Mechanistic insights into the inhibition of serine proteases by monocycliclactams

Although originally discovered as inhibitors of pencillin-binding proteins, beta-lactams have more recently found utility as serine protease inhibitor s. Indeed through their ability to react irreversibly with nucleophilic ser ine residues they have proved extraordinarily successful as enzyme inhibito rs. Consequently there has been much speculation as to the reason for the g eneral effectiveness of beta-lactams as antibacterials or inhibitors of hyd rolytic enzymes. The interaction of analogous beta- and gamma-lactams with a serine protease was investigated. Three series of gamma-lactams based upo n monocyclic beta-lactam inhibitors of elastase [Firestone, R. A. et al. (1 990) Tetrahedron 46, 2255-2262.] but with an extra methylene group inserted between three of the bonds in the ring were synthesized. Their interaction with porcine pancreatic elastase and their efficacy as inhibitors were eva luated through the use of kinetic, NMR, mass spectrometric, and X-ray cryst allographic analyses. The first series, with the methylene group inserted b etween C-3 and C-4 of the p-lactam template, were readily hydrolyzed but we re inactive or very weakly active as inhibitors. The second series, with th e methylene group between C-4 and the nitrogen of the beta-lactam template, were inhibitory and reacted reversibly with PPE to form acyl-enzyme comple xes, which were stable with respect to hydrolysis. The third series, with t he methylene group inserted between C-2 and C-3, were not hydrolyzed and we re not inhibitors consistent with lack of binding to PPE. Comparison of the crystal structure of the acyl-enzyme complex formed between PPE and a seco nd series gamma-lactam and that formed between PPE and a peptide [Wilmouth, R. C., et al. (1997) Nat. Struct. Biol. 4, 456-462.] reveals why the compl exes formed with this series were resistant to hydrolysis and suggests ways in which stable acyl-enzyme complexes might be obtained from monocyclic ga mma-lactam-based inhibitors.