Synthesis of antibiotics is catalyzed by AEHs. The enzyme is acylated by an activated acyl compound, e.g. an ester (d-phenylglycine methyl ester). The resulting acyl enzyme can be hydrolyzed, resulting in a carboxylic acid. Alternatively, the acyl enzyme may be aminolyzed by an amino-beta-lactam (eg 7-amino-desacetoxycephalosporanic acid is (7-ADCA)), resulting in the desired antibiotic. The product (D-Phenylglycine) may also react with free enzyme to form acyl enzyme that can be hydrolyzed.