On the Mechanism of Hormone Recognition and Binding by the CCK-B/Gastrin Receptor

Ligands, their receptors and ... plasma membranes

Ligand-receptor interactions are customarily described by equations that apply to solutes. Yet, most receptors are present in cell membranes so that sufficiently lipophilic ligands could reach the receptor by a two-dimensional approach within the membrane. As summarized in this review, this may affect the ligand-receptor interaction in many ways. Biophysicians calculated that, compared to a three-dimensional approach from the liquid phase, such approach could alter the time the ligands need to find a receptor. Biochemists found that ligand incorporation in lipid bilayers modifies their conformation. This, along with the depth at which the ligands reside in the bilayer, will affect the probability of successful receptor interaction. Novel mechanisms were also introduced, including "exosite" binding and ligand translocation between the receptor's alpha-helical transmembrane domains. Pharmacologists focused attention at ligand concentrations in membrane, their adsorption and release rates and the effects thereof on ligand potency and residence time at the receptor.

Structural characterization of the parathyroid hormone receptor domains determinant for ligand binding

Over the years, the association of peptide ligands to Family B GPCRs (G-protein coupled receptors) has been characterized by a number of experimental and theoretical techniques. For the PTH (parathyroid hormone) ligand-receptor system, important insight has been provided by photoaffinity labelling experiments and the elucidation of direct contact points between ligand and receptor. Our research has focused on the structural elucidation of the receptor domains shown to be involved in the binding of PTH. Employing a combination of carefully designed receptor domains, solution-state NMR carried out in the presence of membrane mimetics and extensive computer simulations, we have obtained a well-resolved model of the ligand-receptor complex for PTH. Here, we review the development of this model and highlight some inherent limitations of the methods employed and their consequences on interpretation of the ligand-receptor model.

Evidence for a C-terminal structural motif in gastrin and its bioactive fragments in membrane mimetic media

The conformational preferences of human little gastrin, [Nle(15)] gastrin-17, and its short analogues, gastrin-4 and [beta-Ala(1)] gastrin-5, which include the C-terminal tetrapeptide sequence Trp-Met-Asp-Phe-NH(2) crucial for gastrin bioactivity, were determined by NMR spectroscopy in aqueous solutions of zwitterionic dodecylphosphocholine micelles. Backbone HN chemical shift temperature variance, Halpha chemical shift deviations and complex non-sequential NOE patterns pointed to the C-terminal of [Nle(15)] gastrin-17 adopting an ordered conformation. Distance geometry calculations and NOE-restrained molecular dynamics simulations in membrane mimetic solvent boxes of decane and water indicated the C-terminal tetrapeptide sequence of all three peptides adopted a similar, well defined structure, with a general type IV beta-turn observed for all three peptides. The conformation of [Nle(15)] gastrin-17 consisted of two short helices between Leu(5)-Glu(9) and Ala(11)-Trp(14), with the one helix terminating in a type I beta-turn spanning Gly(13)-Asp(16). The experimental evidence and conformational characteristics of the three peptides in micellar media support a membrane-associated mechanism of receptor recognition and activation for the gastrin hormone family and furthermore point to a possible biologically relevant structural motif for gastrin activity.

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.

Conformational and Molecular Modeling Studies of β-Cyclodextrin−Heptagastrin and the Third Extracellular Loop of the Cholecystokinin 2 Receptor

The conformational features of a conjugate of the C-terminus of human gastrin (HG[11-17]), the shortest gastrin sequence retaining biological function, with beta-cyclodextrin ([Nle(15)]-HG[11-17]-betaCD) were determined by NMR spectroscopy in an aqueous solution of dodecylphosphocholine (DPC) micelles. The peptide-betaCD conjugate displays a binding affinity and activation profile comparable to those of HG[11-17] at the cholecysokinin 2 (CCK(2)) receptor, the G protein-coupled receptor responsible for the gastrointestinal function of gastrin. The structure of the peptide consisted of a well-defined beta-turn between Gly(13) and Asp(16) of gastrin. The structural preferences of [Nle(15)]-HG[11-17]-betaCD in DPC micelles and the 5-doxylstearate-induced relaxation of the (1)H NMR resonances support a membrane-associated receptor recognition mechanism. Addition of [Nle(15)]-HG[11-17]-betaCD to the third extracellular loop domain of the CCK(2) receptor, CCK(2)-R(352-379), generated a number of intermolecular nuclear Overhauser enhancements (NOEs) and chemical shift perturbations. NOE-restrained MD simulations of the [Nle(15)]-HG[11-17]-betaCD-CCK(2)-R complex produced a topological orientation in which the C-terminus was located in a shallow hydrophobic pocket near the confluence of TM2 and -3. Despite the steric bulk and physicochemical properties of betaCD, the [Nle(15)]-HG[11-17]-betaCD-CCK(2)-R complex is similar to the CCK-8-CCK(2)-R complex determined previously, providing insight into the mode of ligand binding and the role of electrostatic interactions.

The role of segment 32-47 of cholecystokinin receptor type A in CCK8 binding: synthesis, nuclear magnetic resonance, circular dichroism and fluorescence studies

The segment 32-47 of the N-terminal extracellular domain of the type A cholecystokinn receptor, CCK(A)-R(32-47), was synthesized and structurally characterized in a membrane mimicking environment by CD, NMR and molecular dynamics calculations. The region of CCK(A)-R(32-47) encompassing residues 39-46 adopted a well-defined secondary structure in the presence of DPC micelles, whereas the conformation of the N-terminal region (segment 32-37) could not be uniquely defined by the NOE derived distance constraints because of local flexibility. The conformation of the binding domain of CCK(A)-R(32-47) was different from that found for the Intact N-terminal receptor tail, CCK(A)-R(1-47). To assess whether CCK(A)-R(32-47) was still able to bind the nonsulfated cholecystokinin C-terminal octapeptide, CCK8, a series of titrations was carried out in SDS and DPC micelles, and the binding interaction was followed by fluorescence spectroscopy. These titrations gave no evidence for complex formation, whereas a high binding affinity was found between CCK(A)-R(1-47) and CCK8. The different affinities for the ligand shown by CCK(A)-R(32-47) and CCK(A)-R(1-47) were paralleled by different interaction modes between the receptor segments and the micelles.The interaction of CCK(A)-R(32-47) with DPC micelles was much weaker than that of CCK(A)-R(1-47), because the former receptor segment lacks proper stabilizing contacts with the micelle surface. In the case of SDS micelles CCK(A)-R(32-47] was found to form non-micellar adducts with the detergent that prevented the onset of a functionally significant Interaction between the receptor segment and the micelle. It is concluded that tertiary structure interactions brought about by the 1-31 segment play a key role in the stabilization of the membrane bound, biologically active conformation of the N-terminal extracellular tail of the CCKA receptor.

Residue 19 of the parathyroid hormone: structural consequences

Residue 19 of the parathyroid hormone (PTH) has been shown to play an important role in both binding to and activation of the PTH receptor; specifically, Arg(19)-containing analogues have improved biological function over similar Glu(19) peptides [Shimizu et al. (2002) Biochemistry 41, 13224-13233]. Additionally the juxtamembrane portion of the receptor is involved in the different biological responses. Here, we determine the conformational preferences of PTH analogues to provide a structural basis for their biological actions. On the basis of circular dichroism results, the Arg(19) --> Glu(19) mutations within the context of both PTH(1-20) and PTH(1-34) analogues lead to increases in helix content, ranging from a 8-15% increase. High-resolution structures as determined by (1)H NMR and NOE-restrained molecular dynamics simulations clearly illustrate the difference between Arg(19) and Glu(19)-PTH(1-20), particularly with the extent and stability of the C-terminal helix. The Arg(19)-containing analogue has a well defined, stable alpha-helix from Ser(4)-Arg(19), while the Glu(19) analogue is less ordered at the C-terminus. On the basis of these observations, we propose that position 19 of PTH(1-20) must be alpha-helical for optimal interaction with the juxtamembrane portion of the receptor. This mode of binding extends the current view of PTH binding (indeed ligand binding for all class B GPCRs), which invokes a bihelical ligand with the C-terminus of the ligand interacting with the N-terminus of the receptor (responsible for binding) and the N-terminus of the ligand interacting with the seven-helical bundle (leading to receptor activation).

Conformational and molecular modeling studies of sulfated cholecystokinin-15

Conformational features of the C-terminal carboxyamidated pentadecapeptide of CCK (S(19)HRISDRD[SO(4)]-YMGWMDF(33)-NH(2)) were determined by NMR spectroscopy in a zwitterionic membrane-mimetic solvent system, composed of DPC micelles. The C-terminal octapeptide consisted of a well-defined pseudohelix that was nearly identical to the structure previously reported for nonsulfated CCK-8 in the same solvent system. N-terminal amino acids of CCK-15 were highly disordered, with no clear conformational preference. Extensive NOE-restrained molecular dynamics simulations of the CCK-15/CCK(1)-R complex suggested that almost all the experimentally determined intermolecular contact points provided by NMR, site-directed mutagenesis, and photoaffinity labeling could be simultaneously satisfied, when the N-terminus of the ligand is placed in close spatial proximity to the N-terminus of the receptor.

NMR studies of CCK-8/CCK1 complex support membrane-associated pathway for ligand-receptor interaction

The interaction of peptide ligands with their associated G-protein-coupled receptors has been examined by a number of different experimental approaches over the years. We have been developing an approach utilizing high-resolution NMR to determine the structural features of the peptide ligand, well-designed fragments of the receptor, and the ligand-receptor complexes formed upon titration of the peptide hormone. The results from these investigations provide evidence for a membrane-associated pathway for the initial interaction of peptide ligands with the receptor. Here, our results from the investigation of the interaction of CCK-8 with the CCK1 receptor are described. Our spectroscopic results clearly show that both CCK-8 and the regions of CCK1 with which it interacts are closely associated with the zwitterionic interface of the lipids utilized in our solution spectroscopic studies.

Peptide hormone binding to G-protein-coupled receptors: structural characterization via NMR techniques

G-protein-coupled receptors (GPCRs) allow cells to respond to calcium, hormones, and neurotransmitters. Not surprisingly, they currently make up the largest family of validated drug targets. Rational drug design for molecular regulators targeting GPCRs has been limited to theoretical-based computational approaches. X-ray crystallography of intact GPCRs has provided the topological orientation of the seven transmembrane helices, but limited structural information of the extracellular and intracellular loops and protein termini. In this review we detail an NMR-based approach which provides the high-resolution structural features on the extracellular domains of GPCRs and the ligand/receptor complexes formed upon titration of the peptide hormone. The results provide important contact points and a high-resolution description of the ligand/receptor interactions, which may be useful for the rational design of therapeutic agents targeting GPCRs. Recent results from our investigation of the cholecystokinin peptide hormone system are used to highlight this approach.

Structural characterization of lipopeptide agonists for the bradykinin B2 receptor

The conformational features of Pam-Lys(0)-Arg(1)-Pro(2)-Pro(3)-Gly(4)-Phe(5)-Ser(6)-Pro(7)-Phe(8)-Arg(9)-OH (PKD) and Pam-Gly(-1)-Lys(0)-Arg(1)-Pro(2)-Pro(3)-Gly(4)-Phe(5)-Ser(6)-Pro(7)-Phe(8)-Arg(9)-OH (PGKD), the Pam-Lys and Pam-Gly-Lys analogues of bradykinin, have been determined by high-resolution NMR in a zwitterionic lipoid environment. Radical-induced relaxation of the (1)H NMR signals was used to probe the topological orientation of the peptides with respect to the zwitterionic lipid interface. The radical-induced relaxation and molecular dynamics (MD) data indicated that the palmitic acid and N-terminal amino acid residues embed into the micelles, while the rest of the polypeptide chain is closely associated with the water-micelle interface. Throughout the entire nuclear Overhauser effect restrained MD simulation, a nonideal type I beta-turn was observed in the C-terminus of PKD between residues 6 and 9, and a gamma-turn was observed in the C-terminus of PGKD between residues 6 and 7. Therefore, the additional glycine has a dramatic effect on the structural preferences of the biologically important C-terminus, an effect brought about by the interaction with the lipid environment. These structural features are correlated to the biological activity at the bradykinin B2 receptor.

Intermolecular interactions between cholecystokinin-8 and the third extracellular loop of the cholecystokinin A receptor

The interaction of the C-terminal octapeptide of cholecystokinin, CCK-8, with the third extracellular loop of human cholecystokinin-A receptor, CCK(A)-R(329-357), has been probed by high-resolution NMR and extensive computer simulations. The structure of CCK(A)-R(329-357) in the presence of dodecylphosphocholine micelles consists of three alpha-helices, with the first and third corresponding to the extracellular ends of transmembrane (TM) helices 6 and 7. The central helix, residues W335-R345, is found to lie on the zwitterionic surface. Titration with CCK-8 produces a stable complex with a number of intermolecular NOEs between the C-terminus of the ligand (Trp(30), Met(31), Asp(32)) and the interface of TM6 and the third extracellular loop (N333, A334, Y338) of the receptor fragment. The mode of ligand binding based on these intermolecular NOEs is in agreement with a number of published findings from receptor mutagenesis and photoaffinity cross-linking. Utilizing these ligand/receptor points of interaction, the structural features of CCK(A)-R(329-357), and also the structures of CCK-8 and CCK(A)-R(1-47) previously determined, extensive molecular dynamics simulations of the CCK-8/CCK(A)-R complex were carried out. The results provide unique insight into the molecular interactions and forces important for the binding of CCK-8 to CCK(A)-R.

Agonist activity at the kinin B1 receptor: structural requirements of the central tetrapeptide

A series of analogues of desArg(9)-Lys-bradykinin (BK), Lys-Arg-X-Ac(n)c-X-Ser-Pro-Phe, in which the spacer X-Ac(n)c-X replaces the central tetrapeptide Pro-Pro-Gly-Phe of BK, have been synthesized and functionally characterized at the B1 receptor. The 1-aminocycloalkane-1-carboxylic acids (Ac(6)c, Ac(7)c, Ac(8)c, Ac(9)c, Ac(12)c) were incorporated to impart conformational constraint and probe the importance of the hydrophobicity of the residue in the central position. The linker is varied in length (X = Gly, betaAla, gammaAbu) to examine the optimal distance between the biologically important residues at the N- and C-termini. The biological assays indicate that the optimal length is obtained with X = Gly, with reduced activities for the longer linkers. Although the size of the central cyclic amino acid does not significantly alter the biological activity, the hydrophobic residue Ac(n)c which may tether the peptide in the membrane environment is required (Lys-Arg-Gly-Gly-Gly-Ser-Pro-Phe is inactive). Two of the analogues, Lys-Arg-Gly-Ac(7)c-Gly-Ser-Pro-Phe and Lys-Arg-gammaAbu-Ac(7)c-gammaAbu-Ser-Pro-Phe, have been structurally characterized in the presence of a zwitterionic lipid environment by high-resolution NMR. Both compounds have similar structural features, differing greatest in the distance between the termini (9 and 15 A for the Gly- and gammaAbu-containing analogues, respectively). The correlation of the smaller distance with activity at the B1 receptor is in complete accord with the results from our previous examination of Lys-Arg-NH-(CH(2))(11)-CO-Ser-Pro-Phe. With the results from this series of compounds we are beginning to define some of the molecular descriptors important for activity at the B1 BK receptor.

Structure and dynamics of micelle-bound neuropeptide Y: comparison with unligated NPY and implications for receptor selection

The biological importance of the neuropeptide Y (NPY) has steered a number of investigations about its solution structure over the last 20 years. Here, we focus on the comparison of the structure and dynamics of NPY free in solution to when bound to a membrane mimetic, dodecylphosphocholine (DPC) micelles, as studied by 2D (1)H NMR spectroscopy. Both, free in solution and in the micelle-bound form, the N-terminal segment (Tyr1-Glu15) is shown to extend like a flexible tail in solution. This is not compatible with the PP-fold model for NPY that postulates backfolding of the flexible N terminus onto the C-terminal helix. The correlation time (tau(c)) of NPY in aqueous solution, 5.5 (+/-1.0) ns at 32 degrees C, is only consistent with its existence in a dimeric form. Exchange contributions especially enhancing transverse relaxation rates (R(2)) of residues located on one side of the C-terminal helix of the molecule are supposed to originate from dimerization of the NPY molecule. The dimerization interface was directly probed by looking at (15)N-labeled NPY/spin-labeled [TOAC34]-[(14)N]-NPY heterodimers and revealed both parallel and anti-parallel alignment of the helices. The NMR-derived three-dimensional structure of micelle-bound NPY at 37 degrees C and pH 6.0 is similar but not identical to that free in solution. The final set of 17 lowest-energy DYANA structures is particularly well defined in the region of residues 21-31, with a mean pairwise RMSD of 0.23 A for the backbone heavy atoms and 0.85 A for all heavy atoms. The combination of NMR relaxation data and CD measurements clearly demonstrates that the alpha-helical region Ala18-Thr32 is more stable, and the C-terminal tetrapeptide becomes structured only in the presence of the phosphocholine micelles. The position of NPY relative to the DPC micelle surface was probed by adding micelle integrating spin labels. Together with information from (1)H,(2)H exchange rates, we conclude that the interaction of NPY with the micelle is promoted by the amphiphilic alpha-helical segment of residues Tyr21-Thr32. NPY is located at the lipid-water interface with its C-terminal helix parallel to the membrane surface and penetrates the hydrophobic interior only via insertions of a few long aliphatic or aromatic side-chains. From these data we can demonstrate that the dimer interface of neuropeptide Y is similar to the interface of the monomer binding to DPC-micelles. We speculate that binding of the NPY monomer to the membrane is an essential key step preceeding receptor binding, thereby pre-orientating the C-terminal tetrapeptide and possibly inducing the bio-active conformation.

Molecular complex of cholecystokinin-8 and N-terminus of the cholecystokinin A receptor by NMR spectroscopy

The bimolecular complex of the C-terminal octapeptide of cholecystokinin, CCK-8, with the N-terminus of the CCK(A)-receptor, CCK(A)-R(1-47), has been structurally characterized by high-resolution NMR and computational refinement. The conformation of CCK(A)-R(1-47), within the lipid environment used for the spectroscopic studies, consists of a well-defined alpha-helix (residues 3-9) followed by a beta-sheet stabilized by a disulfide linkage between C18 and C29, leading to the first transmembrane alpha-helix (TM1). Titration of CCK(A)-R(1-47) with CCK-8 specifically affects the NMR signals of W39 of the receptor, in a saturable fashion. This association is specific for CCK-8; no association was observed upon titration of CCK(A)-R(1-47) with other peptide hormones. The ligand/receptor complex was characterized by intermolecular NOEs between Tyr(27) and Met(28) of CCK-8 and W39 of CCK(A)-R(1-47). These findings suggest that CCK-8 binds to CCK(A) with the C-terminus within the seven-helical bundle and the N-terminus of the ligand, projecting out between TM1 and TM7, forming specific interactions with the N-terminus of the CCK(A) receptor. This mode of ligand binding, consistent with published mutagenesis studies, requires variation of the interpretation of recent findings from photoaffinity cross-linking studies.

LVV- and VV-hemorphins: comparative levels in rat tissues

Screening of hemorphins in extracts of rat lung, brain, heart and spleen was carried out. The threshold for detection of hemorphins was 0.01 nmol for spleen and 0.05 nmol for other tissues. Both the content and the composition of hemorphins differed significantly in the tissues analyzed. Heart and lung extracts were rich in these peptides, the content of the most abundant components reaching 16-44 nmol/g of tissue. In contrast, spleen and brain contained much lower amounts of hemorphins, i.e. about 0.3-2.6 nmol/g of tissue. The most represented hemorphin in lung, heart and brain was VV-hemorphin-5, while the content of other members of the hemorphin family depended significantly on the tissue analyzed: lung extract was also rich in LVV-hemorphin-5, heart contained similar amounts of LVV-hemorphin-7 and LVV-hemorphin-5 and brain of LVV-hemorphin-6. In contrast, the hemorphin family in spleen was represented mainly by C-terminally shortened VV-hemorphins, i.e. VV-hemorphin-4 and VV-hemorphin-3. The levels of hemorphins in all cases were sufficient to activate the opioid receptors of the respective tissues.