Generation of an LFA-1 antagonist by the transfer of the ICAM-1 immunoregulatory epitope to a small molecule.

The protein-protein interaction between leukocyte functional antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) is critical to lymphocyte and immune system function. Here, we report on the transfer of the contiguous, nonlinear epitope of ICAM-1, responsible for its association with LFA-1, to a small-molecule framework. These LFA-1 antagonists bound LFA-1, blocked binding of ICAM-1, and inhibited a mixed lymphocyte reaction (MLR) with potency significantly greater than that of cyclosporine A. Furthermore, in comparison to an antibody to LFA-1, they exhibited significant anti-inflammatory effects in vivo. These results demonstrate the utility of small-molecule mimics of nonlinear protein epitopes and the protein epitopes themselves as leads in the identification of novel pharmaceutical agents.

A minimal peptide scaffold for beta-turn display: optimizing a strand position in disulfide-cyclized beta-hairpins.

Phage display of peptide libraries has become a powerful tool for the evolution of novel ligands that bind virtually any protein target. However, the rules governing conformational preferences in natural peptides are poorly understood, and consequently, structure-activity relationships in these molecules can be difficult to define. In an effort to simplify this process, we have investigated the structural stability of 10-residue, disulfide-constrained beta-hairpins and assessed their suitability as scaffolds for beta-turn display. Using disulfide formation as a probe, relative free energies of folding were measured for 19 peptides that differ at a one strand position. A tryptophan substitution promotes folding to a remarkable degree. NMR analysis confirms that the measured energies correlate well with the degree of beta-hairpin structure in the disulfide-cyclized peptides. Reexamination of a subset of the strand substitutions in peptides with different turn sequences reveals linear free energy relationships, indicating that turns and strand-strand interactions make independent, additive contributions to hairpin stability. Significantly, the tryptophan strand substitution is highly stabilizing with all turns tested, and peptides that display model turns or the less stable C'-C' ' turn of CD4 on this tryptophan "stem" are highly structured beta-hairpins in water. Thus, we have developed a small, structured beta-turn scaffold, containing only natural L-amino acids, that may be used to display peptide libraries of limited conformational diversity on phage.

From peptide to non-peptide. 3. Atropisomeric GPIIbIIIa antagonists containing the 3,4-dihydro-1H-1,4-benzodiazepine-2,5-dione nucleus.

The benzodiazepinedione class of non-peptidal GPIIbIIIa antagonists has been modified to allow the isolation of noninterconverting rotational isomers, or atropisomers, with the aim of examining their structure-activity relationships as compared to active RGD-containing peptides and other non-peptidal antagonists. Resolution of these antagonists was accomplished by the introduction of a tert-butyl group at N1 and a chlorine at C9 on the 3,4-dihydro-1H-1,4-benzodiazepine-2,5-dione nucleus and enantiospecific substitution on the beta-alanine side chain attached to N4. The relative configuration was determined by single-crystal X-ray analysis. Further, conformational analyses using ab initio calculations were performed to assess the conformational preferences about the beta-alanine side chain. The data support a good topographical correlation between the benzodiazepinedione class of antagonists and the "cupped" presentation of the RGD tripeptide sequence found in the cyclic peptide G4120. The relationship between these compounds with other peptidal and non-peptidal antagonists is discussed.

Model peptide studies demonstrate that amphipathic secondary structures can be recognized by the chaperonin GroEL (cpn60).

The molecular chaperone cpn60 binds many unfolded proteins and facilitates their proper folding. Synthetic peptides have been used to probe the question of how cpn60 might recognize such a diverse set of unfolded proteins. Three hybrid peptides were synthesized encompassing portions of the bee venom peptide, apamin, and the sequence KWLAESVRAGK from an amphipathic helix in the NH2-terminal region of bovine rhodanese. Two disulfides connecting cysteine residues hold the peptides in stable helical conformations with unobstructed faces oriented away from the disulfides. Peptides were designed to present either a hydrophobic or hydrophilic face of the amphipathic helix that is similar to the one near the amino terminus of rhodanese. Aggregation of these peptides was detected by measuring 1,1'-bis(4-anilino)napthalene-5,5'-disulfonic acid (bisANS) fluorescence at increasing peptide concentrations, and aggregation was not apparent below 2 microM. Thus, all experiments with the peptides were performed at a concentration of 1 microM. Reducing agents cause these helical peptides to form random coils. Fluorescence anisotropy measurements of fluorescein-labeled peptide with the exposed hydrophobic face yielded a Kd = approximately 106 microM for binding to cpn60, whereas there was no detectable binding of the reduced form. The peptide with the exposed hydrophilic face did not bind to cpn60 in either the oxidized or reduced states. Fluorescence experiments utilizing bisANS as a probe showed that binding of the helical hydrophobic peptide could induce the exposure of hydrophobic surfaces on cpn60, whereas the same peptide in its random coil form had no effect. Thus, binding to cpn60 is favored by a secondary structure that organizes and exposes a hydrophobic surface, a feature found in amphipathic helices. Further, the binding of a hydrophobic surface to cpn60 can induce further exposure of complementary surfaces on cpn60 complexes, thus amplifying interactions available for target proteins.

Inhibition of HIV type 1 infectivity by constrained alpha-helical peptides: implications for the viral fusion mechanism.

Linear peptides derived from the membrane proximal region of the gp41 ectodomain are effective inhibitors of HIV type 1 (HIV-1)-mediated fusion events. These inhibitory peptides lack structure in solution, rendering mechanistic interpretation of their activity difficult. Using structurally constrained analogs of these molecules, we demonstrate that the peptides inhibit infectivity by adopting a helical conformation. Moreover, we show that a specific face of the helix must be exposed to block viral infectivity. Recent crystal structures show that the region of gp41 corresponding to the inhibitory peptides is helical and uses the analogous face to pack against a groove formed by an N-terminal coiled-coil trimer. Our results provide a direct link between the inhibition of HIV-1 infectivity by these peptides and the x-ray structures, and suggest that the conformation of gp41 observed by crystallography represents the fusogenic state. Other agents that block HIV-1 infectivity by binding to this groove may hold promise for the treatment of AIDS.

Hydroxyl and water molecule orientations in trypsin: comparison to molecular dynamic structures.

A comparison is presented of experimentally observed hydroxyl and water hydrogens in trypsin determined from neutron density maps with the results of a 140ps molecular dynamics (MD) simulation. Experimental determination of hydrogen and deuterium atom positions in molecules as large as proteins is a unique capability of neutron diffraction. The comparison addresses the degree to which a standard force-field approach can adequately describe the local electrostatic and van der Waals forces that determine the orientations of these hydrogens. The molecular dynamics simulation, based on the all-atom AMBER force-field, allowed free rotation of all hydroxyl groups and movement of water molecules making up a bath surrounding the protein. The neutron densities, derived from 2.1A D2O-H2O difference Fourier maps, provide a database of 27 well-ordered hydroxyl hydrogens. Virtually all of the simulated hydroxyl orientations are within a standard deviation of the experimentally-observed positions, including several examples in which both the simulation and the neutron density indicate that a hydroxyl group is shifted from a 'standard' rotamer. For the most highly ordered water molecules, the hydrogen distributions calculated from the trajectory were in good agreement with neutron density; simulated water molecules that displayed multiple hydrogen bonding networks had correspondingly broadened neutron density profiles. This comparison was facilitated by development of a method to construct a pseudo 2A density map based on the hydrogen atom distributions from the simulation. The degree of internal water molecules is shown to result primarily from the electrostatic environment surrounding that water molecule as opposed to the cavity size available to the molecule. A method is presented for comparing the discrete observations sampled in a dynamics trajectory with the time-averaged data obtained from X-ray or neutron diffraction studies. This method is particularly useful for statically-disordered water molecules, in which the average location assigned from a trajectory may represent a site of relatively low occupancy.

In vitro characterization of four novel classes of growth hormone-releasing peptide.

Reexamination of the hexapeptide GH-releasing peptide (GHRP-6) structure/function has lead to the development of four novel classes of compound that stimulate GH release. Each class is represented as follows: a pentapeptide, G-7039; a tetrapeptide, G-7134; a pseudotripeptide, G-7502; and a rigid cyclic heptapeptide, G-7203. The EC50 values for these compounds, determined by GH dose-response curves using primary cultures of rat pituitary cells, were 0.18, 0.34, 10.6, and 0.43 nM, respectively. To demonstrate that these compounds were acting at the putative GHRP receptor, challenges were made using combinations that included GHRP-6 and GH-releasing hormone (GHRH). All four new classes further increased GH release in combination with GHRH, but not with GHRP-6. Homologous desensitization occurred after 45 min of exposure to the new compounds while the cells remained sensitive to GHRH. Somatostatin inhibited all of these compounds. Additionally, G-7039 elevated free calcium, as occurs with GHRP-6. All four classes elicited a robust GH release, a small increase in PRL, and no change in LH, FSH, ACTH, or TSH. We conclude that these novel compounds are potent and direct stimulators of pituitary GH release, with in vitro attributes that suggest mediation via a specific GHRP-like mechanism.

Growth hormone secretagogues: characterization, efficacy, and minimal bioactive conformation.

Another class of growth hormone (GH) secretagogues has been discovered by altering the backbone structure of a flexible linear GH-releasing peptide (GHRP). In vitro and in vivo characterization confirms these GH secretagogues as the most potent and smallest (M(r) < 500) reported. Anabolic efficacy is demonstrated in rodents with intermittent delivery. A convergent model of the bioactive conformation of GHRPs is developed and is supported by the NMR structure of a highly potent cyclic analog of GHRP-2. The model and functional data provide a logical framework for the further design of low-molecular weight secretagogues and illustrate the utility of an interdisciplinary approach to elucidating potential bound-state conformations of flexible peptide ligands.

A comparison of neutron diffraction and molecular dynamics structures: hydroxyl group and water molecule orientations in trypsin.

A comparison is presented of experimentally observed hydroxyl and water hydrogen atoms in trypsin determined from neutron density maps with the results of a 140 ps molecular dynamics simulation. Experimental determination of hydrogen and deuterium atom positions in molecules as large as proteins is a unique capability of neutron diffraction. The comparison addresses the degree to which a standard force-field approach can adequately describe the local electrostatic and van der Waals forces that determine the orientations of these hydrogen atoms. The molecular dynamics simulation, based on the all-atom AMBER force-field, allowed free rotation of all hydroxyl groups and movement of water molecules making up a bath surrounding the protein. The neutron densities, derived from 2.1 A 2H2O-H2O difference Fourier maps, provide a database of 27 well-ordered hydroxyl hydrogen atoms. Virtually all of the simulated hydroxyl orientations are within a standard deviation of the experimentally observed positions, including several examples in which both the simulation and the neutron density indicate that a hydroxyl group is shifted from a "standard" rotamer. For the most highly ordered water molecules, the hydrogen distributions calculated from the trajectory were in good agreement with neutron density; simulated water molecules that displayed multiple hydrogen-bonding networks had correspondingly broadened neutron density profiles. This comparison was facilitated by development of a method to construct a pseudo 2 A density map based on the hydrogen atom distributions from the simulation. This method is particularly useful for statically disordered water molecules, in which the average location assigned from a trajectory may represent a site of relatively low occupancy. The degree of disorder of internal water molecules is shown to result primarily from the electrostatic environment surrounding that water molecule as opposed to the cavity size available to the molecule.

Benzodiazepine peptidomimetic inhibitors of farnesyltransferase.

A structural survey of protein Zn2+ binding geometries was instigated based upon the functional requirement of Ras farnesyltransferase for Zn2+. The Cys-X-X-Cys motif found in Zn(2+)-binding proteins such as aspartate transcarbamylase was used as a template to devise a bidentate-coordination model for Cys-A1-A2-X peptide inhibitors. Accordingly, replacement of the central dipeptide with the hydrophobic scaffold 3-amino-1-carboxymethyl-2,3-dihydro-5- phenyl-1H-1,4-benzodiazepin-2-one (BZA) yielded a peptidomimetic inhibitor, Cys(BZA)Met, of moderate potency (IC50 = 400 nM). N-Methylation of the cysteine amide improved potency almost 100-fold (IC50 = 0.3-1 nM). The increased affinity presumably correlates with a preferred conformation of the inhibitor which maximizes a hydrophobic interaction between the scaffold and the enzyme, and the proper presentation of cysteine and methionine to allow bidentate coordination at Zn2+. These non-peptide inhibitors have been shown to block farnesylation of the Ras protein in intact cells and provide lead compounds for the development of new cancer therapeutic agents.

Solution conformation of an atrial natriuretic peptide variant selective for the type A receptor.

Two-dimensional NMR spectroscopy has been used to characterize the solution conformation of an atrial natriuretic peptide (ANP) variant which is selective for the human natriuretic peptide receptor A (NPR-A) relative to receptor C (NPR-C). The ANP mutant, containing six substitutions, has reduced flexibility in aqueous solution relative to wild-type ANP and allows the observation of sufficient NOE connectivities for structure determination by distance geometry and restrained molecular dynamics calculations. The solution conformation is reasonably well defined, having an average backbone atom rms deviation from the average coordinates of approximately 1.1 A for residues 7-27. The structure is consistent with available functional data and shows a spatial separation between known receptor binding determinants and residues found to be outside the hormone-receptor interface.

Structural and functional aspects of RGD-containing protein antagonists of glycoprotein IIb-IIIa.

In the past few years, knowledge of the structure and function of RGD-containing protein antagonists of the platelet fibrinogen receptor glycoprotein IIb-IIIa has advanced rapidly. The RGD sequence is found at the apex of extended, solvent accessible, and conformationally flexible loops in proteins of vastly different structural frameworks. Conformation of the RGD epitope and the immediate surrounding sequence are critical factors affecting potency and selectivity as integrin antagonists. The RGD sequence has recently been introduced into a number of protein and peptide scaffolds.

Benzodiazepine peptidomimetics: potent inhibitors of Ras farnesylation in animal cells.

Oncogenic Ras proteins transform animal cells to a malignant phenotype only when modified by farnesyl residues attached to cysteines near their carboxyl termini. The farnesyltransferase that catalyzes this reaction recognizes tetrapeptides of the sequence CAAX, where C is cysteine, A is an aliphatic amino acid, and X is a carboxyl-terminal methionine or serine. Replacement of the two aliphatic residues with a benzodiazepine-based mimic of a peptide turn generated potent inhibitors of farnesyltransferase [50 percent inhibitory concentration (IC50) < 1 nM]. Unlike tetrapeptides, the benzodiazepine peptidomimetics enter cells and block attachment of farnesyl to Ras, nuclear lamins, and several other proteins. At micromolar concentrations, these inhibitors restored a normal growth pattern to Ras-transformed cells. The benzodiazepine peptidomimetics may be useful in the design of treatments for tumors in which oncogenic Ras proteins contribute to abnormal growth, such as that of the colon, lung, and pancreas.

Mambin, a potent glycoprotein IIb-IIIa antagonist and platelet aggregation inhibitor structurally related to the short neurotoxins.

The purification, complete amino acid sequence, functional activity, and structural modeling are described for mambin, a platelet glycoprotein GP IIb-IIIa antagonist and potent inhibitor of platelet aggregation from the venom of the Elapidae snake Dendroaspis jamesonii (Jameson's mamba). Mambin is 59 residues in length and contains four disulfide linkages and an RGD amino acid sequence found in protein ligands that bind to GP IIb-IIIa. Mambin inhibits ADP-induced platelet aggregation (IC50 = 172 +/- 22 nM) and inhibits the binding of purified platelet fibrinogen receptor GP IIb-IIIa to immobilized fibrinogen (IC50 = 3.1 +/- 0.8 nM). Mambin has very little sequence similarity to the Viperidae family of platelet aggregation inhibitors, except for the RGD-containing region in the protein. However, mambin does have ca. 47% similarity to the short-chain postsynaptic neurotoxins found in other Elapidae venoms, which do not contain the RGD sequence and do not act as GP IIb-IIIa antagonists. On the basis of its circular dichroism spectrum, mambin has a beta-sheet structure characteristic of the neurotoxins. Molecular modeling of the mambin sequence onto the erabutoxin b structure predicts a very similar structure within the entire protein except for the loop containing the RGD sequence. Mambin may therefore represent a genetic hybrid of neurotoxic and hemotoxic proteins found in snake venoms.

Solar altitude frequency tables.

A table is presented that gives the total number of hours in the year during which the sun's altitude exceeds a given value h, for h = 0-88 degrees in 2 degree increments and for latitudes from the Equator to the North Pole in 2 degree increments. The table also gives corrections to these figures for the effect of atmospheric refraction and the total hours of daylight at each latitude

High-resolution double-resonance spectroscopy of 2v(3) ? v(3) transitions in SF(6).

We assign the 2v(3) ? v(3) absorption transitions seen in a double-resonance pump-probe experiment. We compare the observed frequencies with those derived by calculating the 2v(3) rovibrational energy levels, using spectroscopic constants resulting from a 3v(3) analysis. Some features in the multiple-photon v(3) absorption spectrum of SF(6) can be attributed to two-photon transitions to 2v(3).

CO(2) laser coincidences with nu(3) of SiF(4) near 9.7 microm.

Doppler-limited tunable-diode laser spectra of the stretching fundamental (nu(3)) of (28)SiF(4) at 1031 cm(-1) have been analyzed and the spectroscopic constants determined. Explicit identifications have been made for transitions near CO(2) laser lines between 1023 and 1038 cm(-1); 51 such transitions have been observed in sub-Doppler saturation spectra obtained with a CO(2) laser. The closest observed coincidence is R(53) F(1)(9) of (28)SiF(4), at -1.391 MHz from (12)C(16)O(2)P(30). Implications for isotope-enrichment experiments are discussed.

Spectroscopy of the CF(4) laser.

The frequencies of 28 lines of the (12)CF(4) (nu(2) + nu(4)) ? nu(2) laser have been measured with an accuracy of +/-0.2 cm(-1) for (12)C(16)O(2) pump lines from P(14) to R(24). The CF(4) pump and laser transitions have been identified and the band origin and rotational constant of the nu(2) level determined to be nu(2) = 435.27 +/- 0.06 cm(-1), B(2) = 0.19143 +/- 0.00007 cm(-1). From these constants the laser frequencies expected from any given pumping frequency can be predicted to within +/-0.2 cm(-1).