Mechanism of Inhibition of beta-site amyloid precursor protein-cleaving enzyme (BACE) by a statine-based peptide.

Inhibition of beta-site amyloid precursor protein-cleaving enzyme by a statine-based inhibitor has been studied using steady state and stopped-flow methods. A slow onset rate of inhibition has been observed under steady state conditions, and a K(i) of 22 nm has been derived using progress curves analysis. Simulation of stopped-flow protein fluorescence transients provided an estimate of the K(d) for initial inhibitor binding of 660 nm. A two-step inhibition mechanism is proposed, wherein slower "tightening up" of the initial encounter complex occurs. Two hypotheses have been proposed in the literature to address the nature of the slow step in the inhibition of aspartic proteases by peptidomimetic inhibitors: a conformational change related to the "flap" movement and displacement of a catalytic water. We compared substrate and inhibitor binding rates under pre-steady-state conditions. Both ligands are likely to cause flap movement, whereas no catalytic water replacement occurs during substrate binding. Our results suggest that both ligands bind to the enzyme at a rate significantly lower than the diffusion limit, but there are additional rate limitations involved in inhibitor binding, resulting in a k(on) of 3.5 x 10(4) m(-)1 s(-)1 for the inhibitor compared with 3.5 x 10(5) m(-)1 s(-)1 for the substrate. Even though specific intermediate formation steps might be different in the productive inhibitor and substrate binding to beta-site amyloid precursor protein-cleaving enzyme, a similar final optimized conformation is achieved in both cases, as judged by the comparable free energy changes (DeltaDeltaG of 2.01 versus 1.97 kcal/mol) going from the initial to the final enzyme-inhibitor or enzyme-substrate complexes.

Presenilin-1 and -2 are molecular targets for gamma-secretase inhibitors.

Presenilins are integral membrane protein involved in the production of amyloid beta-protein. Mutations of the presenilin-1 and -2 gene are associated with familial Alzheimer's disease and are thought to alter gamma-secretase cleavage of the beta-amyloid precursor protein, leading to increased production of longer and more amyloidogenic forms of A beta, the 4-kDa beta-peptide. Here, we show that radiolabeled gamma-secretase inhibitors bind to mammalian cell membranes, and a benzophenone analog specifically photocross-links three major membrane polypeptides. A positive correlation is observed among these compounds for inhibition of cellular A beta formation, inhibition of membrane binding and cross-linking. Immunological techniques establish N- and C-terminal fragments of presenilin-1 as specifically cross-linked polypeptides. Furthermore, binding of gamma-secretase inhibitors to embryonic membranes derived from presenilin-1 knockout embryos is reduced in a gene dose-dependent manner. In addition, C-terminal fragments of presenilin-2 are specifically cross-linked. Taken together, these results indicate that potent and selective gamma-secretase inhibitors block A beta formation by binding to presenilin-1 and -2.

Helicobacter pylori-selective antibacterials based on inhibition of pyrimidine biosynthesis.

We report the discovery of a class of pyrazole-based compounds that are potent inhibitors of the dihydroorotate dehydrogenase of Helicobacter pylori but that do not inhibit the cognate enzymes from Gram-positive bacteria or humans. In culture these compounds inhibit the growth of H. pylori selectively, showing no effect on other Gram-negative or Gram-positive bacteria or human cell lines. These compounds represent the first examples of H. pylori-specific antibacterial agents. Cellular activity within this structural class appears to be due to dihydroorotate dehydrogenase inhibition. Minor structural changes that abrogate in vitro inhibition of the enzyme likewise eliminate cellular activity. Furthermore, the minimum inhibitory concentrations of these compounds increase upon addition of orotate to the culture medium in a concentration-dependent manner, consistent with dihydroorotate dehydrogenase inhibition as the mechanism of cellular inhibition. The data presented here suggest that targeted inhibition of de novo pyrimidine biosynthesis may be a valuable mechanism for the development of antimicrobial agents selective for H. pylori.

Molecular basis for the binding of SH3 ligands with non-peptide elements identified by combinatorial synthesis.

BACKGROUND: Protein-structure-based combinatorial chemistry has recently been used to discover several ligands containing non-peptide binding elements to the Src SH3 domain. The encoded library used has the form Cap-M1-M2-M3-PLPPLP, in which the Cap and Mi's are composed of a diverse set of organic monomers. The PLPPLP portion provided a structural bias directing the non-peptide fragment Cap-M1-M2-M3 to the SH3 specificity pocket. Fifteen ligands were selected from > 1.1 million distinct compounds. The structural basis for selection was unknown. RESULTS: The solution structures of the Src SH3 domain complexed with two ligands containing non-peptide elements selected from the library were determined by multidimensional NMR spectroscopy. The non-peptide moieties of the ligands interact with the specificity pocket of Src SH3 domain differently from peptides complexed with SH3 domains. Structural information about the ligands was used to design various homologs, whose affinities for the SH3 domain were measured. The results provide a structural basis for understanding the selection of a few optimal ligands from a large library. CONCLUSIONS: The cycle of protein-structure-based combinatorial chemistry followed by structure determination of the few highest affinity ligands provides a powerful new tool for the field of molecular recognition.