Enzymatic synthesis of ß-lactams: Constraints and control

The constraint in synthesis of ß-lactams by penicillin acylase is the hydrolysis of the activated acyl donor and the synthetic product. The thermodynamically controlled synthesis of ß-lactams deals with direct acylation of a nucleophile (6APA, 7-ACA or 7-ADCA) by free acids in acidic pH and high concentration of solvent to achieve good yield. On the other hand, the kinetically controlled synthesis with esters is much faster. However, success and yield of kinetically controlled synthesis depend on combined application of an insolubilized catalyst, optimum pH of the reaction, addition of suitable solvent and high concentration of actvated acyl donor and ß-lactam nucleus. Water miscible solvents play an important role in the synthesis. The choice of solvent is guided by its inhibitory effect on enzyme activity and deleterious effect on enzyme stability. Synthetic yield of cefazolin in presence of water-miscible solvent is reduced, while the reaction carried in waterimmiscible solvent markedly improves the yield of cefazolin, possibly due to reduction in hydrolysis of acyl donor and the product. Ethylene glycol inhibits more the hydrolysis of ampicillin than the hydrolysis of PGME and the synthetase/amidase ratio varies depending on the concentration of the solvent used. Hydrolytic vs synthetic activity of penicillin acylase is also influenced by water activity of the insolubilized catalyst. Shuffling of PA-encoding genes has been shown to improve the ßlactam synthesis. Delineation of enzyme-solvent interaction needs close scrutiny to ensure further success in biocatalysis and synthesis of ß-lactams in particular.