The Molecular Basis of β-Lactamase Catalysis and Inhibition

The β-lactamases catalyze the hydrolysis of the lactam bond in β-lactams, thus rendering the β-lactam ineffective as an antibiotic. The increasing spread of resistance to β-lactam antibiotics is largely due to this class of enzyme. Mechanistically these enzymes appear to be related to the transpeptidases and carboxypeptidases involved in the synthesis of the bacterial cell wall. Interest in the basic mechanism of action of the β-lactamases has been spurred by the potential for mechanism-based drug design. The past seven years have seen a significant increase in our knowledge of the catalysis and inhibition of the β-lactamases. The presence of an essential, conserved, serine residue which participates in the formation of a covalent acyl-enzyme intermediate in catalysis, inhibition and inactivation by β-lactams has been established. Unfortunately, few additional details regarding the catalytic mechanism are well established. A generalized reaction pathway can be formulated for most β-lactam inhibitors (reversible or irreversible). This scheme involves partitioning of the initially-formed acyl-enzyme by three pathways: 1) hydrolysis leading to turnover, 2) transient inhibition probably involving formation of an imine or enamine acyl-enzyme, or possibly involving a substantial conformational change in some cases, and 3) imine formation followed by additional covalent modification of the enzyme leading to irreversible inactivation. The flux through each of these pathways varies with the nature of the "substrate" and the particular β-lactamase.