Interaction energies between β-lactam antibiotics and E-coli penicillin-binding protein 5 by reversible thermal denaturation

Penicillin-binding proteins (PBPs) catalyze the final stages of bacterial cell wall biosynthesis. PBPs form stable covalent complexes with beta -lactam antibiotics, leading to PBP inactivation and ultimately cell death. To understand more clearly how PBPs recognize beta -lactam antibiotics, it is important to know their energies of interaction. Because beta -lactam antibiotics bind covalently to PBPs, these energies are difficult to measure through binding equilibria. However, the noncovalent interaction energies between beta -lactam antibiotics and a PBP can be determined through reversible denaturation of enzyme-antibiotic complexes. Escherichia coti PBP 5, a D-alanine carboxypeptidase, was reversibly denatured by temperature in an apparently two-state manner with a temperature of melting (T-m) of 48.5 degreesC and a van't Hoff enthalpy of unfolding (DeltaH(VH)) of 193 kcal/mole. The binding of the beta -lactam antibiotics cefoxitin, cloxacillin, moxalactam, and imipenem all stabilized the enzyme significantly, with DeltaT(m) values as high as +4.6 degreesC (a noncovalent interaction energy of +2.7 kcal/mole). interestingly, the noncovalent interaction energies of these ligands did not correlate with their second-order acylation rate constants (k(2)/K'). These rate constants indicate the potency of a covalent inhibitor, but they appear to have Little to do with interactions within covalent complexes, which is the state of the enzyme often used for structure-based inhibitor design.