Fig. 3. Overview of serine β-lactamase mechanism and structure of class A enzymes. (A) Reaction scheme showing the main steps in SBL catalysis. In the acylation step, the SBL nucleophilic serine residue attacks the substrate β-lactam ring, yielding an ester-linked acyl–enzyme complex. In the hydrolysis step, the nucleophilic attack of a water molecule onto the acyl–enzyme complex carbonyl occurs, hydrolyzing the antibiotic and releasing the serine. (B) View of a crystal structure of class A SBL TEM-1 (PDB 1M40). The nucleophilic serine, Ser70, is represented as white sticks. α-Helices are shown in green, β-sheets in yellow, and loops in white. The N- and C-termini are indicated. (C) View of the active site of class A SBL GES-1 in which the serine (Ser64; green sticks) is acylated with the carbapenem imipenem (tan sticks) (PDB 4GOG). Conserved active site residues are represented as sticks and are coloured according to local secondary structure elements.