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The pituitary adenylate cyclase-activating polypeptide (PACAP) receptor is a class II G protein-coupled receptor that contributes to many different cellular functions including neurotransmission, neuronal survival, and synaptic plasticity. The solution structure of the potent antagonist PACAP (residues 6′–38′) complexed to the N-terminal extracellular (EC) domain of the human splice variant hPAC1-R-short (hPAC1-R S ) was determined by NMR. The PACAP peptide adopts a helical conformation when bound to hPAC1-R S with a bend at residue A18′ and makes extensive hydrophobic and electrostatic interactions along the exposed β-sheet and interconnecting loops of the N-terminal EC domain. Mutagenesis data on both the peptide and the receptor delineate the critical interactions between the C terminus of the peptide and the C terminus of the EC domain that define the high affinity and specificity of hormone binding to hPAC1-R S . These results present a structural basis for hPAC1-R S selectivity for PACAP versus the vasoactive intestinal peptide and also differentiate PACAP residues involved in binding to the N-terminal extracellular domain versus other parts of the full-length hPAC1-R S receptor. The structural, mutational, and binding data are consistent with a model for peptide binding in which the C terminus of the peptide hormone interacts almost exclusively with the N-terminal EC domain, whereas the central region makes contacts to both the N-terminal and other extracellular parts of the receptor, ultimately positioning the N terminus of the peptide to contact the transmembrane region and result in receptor activation.

A novel fluorescent substrate was devised for the sirtuin (SIRT) class of human protein deacetylases comprised of a peptide sequence containing a single acetyl-lysine residue, with a fluorescent group (tetramethylrhodamine-6-carboxylic acid, 6-TAMRA) near the carboxyl terminus and a nonfluorescent quenching group (QSY-7) near the amino terminus. The peptide sequence is modeled after the p53 acetylation site but is unreactive toward trypsin because all other lysine and arginine residues have been replaced by serine. However, the SIRT-deacetylated peptide is readily cleaved by trypsin, resulting in a maximal 30-fold enhancement of the 6-TAMRA fluorescence. Nicotinamide at millimolar concentrations stops the deacetylation but does not inhibit trypsin, and a microtiter plate assay of the SIRTs has been devised using the fluorescent substrate and these reagents. Using this method, the kinetics of the reaction of the cosubstrate nicotinamide adenine dinucleotide and the competitive inhibitor nicotinamide with SIRT1 and SIRT2 has been analyzed. Several nicotinamide analogs have also been tested as inhibitors and found to have much lower affinity for these enzymes than does the parent compound.

Indandiones were identified as a novel class of small molecule inhibitors of hepatitis C virus NS3 serine protease from high throughput screening. We further studied the structure activity relationships and the mechanisms of inhibition for this class of compounds. Our studies revealed two similar, yet different, mechanisms accounting for the apparent indandione inhibition of HCV NS3 protease. In one case, the apparent inhibition results from the chemical breakdown of the parent compound and the subsequent redox chemistry of the compound. Oxidation of the cysteine containing substrate A to a disulfide-linked dimer converts this substrate to a potent, slow-binding inhibitor with a K(i) value of 170 nM. The second class of indandiones appears to react directly with the substrate to form an S-phenyl disulfide adduct with the P1 cysteine. This modification converts the substrate to a slow-binding inhibitor with a K(i) value of 110 nM, a k(on) = 2370 M(-1) s(-1), and k(off) = 2.5 x 10(-4) s(-1). A stable analogue of this latter compound was synthesized that contained a CH(2)-S linkage instead of the S-S linkage. The CH(2)-S compound showed no inhibition at concentrations as high as 40 microM, which suggests an important role for the S-S linkage in the inhibitory mechanism. Cysteine 159, which lies near the active site of the HCV protease, was mutated to serine. The C159S mutant displayed wild-type catalytic activity and susceptibility to inhibition by the S-S linked inhibitor. This result argues against a mechanism involving disulfide exchange between the inhibitor and the sulfhydryl group of C159. The mechanism of inhibition for this S-S linked substrate based inhibitor is likely due to oxidation of cysteines involved in chelation of the structural zinc atom.

The ectodomain of HIV-1 gp41 mediates the fusion of viral and host cellular membranes. The peptide-based drug Enfuvirtide 1 is precedent that antagonists of this fusion activity may act as anti HIV-agents. Here, NMR screening was used to discover non-peptide leads against this target and resulted in the discovery of a new benzamide 1 series. This series is non-peptide, low molecular weight, and analogs have activity in a cell fusion assay with EC50 values ranging 3–41 μM. Structural work on the gp41/benzamide 1 complex was determined by NMR spectroscopy using a designed model peptide system that mimics an open pocket of the fusogenic form of the protein.

Two new proteins of approximately 70 amino acids in length, corresponding to an unnaturally-linked N- and C-helix of the ectodomain of the gp41 protein from the human immunodeficiency virus (HIV) type 1, were designed and characterized. A designed tripeptide links the C-terminus of the C-helix with the N-terminus of the N-helix in a circular permutation so that the C-helix precedes the N-helix in sequence. In addition to the artificial peptide linkage, the C-helix is truncated at its N-terminus to expose a region of the N-helix known as the “Trp-Trp-Ile” binding pocket. Sedimentation, crystallographic, and nuclear magnetic resonance studies confirmed that the protein had the desired trimeric structure with an unoccupied binding site. Spectroscopic and centrifugation studies demonstrated that the engineered protein had ligand binding characteristics similar to previously reported constructs. Unlike previous constructs which expose additional, shallow, non-conserved, and undesired binding pockets, only the single deep and conserved Trp-Trp-Ile pocket is exposed in the proteins of this study. This engineered version of gp41 protein will be potentially useful in research programs aimed at discovery of new drugs for therapy of HIV-infection in humans.

A novel and innovative high-throughput screening assay was developed to identify both activators and inhibitors of AMP-activated protein kinase (AMPK) using microarrayed compound screening (microARCS) technology. Test compounds were arrayed at a density of 8640 on a polystyrene sheet, and the enzyme and peptide substrate were introduced into the assay by incorporating them into an agarose gel followed by placement of the gels onto the compound sheet. Adenosine triphosphate (ATP) was delivered via a membrane, and the phosphorylated biotinylated substrate was captured onto a streptavidin affinity membrane (SAM trade mark ). For detection, the SAM trade mark was removed, washed, and imaged on a phosphor screen overnight. A library of more than 700,000 compounds was screened using this format to identify novel activators and inhibitors of AMPK.