Mechanistic basis for the emergence of catalytic competence against carbapenem antibiotics by the GES family of beta-lactamases.

A major mechanism of bacterial resistance to beta-lactam antibiotics (penicillins, cephalosporins, carbapenems, etc.) is the production of beta-lactamases. A handful of class A beta-lactamases have been discovered that have acquired the ability to turn over carbapenem antibiotics. This is a disconcerting development, as carbapenems are often considered last resort antibiotics in the treatment of difficult infections. The GES family of beta-lactamases constitutes a group of extended spectrum resistance enzymes that hydrolyze penicillins and cephalosporins avidly. A single amino acid substitution at position 170 has expanded the breadth of activity to include carbapenems. The basis for this expansion of activity is investigated in this first report of detailed steady-state and pre-steady-state kinetics of carbapenem hydrolysis, performed with a class A carbapenemase. Monitoring the turnover of imipenem (a carbapenem) by GES-1 (Gly-170) revealed the acylation step as rate-limiting. GES-2 (Asn-170) has an enhanced rate of acylation, compared with GES-1, and no longer has a single rate-determining step. Both the acylation and deacylation steps are of equal magnitude. GES-5 (Ser-170) exhibits an enhancement of the rate constant for acylation by a remarkable 5000-fold, whereby the enzyme acylation event is no longer rate-limiting. This carbapenemase exhibits k(cat)/K(m) of 3 x 10(5) m(-1)s(-1), which is sufficient for manifestation of resistance against imipenem.