Increasing the synthetic performance of penicillin acylase PAS2 by structure-inspired semi-random mutagenesis.

A semi-random mutagenesis approach was followed to increase the performance of penicillin acylase PAS2 in the kinetically controlled synthesis of ampicillin from 6-aminopenicillanic acid (6-APA) and activated D-phenylglycine derivatives. We directed changes in amino acid residues to positions close to the active site that are expected to affect the catalytic performance of penicillin acylase: alpha R160, alpha F161 and beta F24. From the resulting triple mutant gene bank, six improved PAS2 mutants were recovered by screening only 700 active mutants with an HPLC-based screening method. A detailed kinetic analysis of the three most promising mutants, T23, TM33 and TM38, is presented. These mutants allowed the accumulation of ampicillin at 4-5 times higher concentrations than the wild-type enzyme, using D-phenylglycine methyl ester as the acyl donor. At the same time, the loss of activated acyl donor due to the competitive hydrolytic side reactions could be reduced to <20% with the mutant enzymes compared >80% wild-type PAS2. Although catalytic activity dropped by a factor of 5-10, the enhanced synthetic performance of the recovered penicillin acylase variants makes them interesting biocatalysts for the production of beta-lactam antibiotics.