Substituted lactam and cyclic azahemiacetals modulate Pseudomonas aeruginosa quorum sensing.

Quorum sensing (QS) is a population-dependent signaling process bacteria use to control multiple processes including virulence that is critical for establishing infection. The most common QS signaling molecule used by Gram-negative bacteria are acylhomoserine lactones. The development of non-native acylhomoserine lactone (AHL) ligands has emerged as a promising new strategy to inhibit QS in Gram-negative bacteria. In this work, we have synthesized a set of optically pure γ-lactams and their reduced cyclic azahemiacetal analogues, bearing the additional alkylthiomethyl substituent, and evaluated their effect on the AHL-dependent Pseudomonas aeruginosa las and rhl QS pathways. The concentration of these ligands and the simple structural modification such as the length of the alkylthio substituent has notable effect on activity. The γ-lactam derivatives with nonylthio or dodecylthio chains acted as inhibitors of las signaling with moderate potency. The cyclic azahemiacetal with shorter propylthio or hexylthio substituent was found to strongly inhibit both las and rhl signaling at higher concentrations while the propylthio analogue strongly stimulated the las QS system at lower concentrations.

Inhibition of LuxS by S-ribosylhomocysteine analogues containing a [4-aza]ribose ring.

LuxS (S-ribosylhomocysteinase) catalyzes the cleavage of the thioether linkage of S-ribosylhomocysteine (SRH) to produce homocysteine and 4,5-dihydroxy-2,3-pentanedione (DPD), the precursor to a small signaling molecule that mediates interspecies bacterial communication called autoinducer 2 (AI-2). Inhibitors of LuxS should interfere with bacterial interspecies communication and potentially provide a novel class of antibacterial agents. In this work, SRH analogues containing substitution of a nitrogen atom for the endocyclic oxygen as well as various deoxyriboses were synthesized and evaluated for LuxS inhibition. Two of the [4-aza]SRH analogues showed modest competitive inhibition (K(I) ∼40 µM), while most of the others were inactive. One compound that contains a hemiaminal moiety exhibited time-dependent inhibition, consistent with enzyme-catalyzed ring opening and conversion into a more potent species (K(I)(∗)=3.5 µM). The structure-activity relationship of the designed inhibitors highlights the importance of both the homocysteine and ribose moieties for high-affinity binding to LuxS active site.

Inhibition of S-ribosylhomocysteinase (LuxS) by substrate analogues modified at the ribosyl C-3 position.

S-ribosylhomocysteinase (LuxS) catalyzes the cleavage of the thioether bond of S-ribosylhomocysteine (SRH) to produce homocysteine and 4,5-dihydroxy-2,3-pentanedione (DPD), which is the precursor of type 2 autoinducer for bacterial cell-cell communication. In this work, we have synthesized several SRH analogues modified at the ribose C3 position as potential inhibitors of LuxS. While removal or methylation of the C3-OH resulted in simple competitive inhibitors of moderate potency, inversion of the C3 stereochemistry or substitution of fluorine for C3-OH resulted in slow-binding inhibitors of improved potency. The most potent inhibitor showed a K(I)(*) value of 0.43 microM.