics; D000617:Aminoglycosides / Q000037:antagonists & inhibitors* / Q000737:chemistry*; D000900:Anti-Bacterial Agents / Q000037:antagonists & inhibitors* / Q000737:chemistry*; D017124:Conserved Sequence; D024881:Drug Resistance, Bacterial; D016984:Enterococcus faecium / Q000201:enzymology* / Q000235:genetics; D004789:Enzyme Activation / Q000235:genetics; D018698:Glutamic Acid / Q000235:genetics; D006639:Histidine / Q000235:genetics; D051548:Histone Acetyltransferases; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007930:Leucine / Q000235:genetics; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D014443:Tyrosine / Q000235:genetics; D000099224:p300-CBP-Associated Factor
chemical_list: D000123:Acetyltransferases; D000617:Aminoglycosides; D000900:Anti-Bacterial Agents; D018698:Glutamic Acid; D006639:Histidine; D051548:Histone Acetyltransferases; D007930:Leucine; D014443:Tyrosine; C120128:tGCN5 histone acetyltransferase; D000099224:p300-CBP-Associated Factor; C056532:aminoglycoside N(6')-acetyltransferase