Cyclic peptides provide attractive lead compounds for drug discovery and excellent molecular probes in biomedical research. Large combinatorial libraries of cyclic peptides can now be routinely synthesized by the split-and-pool method and screened against biological targets. However, post-screening sequence determination of hit peptides has been problematic. In this report, a high-throughput method for the sequence determination of cyclic peptide library members has been developed. TentaGel microbeads (90 mum) were spatially segregated into outer and inner layers; cyclic peptides were displayed on the bead surface, whereas the inner core of each bead contained the corresponding linear peptide as the encoding sequence. After screening of the cyclic peptide library against a macromolecular target, the identity of hit peptides was determined by sequencing the linear encoding peptides inside the bead using a partial Edman degradation/mass spectrometry method. On-bead screening of an octapeptide library (theoretical diversity of 160 000) identified cyclic peptides that bind to streptavidin. A 400-member library of tyrocidine A analogues was synthesized on TentaGel macrobeads and solution-phase screening of the library directly against bacterial cells identified a tyrocidine analogue of improved antibacterial activity. Our results demonstrate that the new method for cyclic peptide sequence determination is reliable, operationally simple, rapid, and inexpensive and should greatly expand the utility of cyclic peptides in biomedical research.
S-Ribosylhomocysteinase (LuxS) cleaves the thioether bond in S-ribosylhomocysteine (SRH) to produce homocysteine (Hcys) and 4,5-dihydroxy-2,3-pentanedione (DPD), the precursor of the type II bacterial quorum sensing molecule (AI-2). The catalytic mechanism of LuxS comprises of three distinct reaction steps. The first step involves carbonyl migration from the C1 carbon of ribose to C2 and the formation of a 2-ketone intermediate. The second step shifts the C=O group from C2 to C3 position to produce a 3-ketone intermediate. In the final step, the 3-ketone intermediate undergoes a β-elimination reaction resulting in the cleavage of the thioether bond. In this work, the 3-ketone intermediate was chemically synthesized and shown to be chemically and kinetically competent in the LuxS catalytic pathway. Substrate analogues halogenated at the C3 position of ribose were synthesized and acted as time-dependent inhibitors of LuxS. The time dependence was caused by enzyme-catalyzed elimination of halide ions. Examination of the kinetics of halide release and decay of the 3-ketone intermediate catalyzed by wild-type and mutant LuxS enzymes revealed that Cys-84 is the general base responsible for proton abstraction in the three reaction steps, whereas Glu-57 likely facilitates substrate binding and proton transfer during catalysis.
S-Ribosylhomocysteinase (LuxS) catalyzes the cleavage of the thioether bond of Sribosylhomocysteine (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 µM.
To further explore the basic structural motifs (3S,6S)‐6‐benzhydryl‐N‐benzyltetrahydro‐2H‐pyran‐3‐amine and (2S,4R,5R)‐2‐benzhydryl‐5‐(benzylamino)tetrahydro‐2H‐pyran‐4‐ol, developed by our research group, for monoamine transport inhibition, we designed and synthesized various structurally altered analogues. The new compounds were tested for their affinities for the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET) in rat brain by measuring their capacity to inhibit the uptake of [3H]DA, [3H]5‐HT, and [3H]NE, respectively. Our results point to novel compounds with a TUI, DNRI, SNRI, or SSRI profile. Among the TUIs, compound 2 g exhibited a balanced potency for all three monoamine transporters (Ki: 60, 79, and 70.3 nM for DAT, SERT, and NET, respectively). In the rat forced swim test, compound 2 g produced a significant decrease in immobility in drug‐treated rats relative to vehicle, indicating a potential antidepressant property.
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