Fluorescence, CD, and NMR studies on spantide, a bombesin and substance P antagonist

The lipid-associated 3D structure of SPA, a broad-spectrum neuropeptide antagonist with anticancer properties

[D-Arg(1), D-Trp(5,7,9), Leu(11)] substance P (SPA) belongs to a family of peptides including antagonist G and SpD that act as broad-spectrum neuropeptide antagonists at several peripheral receptors. The lipid-induced structure of these peptides may be important for the receptor interactions of these analogs. Thus we describe the tertiary structure of SPA in the presence of sodium dodecylsulfate micelles at pH 5.0, and 25 degrees C as determined from two-dimensional (1)H-NMR data recorded at 500 MHz. The resulting three-dimensional structure can be generally described as two type IV nonstandard turns around Arg(1)*, Pro(2), Lys(3), and Pro(4) and Gln(6), Trp(7)*, Phe(8), and Trp(9)* residues, respectively, inserted into the interfacial region of the micelles (the asterisks denote D-form amino acid). These turns juxtapose the N- and C-termini of SPA and may form the basis of this peptide's unique ability to inhibit peptide receptor interactions at multiple receptor types.

Solution conformation of Substance P antagonists-[D-Arg1, D-Trp7,9, Leu11]-SP, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP and [D-Pro2, D-Trp7,9]-SP by CD, NMR and MD simulations

Substance P (SP) is an important neuropeptide involved in pain transmission and induction of inflammation. Its antagonists are being extensively investigated for their non-narcotic analgesic and anti-inflammatory activity. With a view towards better understanding the structural requirements of these analogs for efficient interaction with the SP receptor, the conformation of three SP antagonists [D-Arg1, D-Trp7,9, Leu11]-SP, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP and [D-Pro2, D-Trp7,9]-SP has been studied by CD, NMR and molecular dynamics (MD) simulations. All three peptides exhibit a high dependence of structure on the solvent. The molecules tend to adopt beta-turns in solvents like DMSO and H2O and form helices in a hydrophobic environment. A direct relation between the helix forming potential of these antagonists with their receptor binding potency has been observed.

Conformation induction in melanotropic peptides by trifluoroethanol: fluorescence and circular dichroism study

Conformation induction in the two related peptides, alpha-melanocyte stimulating hormone (alpha-MSH) and delta-melanocyte stimulating hormone (delta-MSH), have been studied in solvent media containing varying percentages of the membrane-mimetic solvent 2,2,2-trifluoroethanol (TFE) using fluorescence and circular dichroism (CD) spectroscopy. Singular value decomposition (SVD) analysis of the CD spectra at different TFE concentrations showed that these spectra can be described as linear combinations of only two distinct basis spectra, corresponding to the peptides in the random-coil and 'folded' conformations. For alpha-MSH the spectrum of the folded state is very similar to the standard spectrum of the alpha-helix, while that for delta-MSH has partial resemblance to the helical spectrum. Fitting the data on ellipticity (at 222 nm) as a function of TFE volume fraction to an equation based on a two-state model describing TFE-induced conformation induction in the peptides gave values of (1.1 +/- 0.4) and (4.2 +/- 0.5) kcal mol-1 for alpha-MSH and delta-MSH, respectively, for the free energy of equilibrium between the helix and coil forms in water. Measurement of fluorescence emission parameters (emission maximum, quantum yield, steady-state anisotropy and mean excited-state lifetime) indicated that the microenvironment around the single tryptophan residues of both peptides changes in like manner with increasing concentration of TFE in the solvent. The similarity of fluorescence behaviour of the peptides suggests that their Trp fluorophores do not participate in secondary structure formation in TFE.

Biological activity and three-dimensional structure of an agonist analog of bombesin

JMV635, a nonapeptide analog of the active terminal nonapeptide segment of bombesin, was tested for its ability to stimulate in vitro amylase release from rat pancreatic acinar cells and to inhibit the binding of gastrin-releasing peptide to rat pancreatic acini. It was found to be a full agonist of bombesin and to recognize the bombesin receptor with moderate potency. The NMR proton assignments of JMV635 were achieved, and the conformations of JMV635 in aqueous solution and in trifluoroethanol at 297 K were determined using two-dimensional COSY, HOHAHA, NOESY and ROESY experiments. In trifluoroethanol, JMV635, like the active part of bombesin, showed a partial alpha-helical structure. These results were confirmed by circular dichroism and refined by restrained molecular dynamic methods. Structure calculations, using the distance and angle restraints obtained from NMR data on JMV635, gave a total of 75 structures which could be aligned to a root mean square deviation of the bond length of 0.007 A and of the valence angle of 1.55 degrees for the backbone atoms of the amino acid residues. The conformation is a well-defined right-handed alpha-helix in the C-terminal Q2-G6 segment and is less structured in the three C-terminal residues.

Conformational structure of bombesin as studied by vibrational and circular dichroism spectroscopy

Raman and Fourier transform infrared (FTIR) spectroscopies and circular dichroism (CD) have been applied to investigate the secondary structure of bombesin in the solid state and in phosphate buffer solution (pH 3.8). At concentrations around 10(-5) M, circular dichroism reveals that bombesin exists as an irregular or disordered conformation. However, the secondary structure of the peptide appears to be a mixture of disordered structure and intermolecular beta-sheets in 0.01 M sodium phosphate buffer when the peptide concentrations are higher than around 6.5 mM. The tendency of bombesin to form aggregated beta-sheet species seems to be originated mainly in the sequence of the residues 7-14, as supported by the Raman spectra and beta-sheet propensities (P beta) of the amino-acid residues. It is the hydrophobic force of this amino-acid sequence, and not a salt bridge effect, that is the factor responsible for the formation of peptide aggregates.

Synthesis and immunological properties of bombesin analogs

Bombesin (Bn, pGlu-Gln-Arg-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2) is one of the most potent peptides, possessing a variety of physiological and pharmacological functions. We find from CD spectroscopy that the eight C-terminal residues of bombesin [Bn(7-14)NH2] have an ordered structure, and replacement of His-12 with Pro of Bn(7-14)NH2 changes the conformation from ordered to a more unordered form. Antibodies to Bn(7-14)NH2 cross-react to Bn and gastrin releasing peptide (GRP) in a dose-dependent manner. Antibodies to the Pro-analog do not recognize Bn or GRP. Substitution of the C-terminal amide by isopropylamide [Bn(7-14)NHC3H7(i)] makes its antibodies more specific to Bn than to GRP. It appears that this region of the peptide is an important antigenic determinant, which makes these antibodies differentiate between BN and GRP.