Kinetic features of L,D-transpeptidase inactivation critical for β-lactam antibacterial activity

PLoS One. 2013 Jul 4;8(7):e67831. doi: 10.1371/journal.pone.0067831. Print 2013.

Abstract

Active-site serine D,D-transpeptidases belonging to the penicillin-binding protein family (PBPs) have been considered for a long time as essential for peptidoglycan cross-linking in all bacteria. However, bypass of the PBPs by an L,D-transpeptidase (Ldt(fm)) conveys high-level resistance to β-lactams of the penam class in Enterococcus faecium with a minimal inhibitory concentration (MIC) of ampicillin >2,000 µg/ml. Unexpectedly, Ldt(fm) does not confer resistance to β-lactams of the carbapenem class (imipenem MIC = 0.5 µg/ml) whereas cephems display residual activity (ceftriaxone MIC = 128 µg/ml). Mass spectrometry, fluorescence kinetics, and NMR chemical shift perturbation experiments were performed to explore the basis for this specificity and identify β-lactam features that are critical for efficient L,D-transpeptidase inactivation. We show that imipenem, ceftriaxone, and ampicillin acylate Ldt(fm) by formation of a thioester bond between the active-site cysteine and the β-lactam-ring carbonyl. However, slow acylation and slow acylenzyme hydrolysis resulted in partial Ldt(fm) inactivation by ampicillin and ceftriaxone. For ampicillin, Ldt(fm) acylation was followed by rupture of the C(5)-C(6) bond of the β-lactam ring and formation of a secondary acylenzyme prone to hydrolysis. The saturable step of the catalytic cycle was the reversible formation of a tetrahedral intermediate (oxyanion) without significant accumulation of a non-covalent complex. In agreement, a derivative of Ldt(fm) blocked in acylation bound ertapenem (a carbapenem), ceftriaxone, and ampicillin with similar low affinities. Thus, oxyanion and acylenzyme stabilization are both critical for rapid L,D-transpeptidase inactivation and antibacterial activity. These results pave the way for optimization of the β-lactam scaffold for L,D-transpeptidase-inactivation.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acylation
  • Ampicillin / chemistry*
  • Bacterial Proteins / antagonists & inhibitors*
  • Bacterial Proteins / chemistry
  • Ceftriaxone / chemistry*
  • Enterococcus faecium / chemistry*
  • Enterococcus faecium / enzymology
  • Imipenem / chemistry*
  • Kinetics
  • Peptidyl Transferases / antagonists & inhibitors*
  • Peptidyl Transferases / chemistry
  • Recombinant Proteins / chemistry
  • Structure-Activity Relationship
  • Substrate Specificity
  • beta-Lactam Resistance

Substances

  • Bacterial Proteins
  • Recombinant Proteins
  • Imipenem
  • Ceftriaxone
  • Ampicillin
  • Peptidyl Transferases