Fig. 23.24 Unsubstituted penicillic acid 23.48 is quickly cleaved by TEM-1β-lactamase (a). By adding a hydroxymethyl group to the 6-position of 23.49 a compound is obtained that forms a hydrolytically stable acyl–enzyme complex with the enzyme (b). A new crystal structure was determined with this compound (b, lower picture). The hydroxyl group is found at the position where the water molecule (orange sphere) starts its nucleophilic attack on the acyl–enzyme intermediate (a, lower part, modulated structure with the coordinates from the crystal structure of the complex with 23.49). The hydrophobic amino acids such as phenylalanine and tryptophan are found at positions 166 and 170 of the transpeptidases, which are structurally related to the β-lactamases.