Importance of the leucine side-chain to the spasmogenic activity and binding of substance P analogues

Conformational analysis of the C-terminal Gly-Leu-Met-NH2 tripeptide of substance P bound to the NK-1 receptor

We examined the effect of simultaneously incorporating proline or proline-amino acid chimeras in positions 9, 10, and/or 11 of substance P, on the affinity for the two NK-1 binding sites and on second-messenger activation. Because these 3-substituted prolines constrain not only the (phi,psi) values of the peptide backbone, but also the chi space of the amino acid side chain, we were able to gather data on the structural requirements for high-affinity binding to the NK-1 receptor. We were able to confirm that this C-terminal component is crucial and that it should adopt an extended conformation close to a polyproline II structure when bound to the receptor. The partial additivity of these constraints, more specifically, for the NK-1M site, suggests that the peptide backbone flexibility around the hinge-point residue Gly9 is essential to subtly position crucial side chains.

Incorporation of vinylogous scaffolds in the C-terminal tripeptide of substance P

Glycine-9 and leucine-10 of substance P (SP) are critical for (NK)-1 receptor recognition and agonist activity. Propsi(Z)-CH=CH(CH3)-CONH)Leu (or Met) and Propsi((E)-CH=CH(CH3)-CONH)Leu (or Met) have been introduced in the sequence of SP, in order to restrict the conformational flexibility of the C-terminal tripeptide, Gly-Leu-Met-NH2, of SP. Propsi((Z)-CH=C(CH2CH(CH3)2)-CONH)Met-NH2, with an isobutyl substituent to mimic the Leu side-chain, was also incorporated in place of the C-terminal tripeptide. The substituted-SP analogs were tested for their affinity to human NK-1 receptor specific binding sites (NK-1M and NK-1m) and their potency to stimulate adenylate cyclase and phospholipase C in Chinese Hamster ovary (CHO) cells transfected with the human NK-1 receptor. The most potent SP analogs [Pro9psi((Z)CH=C(CH3)CONH)Leu10]SP and [Pro9psi ((E)CH=C(CH3)CONH)Leu10]SP, are about 100-fold less potent than SP on both binding sites and second messenger pathways. These vinylogous (Z)- or (E)-CH=C(CH3)- or (Z)-CH=C(CH2CH(CH3)2) moieties hamper the correct positioning of the C-terminal tripeptide of SP within both the NK-1M- and NK-1m-specific binding sites. The origin of these lower potencies is related either to an incorrect peptidic backbone conformation and/or an unfavorable receptor interaction of the methyl or isobutyl group.

A 25 year adventure in the field of tachykinins

Several aspects of our 25 year adventure in the field of tachykinins will be successively described. They concern: substance P (SP) synthesis and release in the basal ganglia, the identification and pharmacological characterization of central tachykinin NK(1), NK(2) and NK(3) binding sites and their topographical distribution, the description of some new biological tests for corresponding receptors, the identification of tachykinin NK(1) receptor subtypes or conformers sensitive to all endogenous tachykinins (substance P, neurokinin A (NKA), neurokinin B (NKB), neuropeptide gamma (NP gamma) and neuropeptide K (NPK)) and finally, the functional involvement of these receptors and their subtypes in tachykinin-induced regulations of dopamine and acetylcholine release in the striatum.

Characterization of the bioactive conformation of the C-terminal tripeptide Gly-Leu-Met-NH2 of substance P using [3-prolinoleucine10]SP analogues

Residue Leu10 of substance P (SP) is critical for NK-1 receptor recognition and agonist activity. In order to probe the bioactive conformation of this residue, cis- and trans-3-substituted prolinoleucines were introduced in position 10 of SP. The substituted SP analogues were tested for their affinity to human NK-1 receptor specific binding sites (NK-1M and NK-1m) and their potency to stimulate adenylate cyclase and phospholipase C in CHO cells transfected with the human NK-1 receptor. [trans-3-prolinoleucine10]SP retained affinity and potency similar to SP whereas [cis-3-prolinoleucine10]SP shows dramatic loss of affinity and potency. To analyze the structural implications of these biological results, the conformational preferences of the SP analogues were analyzed by NMR spectroscopy and minimum-energy conformers of Ac-cis-3-prolinoleucine-NHMe, Ac-trans-3-prolinoleucine-NHMe and model dipeptides were generated by molecular mechanics calculations. From NMR and modeling studies it can be proposed that residue Leu10 of SP adopts a gauche(+) conformation around the chi1 angle and a trans conformation around the chi2 angle in the bioactive conformation. Together with previously published results, our data indicate that the C-terminal SP tripeptide should preferentially adopt an extended conformation or a PPII helical structure when bound to the receptor.

Structural and biological effects of a beta2- or beta3-amino acid insertion in a peptide

Molecular mechanics calculations on conformers of Ac-HGly-NHMe, Ac-beta2-HAla-NHMe and Ac-beta3-HAla-NHMe indicate that low-energy conformations of the beta-amino acids backbone, corresponding to gauche rotamers around the Calpha-Cbeta bond, may overlap canonical backbone conformers observed for alpha-amino acids. Therefore, Substance P (SP) was used as a model peptide to analyse the structural and biological consequences of the substitution of Phe7 and Phe8 by (R)-beta2-HPhe and of Gly9 by HGly (R)-beta2-HAla or (S)-beta3-HAla. [(R)-beta2-HAla9]SP has pharmacological potency similar to that of SP while [HGly9]SP and [(S)-beta3-HAla9]SP show a 30- to 50-fold decrease in biological activities. The three analogues modified at position 9 are more resistant to degradation by angiotensin converting enzyme than SP and [Ala9]SP. NMR analysis of these SP analogues suggest that a beta-amino acid insertion in position 9 does not affect the overall backbone conformation. Altogether these data suggest that [HGly9]SP, [(S)-beta3-HAla9]SP and [(R)-beta2-HAla9]SP could adopt backbone conformations similar to that of SP, [Ala9]SP and [Pro9]SP. In contrast, incorporation of beta2-HPhe in position 7 and 8 of SP led to peptides that are almost devoid of biological activity. Thus, a beta-amino acid could replace an alpha-amino acid within the sequence of a bioactive peptide provided that the additional methylene group does not cause steric hindrance and does not confine orientations of the side chain to regions of space different from those permitted in the alpha-amino acid.

Analogs of Substance P modified at the C-terminus which are both agonist and antagonist of the NK-1 receptor depending on the second messenger pathway

The initial goal of this study was to analyze, using photolabeling, the interactions between Substance P and its tachykinin NK-1 receptor. Therefore, the photoreactive amino acid para-benzoyl-phenylalanine (pBzl)Phe was incorporated into the Substance P sequence from position 4 to 11 leading to Bapa0[(pBzl)Phex]SP analogs. Biotinyl sulfone-5-aminopentanoic acid (Bapa) was introduced in order to purify the covalent complex. These photoreactive SP analogs were first assayed for their affinity for the two binding sites associated with the NK-1 receptor, as well as for their potency in activating the phospholipase C and adenylate cyclase pathways. All analogs photoreactive from position 4 to 11 have moderate to high affinity for the two NK-1 receptor-binding sites, except for the analog modified at position 7. This affinity could be correlated to their potency to activate the phospholipase C and adenylate cyclase pathways, except for the analog photoreactive at position 11. Bapa0[(pBzl)Phe11]SP was found to be an agonist in the phospholipase C pathway and an antagonist in the adenylate cyclase pathway, other analogs modified at position 11 were therefore analyzed. Among these, Bapa0[Pro9, (pBzl)Hcy(O2)11]SP is a partial agonist, whereas Bapa0[Hcy(ethylaminodansyl)11]SP is a full agonist in the phospholipase C pathway, the two analogs being antagonist in the adenylate cyclase pathway. These results show that analogs of SP can be simultaneously agonist at one binding site and antagonist at the other binding site associated with the NK-1 receptor.

Topographic analysis of the S7 binding subsite of the tachykinin neurokinin-1 receptor

Conformationally and configurationally restricted rotameric probes of phenylalanine have been incorporated in the sequence of substance P (SP)-Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2-for analyzing the binding pockets of Phe7 (S7) and Phe8 (S8), in the neurokinin-1 receptor. These analogues of phenylalanine are (2S. 3R)- and (2S, 3S)-indanylglycines, E- and Z-alpha, beta-dehydrophenylalanines, and 2(S)-alpha, beta-cyclopropylphenylalanines [delta E Phe. delta Z Phe. inverted delta E2 (S) Phe, and inverted delta Z 2 (S) Phe]. Binding data obtained with either conformationally (Ing diastereoisomers) or configurationally (delta E Phe, delta Z Phe) probes have unveiled large differences in the binding potencies of these rotameric probes. With the support of nmr data and energy calculations done on these SP-substituted analogues, we attempt to answer questions inherent to such study. First, none of these six probes prevents the formation of bioactive conformation(s) of the backbone of SP. Second, both diastereoisomers (S, S) and (S, R) of indanylglycine preferentially adopt, in the sequence of SP, the gauche (-) and trans side-chain orientations, respectively, as previously postulated from energy calculations with model peptides. However, in solution, the difference in energy between these rotamers included in the sequence of SP, compared to model peptides, is small since the other rotamer can be detected in [(2S, 3R)Ing7]SP. Finally, from this study we can hypothesize that the large variations observed in the affinities of Phe7 substituted analogues of SP must come from steric hindrance in the S7 binding site, which drastically restricts the space filling around the C alpha-C beta bond of residue 7.

Use of conformationally constrained peptides for a topographical analysis of the combing site of a monoclonal anti-substance P antibody

The topography of the binding site of a monoclonal anti-substance P antibody directed toward the C-terminal pentapeptide of substance P, Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2, was analyzed further using a wide range of constrained analogues of substance P. Results obtained in the present study show the following: (a) The binding subsites of Phe7 and Phe8 are large and deep, accommodating various side chains, including nonaromatic amino acids. (b) In contrast, the binding pockets for Gly-Leu-Met-NH2 appear more restrictive. Consequently, five residues in the peptide are necessary for the high binding affinity to the antibody, the C-terminal tripeptide determining the binding specificity. These data, which appear to contradict those previously published, illustrate the limits of conclusions drawn from studies generally carried out using exclusively Ala-substituted peptides. In addition, the present results indicate that the topography of the binding site of this monoclonal antibody differs from that of the specific substance P neurokinin-1 receptor, in agreement with the differences observed in the fine specificities of these two substance P binding macromolecules.

The mammalian tachykinin receptors

The tachykinins (TKs) are a family of small peptides which share the common C-terminal sequence Phe-X-Gly-Leu-MetNH2. Three peptides of this family, substance P, neurokinin A and neurokinin B, have an established role as neurotransmitters in mammals. 2. Three receptors for TKs have been cloned: they are G-protein coupled receptors with seven putative transmembrane spanning segments and have been termed NK1 (substance P-preferring), NK2 (neurokinin A-preferring) and NK3 (neurokinin B-preferring). 3. Synthetic agonists are available to selectively stimulate only one receptor, while natural TKs can act as full agonist at each one of the three receptors, albeit at different concentrations. 4. A number of potent and selective antagonists, both peptide and nonpeptide in nature, have recently been developed. 5. The introduction of these ligands has revealed an unforeseen pharmacological heterogeneity of NK1, NK2 and NK3 receptors which appears largely, if not exclusively, linked to the existence of species homologues of the three receptors.

Highly potent substance P antagonists substituted with beta-phenyl- or beta-benzyl-proline at position 10

In the guinea-pig ileum tissue, [Pro9]substance P, a tachykinin NK1 receptor selective agonist and septide, [pGlu6,Pro9]-substance P-(6-11), do not interact with the same receptor as shown by the different inhibitory profiles of GR 72251 and [D-Pro9,Pro10,Trp11]substance P. Substitution at position 10 of the D-Pro9-Pro10 moiety with bulky N-methylated amino acids increased the antagonist potency for the tachykinin NK1 receptor without affecting that for the 'septide-sensitive receptor'. The incorporation of a trans-beta-L-substituted proline in position 10, for example a benzyl group (beta-benzyl-L-proline), afforded a potent antagonist active in the nanomolar range. For GR 82334, this increase in potency was obtained at the expense of selectivity for tachykinin NK1 and 'septide-sensitive' receptors.