Improvements in the Formation of Cephalosporins from Penicillin G and Other Penicillins by Bioconversion

Chemical ring expansion of penicillin G plus enzymatic removal of the phenylacetyl side chain is currently being used in industry to obtain 7-aminodeacetoxycephalosporanic acid (7-ADCA) that is used for the manufacture of semisynthetic cephalosporins. A biological route requiring only two enzymatic steps (ring expansion and deacylation) might replace this chemical process, thereby reducing costs and environmental problems. Deacetoxycephalosporin C synthase, an iron-dependent and α-ketoglutarate-requiring dioxygenase, catalyses the expansion of the five-membered thiazolidine ring of the penicillin nucleus to form the six-membered dihydrothiazine ring of the cephalosporin nucleus. To improve the substrate specificity of this enzyme on hydrophobic penicillins (e.g., penicillin G, ampicillin, amoxicillin), different strategies have been recently developed. Biotransformation of penicillin G to deacetoxycephalosporin G by resting cells of Streptomyces clavuligerus has been improved by growing the cells in ethanol, eliminating agitation, adding water-immiscible solvents and catalase to the reaction, and using a hybrid strain obtained by directed evolution.