Studies of conformational isomerism in alpha-melanocyte stimulating hormone by design of cyclic analogues

Tetrahydrofuran Calpha-tetrasubstituted amino acids: two consecutive beta-turns in a crystalline linear tripeptide

The synthesis of tetrahydrofuran Calpha-tetrasubstituted amino acids (TAAs) and their effect on the conformation in small peptides are reported. The synthesis starts from the protein amino acid methionine, which is protected at the C and N terminus and converted into the corresponding sulfonium salt by alkylation. Simple base treatment in the presence of an aryl aldehyde leads to the formation of tetrahydrofuran tetrasubstituted Calpha-amino acids in a highly diastereoselective (trans/cis ratio up to 97:3) reaction with moderate to good yields (35-78%) depending on the aldehyde used. Palladium-catalyzed coupling reactions allow a subsequent further functionalization of the TAA. The R,S,S-TAA-Ala dipeptide amide adopts a beta-turn type I conformation, whereas its S,R,S isomer does not. The R,S,S-Gly-TAA-Ala tripeptide amide shows in the solid state and in solution a conformation of two consecutive beta-turn type III structures, stabilized by i+3-->i intramolecular hydrogen bonds.

On-Line Synthesis of Pseudopeptide Library Incorporating a Benzodiazepinone Turn Mimic: Biological Evaluation on MC1 Receptors

Alpha melanocyte stimulating hormone (alpha-MSH) is a widely distributed hormone. This tridecapeptide exhibits various biological activities mediated through different receptors. alpha-MSH binds to the melanocortin-1 receptor (MC1-R), mainly expressed in keratinocytes and melanocytes, inducing melanogenesis and anti-inflammatory processes. The central His-Phe-Arg-Trp tetrapeptide sequence of alpha-MSH is known to form a turn in the bioactive conformation. To find new potent analogs of alpha-MSH, we decided to introduce non-peptide building blocks in the alpha-MSH sequence. Molecular modeling studies showed that two amino acids of the central core sequence could be replaced by the benzodiazepinone building block without loosing the beta-turn conformation. Benzodiazepines are well-known pharmacophores exhibiting a wide scope of biological activities and are described as constrained dipeptide mimics templates. Although numerous synthetic pathways leading to benzodiazepinones have been described in literature, no methodology has 1,4-benzodiazepine-2,5-diones building blocks bearing a free carboxylic acid function and a protected amino function suitable for incorporation into peptide sequences. In this study, we report the synthesis of peptides with a benzodiazepinone moiety obtained directly during the course of solid-phase peptide synthesis (SPPS). This "on-line" strategy leads to the generation of a 54-member pseudo-peptide library of alpha-MSH analogs. After LC/MS purification, binding assays were performed on the MC1 receptor leading to the discovery of several micromolar ligands.

Functional characterization of the modified melanocortin peptides responsible for ligand selectivity at the human melanocortin receptors

The melanocortin system plays an important role in energy homeostasis as well as skin pigmentation, steroidogenesis and exocrine gland function. In this study, we examined eight Ac-His-Phe-Arg-Trp-NH(2) tetrapeptides that were modified at the Phe position and pharmacologically characterized their activities at the human MCR wild-types and their mutants. Our results indicate that at the hMC1R, all D stereochemical modified residues at the Phe position of peptides increase cAMP production in a dose-dependent manner. At the hMC3R, the DPhe peptide dose dependently increases cAMP production but all other three tetrapeptides were not. At the hMC4R, both the DPhe and DNal(1') peptides induce cAMP production. However, both DTyr and DNal(2') were not able to induce cAMP production. Further studies indicated that at the hMC1R M128L mutant receptor, the all D-configured tetrapeptides reduce their potencies as compared to that of hMC1R wild-type. However, at the hMC3R and hMC4R L165M and L133M mutant receptors, the DNal(2') and DTyr tetrapeptides possess agonist activity. These findings indicate that DPhe in tetrapeptide plays an important role in ligand selectivity and specific residue TM3 of the melanocortin receptors is crucial for ligand selectivity.

Interactions of human melanocortin 4 receptor with nonpeptide and peptide agonists

Specific interactions of human melanocortin-4 receptor (hMC4R) with its nonpeptide and peptide agonists were studied using alanine-scanning mutagenesis. The binding affinities and potencies of two synthetic, small-molecule agonists (THIQ, MB243) were strongly affected by substitutions in transmembrane alpha-helices (TM) 2, 3, 6, and 7 (residues Glu(100), Asp(122), Asp(126), Phe(261), His(264), Leu(265), and Leu(288)). In addition, a I129A mutation primarily affected the binding and potency of THIQ, while F262A, W258A, Y268A mutations impaired interactions with MB243. By contrast, binding affinity and potency of the linear peptide agonist NDP-MSH were substantially reduced only in D126A and H264A mutants. Three-dimensional models of receptor-ligand complexes with their agonists were generated by distance-geometry using the experimental, homology-based, and other structural constraints, including interhelical H-bonds and two disulfide bridges (Cys(40)-Cys(279), Cys(271)-Cys(277)) of hMC4R. In the models, all pharmacophore elements of small-molecule agonists are spatially overlapped with the corresponding key residues (His(6), d-Phe(7), Arg(8), and Trp(9)) of the linear peptide: their charged amine groups interact with acidic residues from TM2 and TM3, similar to His(6) and Arg(6) of NDP-MSH; their substituted piperidines mimic Trp(9) of the peptide and interact with TM5 and TM6, while the d-Phe aromatic rings of all three agonists contact with Leu(133), Trp(258), and Phe(261) residues.

Structure-activity relationships of the unique and potent agouti-related protein (AGRP)-melanocortin chimeric Tyr-c[beta-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH2 peptide template

The melanocortin receptor system consists of endogenous agonists, antagonists, G-protein coupled receptors, and auxiliary proteins that are involved in the regulation of complex physiological functions such as energy and weight homeostasis, feeding behavior, inflammation, sexual function, pigmentation, and exocrine gland function. Herein, we report the structure-activity relationship (SAR) of a new chimeric hAGRP-melanocortin agonist peptide template Tyr-c[beta-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) that was characterized using amino acids previously reported in other melanocortin agonist templates. Twenty peptides were examined in this study, and six peptides were selected for (1)H NMR and computer-assisted molecular modeling structural analysis. The most notable results include the identification that modification of the chimeric template at the His position with Pro and Phe resulted in ligands that were nM mouse melanocortin-3 receptor (mMC3R) antagonists and nM mouse melanocortin-4 receptor (mMC4R) agonists. The peptides Tyr-c[beta-Asp-His-DPhe-Ala-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) and Tyr-c[beta-Asp-His-DNal(1')-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) resulted in 730- and 560-fold, respectively, mMC4R versus mMC3R selective agonists that also possessed nM agonist potency at the mMC1R and mMC5R. Structural studies identified a reverse turn occurring in the His-DPhe-Arg-Trp domain, with subtle differences observed that may account for the differences in melanocortin receptor pharmacology. Specifically, a gamma-turn secondary structure involving the DPhe(4) in the central position of the Tyr-c[beta-Asp-Phe-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) peptide may differentiate the mixed mMC3R antagonist and mMC4R agonist pharmacology.

Solution structures of cyclic melanocortin agonists and antagonists by NMR

The melanocortin receptors are involved in many physiological functions, including pigmentation, sexual function, feeding behavior, and energy homeostasis, making them potential targets for drugs to treat obesity, sexual dysfunction, etc. Understanding the conformational basis of the receptor-ligand interactions is crucial to the design of potent and selective ligands for these receptors. The solution structures of the cyclic melanocortin agonists, partial agonist, and antagonists MTII, VJH085, SHU9119, MK5, and MK9 were determined by two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy at pH 4.5 and 25 degrees C in water (90% H(2)O/10% D(2)O). The overall backbone structures of these cyclic alpha-melanocyte-stimulating hormone (alpha-MSH) analogues around the message sequence (His(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)) were similar and reasonably well defined. beta-Turns spanning His(6) and D-Phe(7)/D-Nal(2')(7) were identified in all analogues, and an amphiphilic molecular surface was obtained for the message sequence residues in most structures within the NMR ensembles. The beta-turn, which most closely resembles a type II beta-turn, leads to stacking between the aromatic rings of His(6) and D-Phe(7) in MTII and VJH085. However, no aromatic stacking between His(6) and D-Nal(2')(7) was found in structures of the D-Nal(2')(7)-containing analogues. The difference in the side-chain dispositions of His(6) and D-Nal(2')(7) may be responsible for the reduced potency or antagonist activity of the D-Nal(2')(7)-containing analogues. In addition, our results suggest that the side-chain orientations may also modulate the receptor selectivity. The information found in this study will be useful for the further design of ligands for melanocortin receptors.

Extensive structure-activity studies of lactam derivatives of MT-II and SHU-9119: their activity and selectivity at human melanocortin receptors 3, 4, and 5

The melanocortin system is involved in the regulation of several diverse physiologic pathways. Recently we have demonstrated that replacing His6 by Pro6 in the well-known antagonist SHU-9119 resulted in a potent agonist at the hMC5R (EC50 = 0.072 nm) with full antagonist activity at the hMC3R and the hMC4R. We have designed, synthesized, and pharmacologically characterized a series of peptide analogs of MT-II and SHU-9119 at the human melanocortin receptors MC3R, MC4R and MC5R. All these peptides were modified at position 6 with a Pro instead of a His residue. In this study, we have identified new scaffolds which are antagonists at the hMC4R and hMC3R. Additionally, we have discovered a new selective agonist at the hMC4R, Ac-Nle-c[Asp-Pro-D-Phe-Arg-Trp-Lys]-Pro-Val-NH2 (6, PG-931) which will be useful in further biologic investigations of the hMC4R. PG-931 was about 100-fold more selective for the hMC4R vs. the hMC3R (IC50 = 0.58 and 55 nm, respectively). Some of these new analogs have exceptional biologic potencies at the hMC5R and will be useful in further efforts to differentiate the substructural features responsible for selectivity at the hMC3R, hMC4R, and hMC5R.

A new tri-orthogonal strategy for peptide cyclization

A solid phase tri-orthogonal protection/cleavage strategy that uses acidic, basic, and neutral conditions is described. Strategically protected alpha-azido-gamma-9-fluorenylmethyl-L-glutamate (1) and alpha-azido-epsilon-N-Fmoc-L-lysine (2) were incorporated into growing peptides on Wang resin using a novel azide protection strategy. These residues, separated by 1-3 monomers, were deprotected at the side chains and cyclized via lactam formation. The N-terminus was further functionalized to extend the chain. This method represents a straightforward protocol for peptide cyclization on solid support.

Design and synthesis of highly potent and selective melanotropin analogues of SHU9119 modified at position 6

The melanocortin receptors are involved in several important physiological functions. The potent and enzymatically stable analogues MT-II (Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH(2)) and SHU9119 (Ac-Nle-c[Asp-His-DNal(2')-Arg-Trp-Lys]-NH(2)) are important ligands of these receptors but are relatively nonselective. To differentiate between the physiological functions of these receptors, agonists, and antagonists with improved receptor selectivities are needed. We report here analogues of the well-characterized antagonist SHU9119 in which we replaced His(6) with conformationally constrained amino acids. By this structure-activity study we discovered two important compounds, PG-901 (Ac-Nle(4)-c[Asp(5)-Pro(6)-DNal(2')(7)-Arg(8)-Trp(9)-Lys(10)]-NH(2)) and PG-911 (Ac-Nle(4)-c[Asp(5)-Hyp(6)-DNal(2')(7)-Arg(8)-Trp(9)-Lys(10)]-NH(2)), characterized to be full agonists at the hMC5R (EC(50) = 0.072 nM and 0.031 nM, respectively), but full antagonists at the hMC3R and the hMC4R. We also demonstrated that the relative stereochemistry of the amino acid at the 6-position is critical for activity, and could play an important role in potency as well as in selectivity for the melanocortin receptors.

Membrane-active properties of alpha-MSH analogs: aggregation and fusion of liposomes triggered by surface-conjugated peptides

Reaction of the melanotropin hormone analogs [Nle(4),D-Phe(7)]-alpha-MSH and [Nle(4),D-Phe(7)]-alpha-MSH(4-10), which were extended at their N-terminus by a thiol-functionalized spacer arm, with preformed liposomes containing thiol-reactive (phospho)lipid derivatives resulted in the aggregation of the vesicles and in a partial leakage of their inner contents. This aggregation/leakage effect, which was only observed when the peptides were covalently conjugated to the surface of the liposomes, was correlated with the fusion of the vesicles as demonstrated by the observed decrease in resonance energy transfer between probes in a membrane lipid mixing assay. A limited fusion was confirmed by monitoring the mixing of the liposome inner contents (formation of 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylene bis(pyridinium bromide) complex). The membrane-active properties of the peptides could be correlated with changes in the fluorescence emission spectra of their tryptophan residue, which suggested that after their covalent binding to the outer surface of the liposomes they can partition within the core of the bilayers. A blue shift of 10 nm was observed for [Nle(4),D-Phe(7)]-alpha-MSH which was correlated with an increase in fluorescence anisotropy and with changes in the accessibility of the coupled peptide as assessed by the quenching of fluorescence of its tryptophan residue by iodide (Stern-Volmer plots). These results should be related to the previously described capacity of alpha-MSH, and analogs, to interact with membranes and with the favored conformation of these peptides which, via a beta-turn, segregate their central hydrophobic residues into a domain that could insert into membranes and, as shown here, trigger their destabilization.

Conformational analysis of tripeptide Ac-Lys-Pro-Val-NH2, COOH-terminal sequence of alpha-MSH

Alpha-melanocyte stimulating hormone (alpha-MSH) is an endogenous linear tridecapeptide which interacts with the melanocortin receptors (MC1-R to MC5-R) to mediate its biological effects. Antipyretic and anti-inflammatory activities of alpha-MSH are due to the COOH-terminal peptide sequence, Lys-Pro-Val (alpha-MSH[11-13]). This tripeptide might be useful as a therapeutic agent in the control of fever and inflammatory reactions. With this aim, a theoretical conformational study of the tripeptide has been carried out using molecular dynamics. The obtained conformational space has been classified into families according to the letter-code convention to partition the phi-psi map. The lowest energy conformations of each family were used as templates to design six models of conformationally constrained nonpeptide analogues.

Design of new small cyclic melanocortin receptor-binding peptides using molecular modelling: Role of the His residue in the melanocortin peptide core

The conserved core of melanocyte stimulating hormones (MSH), His-Phe-Arg-Trp, was probed by comparing a cyclic pentapeptide containing His-DPhe-Arg-Trp, with three structurally similar cyclic peptides, that lacked the His residue. All three peptides bound to the MC(1), MC(3), MC(4) and MC(5) receptors with similar affinities. Molecular modelling indicated that the 3D structure of the DPhe-Arg-Trp of all three peptides were closely similar. The data indicate that the His residue of the small rigid cyclic MSH core peptides does not participate in binding with the melanocortin receptors.

Chemical syntheses and biological activities of lactam analogues of alpha-conotoxin SI

Bicyclization represents an effective method for the introduction of conformational constraints into small, biologically important peptides. Several strategies have been developed for the preparation of bicyclic lactam analogues of alpha-conotoxin SI, a 13-residue peptide neurotoxin found in cone snail venom. Four analogues of the natural regioisomer of alpha-conotoxin SI were designed and synthesized, each with one of the two paired cysteines of the parent peptide being replaced by a side-chain lactam bridged glutamic acid/lysine pair. Solid-phase lactamization was studied to determine rates of formation of the two possible loops and to document the extent of dimerization and higher oligomerization. Radioligand binding assays were carried out on all synthesized peptides, including the naturally occurring two-disulfide form, in order to determine their affinities for nicotinic acetylcholine receptors (nAChRs). Replacement of the Cys(2)-Cys(7) loop of alpha-conotoxin SI with a lactam bridge resulted in complete loss of activity, whereas replacement of the Cys(3)-Cys(13) disulfide loop resulted in a approximately 60-fold reduction in affinity for one orientation and a approximately 70-fold increase in affinity for the other. The two active lactam analogues retain the selectivity exhibited by the naturally occurring peptide for the alpha/delta subunit of nAChRs, as judged by competition experiments with the curariform antagonist metocurine.

Modeling of alpha-MSH conformations with implicit solvent

A conformational search for the most probable structures of the hormone alpha-MSH in aqueous solution was performed in order to help determine the structural features necessary for biological activity. The free-energy surface was modeled using methods from integral equation theory, and high-temperature molecular dynamics was used to enhance conformational sampling. Families of low free-energy structures have been found. The minimum energy structure shows a stable beta-turn conformation in the putative message region that is stabilized by a salt bridge between Glu5 and Lys11. The orientation of the side chains reflects the amphiphilic nature of the peptide, and a close interaction between the side chains of the His6, Phe7 and Trp9 was observed. Several structural features observed in the minimum energy structure agree well with experimental results. The conformational features led to a hypothesis of a receptor-hormone interaction model in which the hydrophobic side chains of Phe7 and Trp9 interact with the transmembrane portion of the human melanocortin (MC1) receptor. Also, the positively charged side chain of Arg8 and the imidazole side chain of His6 may interact with the negatively charged portions of the receptor which may even be on the receptor's extracellular loops.

Structure and dynamics of alpha-MSH using DRISM integral equation theory and stochastic dynamics

The structural and dynamical features of the hormone alpha-MSH in solution have been examined over a 100 ns time scale by using free energy molecular mechanics models at room temperature. The free energy surface has been modeled using methods from integral equation theory and the dynamics by the Langevin equation. A modification of the accessible surface area friction drag model was used to calculate the atomic friction coefficients. The molecule shows a stable beta-turn conformation in the message region and a close interaction between the side chains of His6, Phe7, and Trp9. A salt bridge between Glu5 and Arg8 was found not to be a preferred interaction, whereas a Glu5 and Lys11 salt bridge was not sampled, presumably due to relatively high free energy barriers. The message region was more conformationally rigid than the N-terminal region. Several structural features observed here agree well with experimental results. The conformational features suggest a receptor-hormone interaction model where the hydrophobic side chains of Phe7 and Trp9 interact with the transmembrane portion of the MC1 receptor. Also, the positively charged side chain of Arg8 and the imidazole side chain of His6 may interact with the negatively charged portions of the receptor which may even be on the receptor's extracellular loops.