Properties of a class C β-lactamase from Serratia marcescens

Abstract

A β-lactamase produced by a penicillin-resistant strain of Serratia marcescens was isolated and purified. The κ(cat.) value for benzylpenicillin was about 5% of that observed for the best cephalosporin substrates. However, the low K(m) of the penam resulted in a high catalytic efficiency (κ(cat.)/K(m)) and the classification of the enzyme as a cephalosporinase might not be completely justified. It also exhibited a low but measurable activity against cefotaxime, cefuroxime, cefoxitin and moxalactam. Substrate-induced inactivation was observed both with a very good (cephalothin) or a very bad (moxalactam) substrate. The active site was labelled by β-iodopenicillanate. Trypsin digestion produced a 19-residue active-site peptide whose sequence clearly allowed the classification of the enzyme as a class C β-lactamase.