The mechanism for isopenicillin N synthase from density-functional modeling highlights the similarities with other enzymes in the 2-His-1-carboxylate family.
Isopenicillin N synthase (IPNS) catalyzes a key step in the biosynthesis of the important beta-lactam antibiotics penicillins and cephalosporins. Density-functional calculations with the B3LYP functional are used to propose a detailed mechanism for this reaction. The results support the general scheme outlined from experimental observations, with formation of a four-membered beta-lactam ring followed by formation of a five-membered thiazolidine ring. However, an alternative mechanism for the heterolytic O-O bond cleavage and beta-lactam ring formation steps is proposed. The former part involves protonation of the distal oxygen by an iron-bound water ligand. This mechanism highlights the strong similarities that exist between IPNS and other enzymes of the 2-histidine-1-carboxylate family, especially pterin-dependent amino acid hydroxylases and alpha-keto acid-dependent dioxygenases. Both activation of the cysteine beta-C-H bond by an iron-bound superoxo radical and activation of the valine beta-C-H bond by a ferryl-oxo species show reaction barriers close to the experimentally measured one. These results are in agreement with kinetic isotope experiments that suggest both C-H bond activation steps to be partially rate limiting. The ring formation sequence is determined by the relative strengths of the two C-H bonds. Only the ferryl-oxo intermediate is capable of activating the stronger valine beta-C-H bond.
Lundberg
Marcus
M
Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano Nishihiraki-cho, Sakyo, Kyoto 606-8103, Japan.
Siegbahn
Per E M
PE
Morokuma
Keiji
K
D002384:Catalysis; D020134:Catalytic Domain; D004798:Enzymes / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D009842:Oligopeptides / Q000737:chemistry; D010088:Oxidoreductases / Q000737:chemistry*; D010406:Penicillins / Q000138:chemical synthesis / Q000737:chemistry; D013816:Thermodynamics; D053778:Thiazolidines / Q000737:chemistry
D004798:Enzymes; D009842:Oligopeptides; D010406:Penicillins; D053778:Thiazolidines; C011638:5-(2-aminoadipyl)cysteinylvaline; D010088:Oxidoreductases; C038370:isopenicillin N synthetase; C006414:penicillin N