Electrosynthesis offers a powerful tool for the formation of anion and cation radical intermediates and for driving clean synthetic reactions without the need for additional chemical reagents.

No abstract

Although originally discovered as inhibitors of pencillin-binding proteins, beta-lactams have more recently found utility as serine protease inhibitors. Indeed through their ability to react irreversibly with nucleophilic serine residues they have proved extraordinarily successful as enzyme inhibitors. Consequently there has been much speculation as to the reason for the general effectiveness of beta-lactams as antibacterials or inhibitors of hydrolytic enzymes. The interaction of analogous beta- and gamma-lactams with a serine protease was investigated. Three series of gamma-lactams based upon monocyclic beta-lactam inhibitors of elastase [Firestone, R. A. et al. (1990) 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 evaluated through the use of kinetic, NMR, mass spectrometric, and X-ray crystallographic analyses. The first series, with the methylene group inserted between C-3 and C-4 of the beta-lactam template, were readily hydrolyzed but were inactive or very weakly active as inhibitors. The second series, with the methylene group between C-4 and the nitrogen of the beta-lactam template, were inhibitory and reacted reversibly with PPE to form acyl-enzyme complexes, which were stable with respect to hydrolysis. The third series, with the methylene group inserted between C-2 and C-3, were not hydrolyzed and were not inhibitors consistent with lack of binding to PPE. Comparison of the crystal structure of the acyl-enzyme complex formed between PPE and a second 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 complexes formed with this series were resistant to hydrolysis and suggests ways in which stable acyl-enzyme complexes might be obtained from monocyclic gamma-lactam-based inhibitors.

A homology derived molecular model of prostate specific antigen (PSA) was created and refined. The active site region was investigated for specific interacting functionality and a binding model postulated for the novel 2-azetidinone acyl enzyme inhibitor 1 (IC(50) = 8.98 +/- 0.90 microM) which was used as a lead compound in this study. A single low energy conformation structure II (Figure 2) was adopted as most likely to represent binding after minimization and dynamics calculations. Systematic analysis of the binding importance of all three side chains appended to the 2-azetidinone was conducted by the synthesis of several analogues. A proposed salt bridge to Lys-145 with 4 (IC(50) = 5.84 +/- 0.92 microM) gave improved inhibition, but generally the binding of the N-1 side chain in a specific secondary aromatic binding site did not tolerate much structural alteration. A hydrophobic interaction of the C-4 side chain afforded inhibitor 6 (IC(50) = 1.43 +/- 0.19 microM), and polar functionality could also be added in a proposed interaction with Gln-166 in 5 (IC(50) = 1.34 +/- 0.05 microM). Reversal of the C-4 ester connectivity furnished inhibitors 7 (IC(50) = 1.59 +/- 0.15 microM), 11 (IC(50) = 3.08 +/- 0.41 microM), and 13 (IC(50) = 2.19 +/- 0.36 microM) which were perceived to bind to PSA by a rotation of 180 degrees relative to the C-4 ester of normal connectivity. Incorporation of hydroxyl functionality into the C-3 side chain provided 16 (IC(50) = 348 +/- 50 nM) with the greatest increase in PSA inhibition by a single modification. Multiple copy simultaneous search (MCSS) analysis of the PSA active site further supported our model and suggested that 18 would bind strongly. Asymmetric synthesis yielded 18 (IC(50) = 226 +/- 10 nM) as the most potent inhibitor of PSA reported to date. It is concluded that our design approach has been successful in developing PSA inhibitors and could also be applied to the inhibition of other enzymes, especially in the absence of crystallographic information.