Stabilized NMR structure of human parathyroid hormone(1-34)

The structure of secretin family GPCR peptide ligands: implications for receptor pharmacology and drug development

The secretin family G protein-coupled receptors, characterized by a large N-terminal extracellular domain and seven transmembrane helices, are drug targets in many diseases, including migraine, cardiovascular disease, diabetes, osteoporosis and inflammatory disorders. Their activating ligands are peptides with an average length of 30 amino acids. In this article we review the available structural data for these peptides and how this explains their activity. We emphasize how this information may be used to accelerate the development of new drugs against these receptors.

Backbone dynamics of human parathyroid hormone (1-34): flexibility of the central region under different environmental conditions

The presence of a stable tertiary structure in the bioactive N-terminal portion of parathyroid hormone (PTH), a major hormone in the maintenance of extracellular calcium homeostasis, is still debated. In this work, 15N relaxation parameters of the 33 backbone amides of human PTH(1-34) were determined in phosphate-buffered saline solution (PBS) and in the presence of dodecylphosphocholine (DPC) micelles. The relaxation parameters were analyzed using both the model-free formalism (G. Lipari and A. Szabo, Journal of the American Chemical Society, 1982, Vol. 104, pp. 4546-4549) and the reduced spectral density functions approach (J.-F. Lefevre, K. T. Dayie, J. W. Peng, and G. Wagner, Biochemistry, 1996, Vol. 35, pp. 2674-2686). In PBS, the region around Gly12 possesses a high degree of flexibility and the C-terminal helix is less flexible than the N-terminal one. In the presence of DPC micelles, the mobility of the entire molecule is reduced, but the stability of the N-terminal helix increases relative to the C-terminal one. A point of relatively higher mobility at residue Gly12 is still present and a new site of local mobility at residues 16-17 is generated. These results justify the lack of experimental nuclear Overhauser effect (NOE) restraints with lack of tertiary structure and support the hypothesis that, in the absence of the receptor, the relative spatial orientation of the two N- and C-terminal helices is undefined. The flexibility in the midregion of PTH(1-34), maintained in the presence of the membrane-mimetic environment, may enable the correct relative disposition of the two helices, favoring a productive interaction with the receptor.

Structural studies on 26RFa, a novel human RFamide-related peptide with orexigenic activity

A novel hypothalamic neuropeptide of the RFamide family, comprising 26 amino acids residues and thus termed 26RFa, has been recently characterized in human, and was found to be the endogenous ligand for the orphan G protein-coupled receptor GPR103. Intracerebroventricular injection of 26RFa provokes a robust increase in food intake in rodents. In the present study, we have investigated the solution conformation of 26RFa by using two-dimensional NMR spectroscopy in different media. In water, 26RFa exhibits mainly a random coil conformation although the presence of a nascent helix was detected between residues 6 and 15. In methanol, 26RFa adopts a well-defined conformation consisting of an amphipathic alpha-helical structure (Pro4-Arg17), flanked by two N- and C-terminal disordered regions. The strong conservation, from amphibians to mammals, of the amino acid sequence corresponding to the amphipathic helix and to the C-terminal flexible octapeptide of 26RFa, suggests that these two domains are crucial for the interaction of the peptide with its receptor.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

A structural and mechanistic study of the oxidation of methionine residues in hPTH(1-34) via experiments and simulations

The relationship between the conformational properties of 1-34 human parathyroid hormone [hPTH(1-34)] and the oxidation of its methionine residues, Met8 and Met18, by hydrogen peroxide is analyzed as a function of pH by measuring the rates of oxidation and by performing MD simulations with an explicit representation of water molecules. Between pH 4 and pH 8, both Met8 and Met18 have nearly pH independent rates of oxidation, and Met18 is oxidized at a rate that is 90-100% of that of freeMet and 10-20% faster than that of Met8. We also found that average 2SWCNs calculated from MD simulations correlate well to the rates of oxidation of Met8 and Met18. The use of 2SWCNs is based on the mechanism that we proposed, the water-mediated mechanism, in which water molecules stabilize the transition state via specific interactions, but the transfer of protons (acid-catalyzed mechanism) does not play a role [Chu, J. W., and Trout, B. L. (2004) J. Am. Chem. Soc. 126 (3), 900-908]. Only at very low pH values, pH 1 for the oxidation of freeMet, does the acid-catalyzed oxidation mechanism become important. For the oxidation of Met8 and Met18 in hPTH(1-34), the acid-catalyzed mechanism becomes significant at a higher pH value, pH 2, probably due to the proximity of nearby acidic residues to Met8 (Glu4) and Met18 (Glu22). In this study, we have demonstrated that the chemistry of oxidation and the structure of polypeptides can be correlated via a detailed understanding of the reaction mechanism, appropriate sampling of configurational space, and a suitable choice of a structural property, water coordination number.

Conformational and biological characterization of human parathyroid hormone hPTH(1-34) analogues containing beta-amino acid residues in positions 17-19

The N-terminal 1-34 fragment of parathyroid hormone (PTH) elicits the full spectrum of bone-related biological activities of the intact native sequences. It has been suggested that the structural elements essential for bioactivity are two helical segments located at the N-terminal and C-terminal sequences, connected by hinges or flexible points around positions 12 and 19. In order to assess the relevance of the local conformation around Gly(18) upon biological function, we synthesized and characterized the following human (h) PTH(1-34) analogues containing beta-amino acid residues: [analogues: see text]. Biological activity and binding affinity of analogue I are one order of magnitude lower than those of the parent compound. In analogue II, both binding affinity and biological activity are partially recovered. Analogues III and V have no binding affinity and very low biological activity. Both bioactivity and binding affinity are partially recovered in analogue IV. The conformational properties of the analogues in aqueous solution containing dodecylphosphocholine micelles were studied by CD, 2D-nuclear magnetic resonance and molecular dynamics calculations. The results confirmed the presence in all analogues of two helical segments located at the N-terminal and C-terminal sequences. The insertion of beta-amino acid residues around position 18 does not cause appreciable conformational differences in the five analogues. The differences in biological activity and binding affinity among the five analogues cannot be related to structural differences in the membrane mimetic environment reported in this study. Our results stress the importance of the side-chain functionalities in the sequence 17-19 for biological function.

Structure-function relationship studies of bovine parathyroid hormone [bPTH(1-34)] analogues containing alpha-amino-iso-butyric acid (Aib) residues

The N-terminal 1-34 fragments of the parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) elicit the full spectrum of bone-related biological activities of the intact native sequences. It has been suggested that the structural elements essential for bioactivity are two helical segments located at the N-terminal and C-terminal sequences, connected by hinges or flexible points around positions 12 and 19. In order to assess the relevance of the local conformation around Gly(12) upon biological function, we synthesized and characterized the following PTH(1-34) analogues containing Aib residues: (I) A-V-S-E-I-Q-F-nL-H-N-Aib-G-K-H-L-S-S-nL-E-R-V-E-Nal-L-R-K-K-L-Q-D-V-H-N-Y-NH(2) ([Nle(8,18), Aib(11), Nal(23),Tyr(34)]bPTH(1-34)-NH(2)); (II) A-V-S-E-I-Q-F-nL-H-N-L-Aib-K-H-L-S-S-nL-E-R-V-E-Nal-L-R-K-K-L-Q-D-V-H-N-Y-NH(2) ([Nle(8,18), Aib(12),Nal(23),Tyr(34)]bPTH(1-34)-NH(2)); (III) A-V-S-E-I-Q-F-nL-H-N-L-G-Aib-H-L-S-S-nL-E-R-V-E-Nal-L-R-K-K-L-Q-D-V-H-N-Y-NH(2) ([Nle(8,18), Aib(13), Nal(23),Tyr(34)]bPTH(1-34)-NH(2)); (IV) A-V-S-E-I-Q-F-nL-H-N-Aib-Aib-K-H-L-S-S-nL-E-R-V-E-Nal-L-R-K-K-L-Q-D-V-H-N-YNH(2) ([Nle(8,18), Aib(11,12), Nal(23),Tyr(34)]bPTH(1-34)-NH(2)); (V) A-V-S-E-I-Q-F-nL-H-N-L-Aib-Aib-H-L-S-S-nL-E-R-V-E-Nal-L-R-K-K-L-Q-D-V-H-N-Y-NH(2) ([Nle(8,18), Aib(12,13),Nal(23),Tyr(34)]bPTH(1-34)-NH(2)). (nL= Nle; Nal= L-(2-naphthyl)-alanine; Aib= alpha-amino-isobutyric acid.) The introduction of Aib residues at position 11 in analogue I or at positions 11 and 12 in analogue IV resulted in a 5-20-fold lower efficacy and a substantial loss of binding affinity compared to the parent compound [Nle(8,18), Nal(23),Tyr(34)]bPTH(1-34)-NH(2). Both binding affinity and adenylyl cyclase stimulation activity are largely restored when the Aib residues are introduced at position 12 in analogue II, 13 in analogue III, and 12-13 in analogue V. The conformational properties of the analogues in aqueous solution containing dodecylphosphocholine micelles were studied by CD, two-dimensional (2D) NMR and computer simulations. The results indicated the presence of two helical segments in all analogues, located at the N-terminal and C-terminal sequences. Insertion of Aib residues at positions 12 and 13, or of Aib dyads at positions 11-12 and 12-13, enhances the stability of the N-terminal helix of all analogues. In all analogues the Aib residues are included in the helical segments. These results confirmed the importance of the helical structure in the N-terminal activation domain, as well as of the presence of the Leu(11) hydrophobic side chain in the native sequence, for PTH-like bioactivity.

Purification and characterization of a receptor for human parathyroid hormone and parathyroid hormone-related peptide

The human parathyroid hormone (PTH) receptor (hPTH1R), containing a 9-amino acid sequence of rhodopsin at its C terminus, was transiently expressed in COS-7 cells and solubilized with 0.25% n-dodecyl maltoside. Approximately 18 microg of hPTH1R were purified to homogeneity per mg of crude membranes by single-step affinity chromatography using 1D4, a monoclonal antibody to a rhodopsin epitope. The N terminus of the hPTH1R is Tyr(23), consistent with removal of the 22-amino acid signal peptide. Comparisons of hPTH1R by quantitative immunoblotting and Scatchard analysis revealed that 75% of the receptors in membrane preparations were functional; there was little, if any, loss of functional receptors during purification. The binding affinity of the purified hPTH1R was slightly lower than membrane-embedded hPTH1R (K(d) = 16.5 +/- 1.3 versus 11.9 +/- 1.9 nm), and the purified receptors bound rat [Nle(8,21),Tyr(34)]PTH-(1-34)-NH(2) (PTH-(1-34)), and rat [Ile(5),Trp(23),Tyr(36)]PTHrP-(5-36)-NH(2) with indistinguishable affinity. Maximal displacement of (125)I-PTH-(1-34) binding by rat [alpha-aminoisobutyric acid (Aib)(1,3),Nle(8),Gln(10),Har(11),Ala(12),Trp(14),Arg(19),Tyr(21)]PTH-(1-21)-NH(2) and rat [Aib(1,3),Gln(10),Har(11),Ala(12),Trp(14)]PTH-(1-14)-NH(2) of 80 and 10%, respectively, indicates that both N-terminal and juxtamembrane ligand binding determinants are functional in the purified hPTH1R. Finally, PTH stimulated [(35)S]GTP gamma S incorporation into G alpha(s) in a time- and dose-dependent manner, when recombinant hPTH1R, G alpha(s)-, and beta gamma-subunits were reconstituted in phospholipid vesicles. The methods described will enable structural studies of the hPTH1R, and they provide an efficient and general technique to purify proteins, particularly those of the class II G protein-coupled receptor family.

Bioactivity of PTH/PTHrP analogs lacking the 1-14 N-terminal domain

The N-terminal regions of 1-34 parathyroid hormone (PTH) and 1-34 parathyroid hormone related protein (PTHrP) are thought to be required for full agonist activity of these molecules and for signal transduction by cyclic AMP (cAMP). The C-terminal regions are thought to be involved in receptor binding and protein kinase C activation. In this study, two analogs of PTH/PTHrP lacking the segment 1-14 exhibited agonist activity in opossum kidney (OK) 3B2 cells. Analogs cPTHrP(15-34) and ANA NPY(13-36), an analog of neuropeptide Y, which both have amphipathic alpha helices, inhibited phosphate uptake and stimulated cAMP production in a dose-dependent manner, with half maximal activity in the microM range, compared to the nM range for hPTHrP(1-34) and hPTH(1-34). They also exhibited proportionately lower receptor binding affinities. cAMP production by these analogs was suppressed by the antagonist hPTHrP(7-34). Inhibition of phosphate uptake in response to the analogs was partially suppressed by H-89, but not by bisindolylmaleimide. The analogs also inhibited phosphate uptake and stimulated cAMP in parent OK cells and stimulated cAMP production in UMR-106 cells. These studies present the novel finding that in these cell types, a C-terminal region encompassing PTH/PTHrP(24-31), with the alpha-helical structure maintained, is sufficient for full activity at reduced potency.

Molecular properties of the PTH/PTHrP receptor

The receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) is a G protein-coupled receptor (GPCR) that plays a key role in controlling blood Ca(2+) concentration and endochondral bone formation. This review focuses on the molecular mechanisms by which the receptor recognizes the PTH and PTHrP peptide ligands and transmits their signal across the cell membrane. The available data suggest that there are two principal components to the ligand-receptor interaction. First, a docking interaction between the C-terminal portion of PTH(1-34) and the N-terminal extracellular domain of the receptor; and second, a weaker interaction between the N-terminal portion of the ligand and the juxtamembrane region of the receptor, which induces signal transduction. A full understanding of these processes could lead to new PTH/PTHrP receptor ligands that are effective in controlling diseases of bone and mineral metabolism, such as osteoporosis.

Conformational studies of parathyroid hormone (PTH)/PTH-related protein (PTHrp) chimeric peptides

The N-terminal 1-34 segments of both parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) bind and activate the same membrane-embedded G protein-coupled receptor (PTH1 Rc) present on the surface of cells in target tissues such as bone and kidney. This binding occurs in spite of major differences between the two hormones in their amino acid sequence. Recently, it was shown that in (1-34) PTH/PTHrP hybrid peptides, the N-terminal 1-14 segment of PTHrP is incompatible with the C-terminal 15-34 region of PTH in terms of bioactivity. The sites of incompatibility were identified at positions 5 in PTHrP and 19 in PTH. In the present paper we describe the synthesis, biological evaluation, and conformational characterization of two segmental hybrids: PTHrP(1-27)-[Tyr(34)]bPTH(28-34)-NH(2) (hybrid I) and PTHrP(1-18)-[Nal(23), Tyr(34)]bPTH(19-34)-NH(2) (hybrid II). Hybrid I is as active as PTH(1-34)NH(2) and more than two orders of magnitude more active than hybrid II. The conformational properties of the hybrids were studied in water/trifluoroethanol (TFE) mixtures and in aqueous solutions containing dodecylphosphocholine (DPC) micelles by CD, two-dimensional nmr and computer simulations. Upon addition of TFE to the aqueous solution, both hybrids undergo a coil-helix transition. The helix content in 1:1 water/TFE obtained by CD data is about 75% for both hybrids. In the presence of DPC, helix formation is observed at detergent concentrations above critical micellar concentration and the maximum helix content is of approximately 35 and approximately 30% for hybrid I and II, respectively. Combined nmr analysis, distance geometry, and molecular dynamics calculations suggest that, in both solvent systems, the biologically active hybrid I exhibits two flexible sites, centered at residues 12 and 19, connecting helical segments. The flexibility point at position 19 is not present in the poorly active hybrid II. Our findings support the hypothesis, proposed in our previous work, that in bioactive PTH analogues the presence and location of flexibility points between helical segments are essential for enabling them to fold into the bioactive conformation upon interaction with the PTH1 receptor.

Solution structure of the osteogenic 1-31 fragment of the human parathyroid hormone

The solution conformations of a selectively osteogenic 1-31 fragment of the human parathyroid hormone (hPTH), hPTH(1-31)NH(2), have been characterized by use of very high field NMR spectroscopy at 800 MHz. The combination of the CalphaH proton and (13)Calpha chemical shifts, (3)J(NH)(alpha) coupling constants, NH proton temperature coefficients, and backbone NOEs reveals that the hPTH(1-31)NH(2) peptide has well-formed helical structures localized in two distinct segments of the polypeptide backbone. There are also many characteristic NOEs defining specific side-chain/backbone and side-chain/side-chain contacts within both helical structures. The solution structure of hPTH(1-31)NH(2) contains a short N-terminal helical segment for residues 3-11, including the helix capping residues 3 and 11 and a long C-terminal helix for residues 16-30. The two helical structures are reinforced by well-defined capping motifs and side-chain packing interactions within and at both ends of these helices. On one face of the C-terminal helix, there are side-chain pairs of Glu22-Arg25, Glu22-Lys26, and Arg25-Gln29 that can form ion-pair and/or hydrogen bonding interactions. On the opposite face of this helix, there are characteristic hydrophobic interactions involving the aromatic side chain of Trp23 packing against the aliphatic side chains of Leu15, Leu24, Lys27, and Leu28. There is also a linear array of hydrophobic residues from Val2, to Leu7, to Leu11 and continuing on to residues His14 and Leu15 in the hinge region and to Trp23 in the C-terminal helix. Capping and hydrophobic interactions at the end of the N-terminal and at the beginning of the C-terminal helix appear to consolidate the helical structures into a V-shaped overall conformation for at least the folded population of the hPTH(1-31)NH(2) peptide. Stabilization of well-folded conformations in this linear 1-31 peptide fragment and possibly other analogues of human PTH may have a significant impact on the biological activities of the PTH peptides in general and specifically for the osteogenic/anabolic activities of bone-building PTH analogues.

Structural characterization of peptide hormone/receptor interactions by NMR spectroscopy

The structural characterization of peptide hormones and their interaction with G-protein (guanine nucleotide-binding regulatory protein) coupled receptors by high-resolution nmr is described. The general approaches utilized can be categorized into three different classes based on their target: the ligand, the receptor, and the ligand/receptor complex. Examples of these different approaches, aimed at facilitating the rational design of peptides and peptidomimetics with improved pharmacological profiles, based on work carried out in our own laboratory, are given. In the ligand-based approach, the high-resolution structures of bradykinin analogues allowing for the development of a structure-activity relationship for activation of the B1 receptor are described. Studies targeting the receptor are to a large extent theoretical, based on computational molecular modeling. However, experimentally based structural features provided by high-resolution nmr can be used to great advantage, providing insight into the mechanism of receptor function, as illustrated here with results from parathyroid hormone. A similar combination of theoretical methods, supplemented by high-resolution structures from nmr has been utilized to probe the formation and stabilization of the ligand/receptor complex both for parathyroid hormone and cholecystokinin. In each of these three approaches, the importance of well-designed peptide mimetics and accurate structural analysis by high-resolution nmr, will be highlighted.

Active variants of human parathyroid hormone (1-34) with multiple amino acid substitutions

We used site-directed mutagenesis to construct 55 single-site variants of rhPTH, a recombinantly-expressed form of human parathyroid hormone (1-34) containing three amino acid changes compared to the natural sequence (ML8, ML18 and FY34). We identified several mutations, at residues Lys(13), Glu(19), Val(21), Glu(22), Lys(27) and Asp(30), that increase biological activity by up to 2. 5-fold, as measured by stimulation of adenylate cyclase activity in rat UMR-106 cells. We constructed a series of 15 variants in which two to eight substitutions at these positions were combined, and found that the mutations behaved additively, leading to peptides with significantly enhanced potency. The most active combination variant, with six substitutions (KS13, ES19, VQ21, ES22, KQ27 and DN30), is 15 times more active than the parent molecule. However, the extent to which such combinations increase the activity of the peptide depends critically on the identity of the residues at positions 8 and 18. We constructed two of the combination variants in a variety of sequence backgrounds containing different combinations of leucine, methionine and norleucine at positions 8 and 18. Enhancements in potency were significantly reduced when Met or Nle was present at either of these positions, both in UMR-106 cells and human SaOS-2 cells. A corresponding non-additivity was observed in direct measurements of receptor binding affinity on UMR-106 cells. These results suggest that interactions, either direct or indirect, between certain PTH side chains prevent these mutations from behaving in an additive manner.

Solution structures of human parathyroid hormone fragments hPTH(1-34) and hPTH(1-39) and bovine parathyroid hormone fragment bPTH(1-37)

Parathyroid hormone (PTH) is involved in regulation of the calcium level in blood and has an influence on bone metabolism, thus playing a role in osteoporosis therapy. In this study, the structures of the human PTH fragments (1-34) and (1-39) as well as bovine PTH(1-37) in aqueous buffer solution under near physiological conditions were determined using two-dimensional nuclear magnetic resonance spectroscopy. The overall structure of the first 34 amino acids of these three peptides is virtually identical, exhibiting a short NH(2)-terminal and a longer COOH-terminal helix as well as a defined loop region from His14 to Ser17, stabilized by hydrophobic interactions. bPTH(1-37), which has a higher biological activity, shows a better-defined NH(2)-terminal part. In contrast to NH(2)-terminal truncations, which cause destabilization of helical structure, neither COOH-terminal truncation nor elongation significantly influences the secondary structure. Furthermore, we investigated the structure of hPTH(1-34) in 20% trifluoroethanol solution. In addition to its helix-stabilizing effect, trifluorethanol causes the loss of tertiary hydrophobic interactions.

Conformational studies of parathyroid hormone (PTH)/PTH-related protein (PTHrP) point-mutated hybrids

The N-terminal 1-34 segments of both parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) bind and activate the same membrane receptor in spite of major differences between the two hormones in their amino acid sequence. Recently, it was shown that in (1-34)PTH/PTHrP segmental hybrid peptides, the N-terminal 1-14 segment of PTHrP is incompatible with the C-terminal 15-34 region of PTH leading to substantial reduction in potency. The sites of incompatibility were identified as positions 5 in PTH and 19 in PTHrP. In the present paper we describe the synthesis, biological evaluation, and conformational characterization of two point-mutated PTH/PTHrP 1-34 hybrids in which the arginine residues at positions 19 and 21 of the native sequence of PTHrP have been replaced by valine (hybrid V(21)) and glutamic acid (hybrid E(19)), respectively, taken from the PTH sequence. Hybrid V(21) exhibits both high receptor affinity and biological potency, while hybrid E(19) binds weakly and is poorly active. The conformational properties of the two hybrids were studied in aqueous solution containing dodecylphosphocholine (DPC) micelles and in water/2,2, 2-trifluoroethanol (TFE) mixtures. Upon addition of TFE or DPC micelles to the aqueous solution, both hybrids undergo a coil-helix transition. The maximum helix content in 1 : 1 water/TFE, obtained by CD data for both hybrids, is approximately 80%. In the presence of DPC micelles, the maximum helix content is approximately 40%. The conformational properties of the two hybrids in the micellar system were further investigated by combined 2D-nmr, distance geometry (DG), and molecular dynamics (MD) calculations. The common structural motif, consisting of two helical segments located at N- and C-termini, was observed in both hybrids. However, the biologically potent hybrid V(21) exhibits two flexible sites, centered at residues 12 and 19 and connecting helical segments, while the flexibility sites in the weakly active hybrid E(19) are located at position 11 and in the sequence 20-26. Our findings support the hypothesis that the presence and location of flexibility points between helical segments are essential for enabling the active analogs to fold into the bioactive conformation upon interaction with the receptor.

Conformational studies of a bicyclic, lactam-constrained parathyroid hormone-related protein-derived agonist

The N-terminal 1-34 segments of both parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) bind and activate the same membrane receptor in spite of major differences in their amino acid sequence. The hypothesis was made that they share the same bioactive conformation when bound to the receptor. A common structural motif in all bioactive fragments of the hormone in water/trifluoroethanol mixtures or in aqueous solution containing detergent micelles is the presence of two helical segments at the N- and C-termini of the sequence. In order to stabilize the helical structures, we have recently synthesized and studied the PTHrP(1-34) analog [(Lys13-Asp17, Lys26-Asp30)]PTHrP(1-34)NH2, which contains lactam-constrained Lys-Asp side chains at positions i, i+4. This very potent agonist exhibits enhanced helix stability with respect to the corresponding linear peptide and also two flexible sites at positions 12 and 19 in 1:1 trifluoroethanol/water. These structural elements have been suggested to play a critical role in bioactivity. In the present work we have extended our conformational studies on the bicyclic lactam-constrained analog to aqueous solution. By CD, 2D-NMR and structure calculations we have shown that in water two helical segments are present in the region of the lactam bridges (13-18, and 26-31) with high flexibility around Gly12 and Arg19. Thus, the essential structural features observed in the aqueous-organic medium are maintained in water even if, in this solvent, the overall structure is more flexible. Our findings confirm the stabilizing effect of side-chain lactam constraints on the alpha-helical structure.

Structure study of osteostatin PTHrP[Thr107](107-139)

The structure of chicken osteostatin or parathyroid hormone-related protein (PTHrP) (residues 107-139) containing an Ala/Thr substitution at the N-terminus was studied using two-dimensional proton NMR spectroscopy in an aqueous environment. Osteostatin is a separate circulating domain responsible for a range of activities related to the modulation of bone formation as well as keratinocyte proliferation. Anti-mitogenic properties of osteostatin have been detected in breast cancer cells and cytosolic calcium is used by osteostatin to signal in some neurons through a non-PTH receptor, unlike the separate circulating N-terminal domain. A structural basis for the activity is presented with particular emphasis given to the conformation of the bioactive segment 107-111, forming part of a finger-like projection capable of binding to the non-PTH receptor both in the presence and absence of the remainder of the molecule which appears simply to act as a largely globular carrier.

Molecular characterization of the receptor-ligand complex for parathyroid hormone

Molecular models for the interaction of parathyroid hormone (PTH) with its G-protein-coupled receptors (PTH1 and PTH2) have been developed. The proposed ligand-receptor complex is based on experimental data from spectroscopic investigations of the hormone and receptor fragments as well as theoretical structure predictions based on homology analysis with proteins of known structure. From the insight afforded by the models, biochemical and pharmacological observations can be correlated with specific molecular or atomic interactions. The ligand selectivity of PTH2, specifically the lack of binding of His5-containing analogues, can be ascribed to unfavorable steric interactions (the binding pocket is markedly smaller in PTH2 than PTH1) as well as repulsive Coulombic forces between amino acids of like-charge (a positively charged H384 is located in the binding pocket in PTH2). The model of PTH1 suggests that the constitutive activity observed from the incorporation of a positively charged amino acid at position 223, found at the cytoplasmic end of TM2, is caused by a Coulombic attraction to E465, at the cytoplasmic end of TM7, leading to an association of TM2 and TM7 and thereby ligand-free activation. Additionally, a number of important interactions in the ligand-receptor complex are described along with predictions of the pharmacological profile which will result from specific modifications at these sites. In this regard, the models described here allow for atomic insight into the biochemical data currently available and allow targeting of future mutations to probe specific ligand/receptor interactions and thereby further our understanding of the functioning of this important hormone system.

Comparison of recombinant human PTH(1-34) (LY333334) with a C-terminally substituted analog of human PTH-related protein(1-34) (RS-66271): In vitro activity and in vivo pharmacological effects in rats

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are believed to exert their biological actions through binding and activation of a common cell surface receptor. Recently, an analog of PTHrP (RS-66271), was described that demonstrated reduced binding affinity for the PTH/PTHrP receptor compared with bovine PTH(1-34) but retained equal biological activity. The present study investigated the receptor binding affinities of synthetic RS-66271 and recombinant human PTH(1-34) (LY333334) and compared their in vitro and in vivo pharmacological effects. RS-66271 had one hundredth the activity of PTH(1-34) in competing for the binding of [125I] [Nle8,18, Tyr34]human PTH(1-34) to the human PTH/PTHrP receptor stably expressed in a human kidney cell line. Despite this reduced binding affinity, RS-66271 had equivalent activity in increasing both cAMP production in osteoblast-like cells and bone resorption in neonatal mouse calvariae. However, RS-66271 was 7. 6-fold less active in stimulating inositol phosphate production. For in vivo studies, young, male Fisher rats received a daily subcutaneous dose of either 10 or 40 microg/kg of peptide for 1, 2, or 4 weeks. Volumetric bone mineral density and total bone mineral content of the proximal tibia were determined by peripheral quantitative computerized tomography. Trabecular and cortical bone of the distal femur were analyzed for calcium and dry weight. Lumbar vertebrae (L4-L6) were analyzed by histomorphometry. Trabecular and cortical bone mass showed a dose- and time-dependent increase in the treated animals compared with the controls. These increases were evident as early as 1 week after initiation of dosing. There were no consistent significant differences in the comparative effects of PTH(1-34) and RS-66271 on the measured bone parameters. In conclusion, despite the reduced binding affinity of RS-66271 for the PTH/PTHrP receptor compared with human PTH(1-34), both peptides displayed similar in vitro and in vivo pharmacological effects.

Structure-activity relation of NH2-terminal human parathyroid hormone fragments

Human parathyroid hormone (hPTH) is involved in the regulation of the calcium level in blood. This hormone function is located in the NH2-terminal 34 amino acids of the 84-amino acid peptide hormone and is transduced via the adenylate cyclase and the phosphatidylinositol signaling pathways. It is well known that truncation of the two NH2-terminal amino acids of the hormone leads to complete loss of in vivo normocalcemic function. To correlate loss of calcium level regulatory activity after stepwise NH2-terminal truncation and solution structure, we studied the conformations of fragments hPTH-(2-37), hPTH-(3-37), and hPTH-(4-37) in comparison to hPTH-(1-37) in aqueous buffer solution under near physiological conditions by circular dichroism spectroscopy, two-dimensional nuclear magnetic resonance spectroscopy, and restrained molecular dynamics calculations. All peptides show helical structures and hydrophobic interactions between Leu-15 and Trp-23 that lead to a defined loop region from His-14 to Ser-17. A COOH-terminal helix from Met-18 to at least Leu-28 was found for all peptides. The helical structure in the NH2-terminal part of the peptides was lost in parallel with the NH2-terminal truncation and can be correlated with the loss of calcium regulatory activity.

Solution structure of parathyroid hormone related protein (residues 1-34) containing an Ala substituted for an Ile in position 15 (PTHrP[Ala15]-(1-34))

The structure of human parathyroid hormone (PTH) related protein (residues 1-34) containing an Ala substituted for an Ile in position 15 was studied by two-dimensional proton nuclear magnetic resonance spectroscopy. This mutant retains quite high levels of adenylate cyclase activity based on slightly reduced PTH receptor binding capacity. Three segments of helix were revealed extending from His5 to Lys11, Lys13 to Arg19, and from Phe22 to Thr33/Ala34, with a decided kink between the first two helices around Gly12. N- and C-terminal helices were stabilized by charged and hydrophobic side chain interactions between His5 and Glu30, Asp17 and both His9 and His25, and between Leu8 and Ala29, resulting in a globular molecule occupying a single conformation. While the structure of the entire mid-molecule region differed greatly from the structure of the native peptide, the structure of both N- and C-terminal regions remains essentially unaltered. The residues responsible for initiating signal transduction in the mutant are located in the vicinity of the residues responsible for receptor binding. The C-terminal amphipathic helix forming the receptor binding site exhibits reduced binding as a result of the closely applied N-terminal signal transduction-activating region. Although not contributing directly to receptor binding, the N-terminal region can sterically affect hormone binding through modifications to certain N-terminal side chains.

Comparative study of chicken and human parathyroid hormone-(1-34)-peptides in solution with SDS

Molecular conformations of chicken [cPTH-(1-34)] and human [hPTH-(1-34)] parathyroid hormone fragments in aqueous solutions with various concentrations of SDS were investigated by CD, fluorescence and NMR spectroscopy techniques. In the presence of SDS, chicken and human PTH-(1-34) adopt an a-helical structure making up 32-38% of all the peptide amino acids. The process of the a-helical formation of these two fragments is considerably different. The CD spectral change of hPTH-(1-34) was characteristic of a monotonous increase in the negative peak at 222 nm with increasing SDS concentrations. However, for cPTH-(1-34) a beta-turn is formed first, followed by alpha-helix formation upon an increase in SDS concentrations. The change of the tryptophan fluorescence spectra of cPTH-(1-34) is well correlated with the changes in CD spectra, suggesting that the side chain of Trp23 is involved in the conformational change from random coil to alpha-helix via beta-turn. The three-dimensional structure of cPTH-(1-34) with SDS micelle was elucidated by 1H-NMR at pH 3.8 and 300 K, with the combined use of distance geometry and restrained molecular dynamics calculations. NMR results indicated that it contains two helices encompassing residues 7-12 and 24-30, respectively. The C-terminal helix in the residue range of 24-30 is amphiphilic, which is stabilized by the hydrophobic interactions among Trp23, Leu24 and Lys27.

Conformational studies of RS-66271, an analog of parathyroid hormone-related protein with pronounced bone anabolic activity

Both the parathyroid hormone (PTH) and the functionally similar parathyroid hormone-related protein (PTHrP) have served as templates for the development of novel bone anabolic agents for the treatment of osteoporosis. The PTHrP analog RS-66271 (Vickery, B. H.; Avnur, Z.; Cheng Y.; Chiou, S.-S.; Leaffer, D.; Caulfield, J. P.; Kimmel, D. B.; Ho, T.; Krstenansky, J. L. J. Bone Miner. Res. 1996, 11, 1943-1951), in which the amino acids 22-31 have been substituted by the sequence E22-L-L-E-K-L-L-E-K-L31 (a model amphiphilic peptide), is a potent bone anabolic agent in vivo. Therefore, RS-66271 is a good candidate for structural analysis with the aim of developing a structure-activity relationship. The structural characterization described here was carried out in aqueous solution employing circular dichroism and nuclear magnetic resonance spectroscopy. We find that the incorporated amphiphilic decapeptide is indeed helical. In addition, it induces the adjacent residues, up to residue 16, to adopt the helical conformation. The helical domain, including residues 16-32, incorporates most of the previously identified principal receptor binding domain PTHrP(25-34). We discuss the relevance of the distinct and extensive helicity in light of the reduced in vitro receptor affinity/ activity and the enhanced in vivo bone anabolic efficacy of RS-66271.

Bioactivities and secondary structures of constrained analogues of human parathyroid hormone: cyclic lactams of the receptor binding region

In a search for analogues of human parathyroid hormone (hPTH) with improved activities and bioavailabilities, we have prepared the following three lactam analogues of hPTH-(1-31)-NH2 (1) or [Leu27]hPTH-(1-31)-NH2 (2): [Leu27]cyclo(Glu22-Lys26)-hPTH-(1-31)-NH2 (3), [Leu27]cyclo(Lys26-Asp30)-hPTH-(1-31)-NH2 (4), and cyclo(Lys27-Asp30)-hPTH-(1-31)-NH2 (5). Analogues 1, 2, and 5 had seven or eight residues of alpha-helix, as estimated from their circular dichroism (CD) spectra, in contrast to 12 residues in cyclic analogues 3 and 4. Thus, lactams 3 and 4 stabilized a helix previously shown to exist within residues 17-29. The adenylyl cyclase activity (EC50), measured in rat osteosarcoma 17/2 cells, of 5 (40.3 +/- 2.3 nM) was half that of its linear form 1 (19.9 +/- 3.9 nM). The linear Leu27 mutant 2 was twice as active (11.5 +/- 5.2) as analogue 1, and lactam analogue 3 was 6-fold more active (3.3 +/- 0.3 nM). Lactam analogue 4 had less activity (16.9 +/- 3.3 nM) than 2, its linear form. Peptides hPTH-(1-30)-NH2 (6), [Leu27]hPTH-(1-30)-NH2 (7), and [Leu27]cyclo(Glu22-Lys26)-hPTH-(1-30)-NH2 (8) all had AC-stimulating activities similar to that of 1. When injected intravenously, with a dose of 0.8 nmol/100 g of analogue in acid saline, hypotensive effects paralleled their adenylyl cyclase activities. They behaved quite differently when applied subcutaneously. Analogues 1, 5, and 6, the weakest, showed about half the drop in blood pressure observed with 3 and 4, the most active. In contrast, the time required to reach a maximum drop in blood pressure of 4-8, after subcutaneous administration, was 2-4 times that of the other analogues. Thus, the bioavailabilities of the lactam analogues, unlike their adenylyl cyclase-stimulating activities, were highly dependent on the presence or conformation of Val31.

Structure of the pseudosubstrate recognition site of chicken smooth muscle myosin light chain kinase

The structure of the chicken smooth muscle myosin light chain kinase pseudosubstrate sequence MLCK(774-807)amide was studied using two-dimensional proton NMR spectroscopy. Resonance assignments were made with the aid of totally correlated and nuclear Overhauser effect spectroscopy. A distance geometry algorithm was used to process the body of NMR distance and angle data and the resulting family of structures was further refined using dynamic simulated annealing. The major structural features determined include two helical segments extending from Asp-777 to Lys-785 and from Arg-790/Met-791 to Trp-800 connected by a turn region from Leu-786 to Asp-789 enabling the helices to interact in solution. The C-terminal helix incorporates the bulk of the pseudosubstrate recognition site which is partially overlapped by the calmodulin binding site while the N-terminal helix forms the bulk of the connecting peptide. The demonstrated turn between the helices may assist in enabling the autoregulatory or pseudosubstrate recognition sequence to be rotated out of the active site of the catalytic core following calmodulin binding.

Conformation of parathyroid hormone antagonists by CD, NMR, and molecular dynamics simulations

The conformation of two highly potent parathyroid hormone (PTH) antagonists was investigated in water/2,2,2-trifluoroethanol mixtures. The two peptides are derived from the sequence (7-34) of PTH and of PTH-related protein (PTHrP) and have a D-Trp replacing Gly in position 12. In the analogue derived from PTHrP, Lys11 was replaced by Leu to remove the residual agonist activity. The study was conducted by CD and two-dimensional proton magnetic resonance spectroscopy, and the nuclear Overhauser effects found were utilized in restrained distance geometry and molecular dynamics simulations. Both peptides adopt a helical C-terminal conformation, which seems more stable in the case of the PTHrP analogue. A type II' beta-turn centered around D-Trp12 and Lys13 is present in both structures.

Structure of human parathyroid hormone 1-37 in solution

Human parathyroid hormone (hPTH), amino acids Ser1 to Leu37, is biologically active with respect to both receptor binding and activation of adenylate cyclase to influence the serum calcium concentration. It induces DNA synthesis via an unknown signal pathway. We investigated the structure of hPTH(1-37) in H2O/buffer solution under near physiological conditions, that is pH 6.0 and 270 mM salt, by circular dichroism, ultracentrifugation, nuclear magnetic resonance spectroscopy, and molecular dynamics calculations. Complete sequence specific assignments of all 1H resonances were performed by using 1H two-dimensional NMR measurements (double quantum-filtered correlated spectroscopy, nuclear Overhauser effect spectroscopy (NOESY), and total correlation spectroscopy with suppression of NOESY-type cross-peaks spectra). hPTH(1-37) obtained helical structure and showed hydrophobic interactions defining a tertiary structure. The NH2-terminal four amino acids of hPTH(1-37) did not show a stable conformation. Evidence for an alpha-helical region between Ile5 and Asn10 was found. This region was followed by a flexible link (Gly12, Lys13) and a well defined turn region, His14 to Ser17. The latter was stabilized by hydrophobic interactions between Trp23 and Leu15. Ser17 through at least Leu28 formed an alpha-helix. Arg20 and Lys27 were involved in the core built by His14 to Ser17. Unrestrained molecular dynamics simulations indicated that the structure was stable on the 200 ps time scale.

Parathyroid hormone (PTH)-PTH-related peptide hybrid peptides reveal functional interactions between the 1-14 and 15-34 domains of the ligand

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) bind to a common PTH/PTHrP receptor. To explore structure-function relations in these ligands, we synthesized and functionally evaluated PTH-PTHrP hybrid peptides in which the homologous 1-14 portions were exchanged. Hybrid-2, PTH-(1-14)-PTHrP-(15-34)NH2, bound to LLC-PK1 cells expressing the cloned rat PTH/PTHrP receptor with high affinity (IC50 approximately equal to 7 nM). In contrast, hybrid-1, PTHrP-(1-14)-PTH-(15-34)NH2, bound with much weaker affinity (IC50 approximately equal to 8,700 nM). Thus, the 1-14 region of PTHrP is incompatible with the 15-34 region of PTH. The carboxyl-terminal incompatibility site was identified as residues 19-21 (Glu-Arg-Val in PTH and Arg-Arg-Arg in PTHrP); extending the amino-terminal PTHrP sequence to residue 21 but not to 18 cured the hybrid's binding defect. The amino-terminal incompatibility site was identified as position 5 (Ile in PTH and His in PTHrP), because Ile5-hybrid-1 bound with high affinity (IC50 approximately equal to 20 nM). The importance of these identified residues in the native ligands was established by evaluating the effects of substitutions at these sites in a series of PTH and PTHrP analog peptides. Overall, the results are consistent with the hypothesis that, in both PTH and PTHrP, the 1-14 and 15-34 domains interact when binding to the receptor and that residues 5, 19, and 21 contribute either directly or indirectly to this interaction.

Expression and characterization of a recombinant human parathyroid hormone partial agonist with antagonistic properties: Gly-hPTH(-1-->+84)

We have produced and characterized a hPTH analogue with an amino-terminal extension of glycine, Gly-hPTH(-1-->+84) (denoted Gly-hPTH). The hormone analogue was synthesized in E. coli strain BJ5183 transformed with the expression plasmid pKKPTH, extracted from the bacterial pellet and purified by reverse-phase high performance liquid chromatography. Its chemical nature, as determined by amino acid composition analysis, N-terminal amino acid analysis, and mass spectrometry, showed the 9480-Da Gly-hPTH as the predominant species. Because f-Met-Gly-hPTH was the expected form encoded by the plasmid construct, the results indicate that the f-Met residue was efficiently removed from the precurser form. The following functional characteristics of Gly-hPTH were demonstrated. 1) In cells transfected with the human PTH/PTHrP receptor, the receptor binding affinity was reduced threefold compared to the authentic hPTH(1-84) produced by Saccharomyces cerevisiae (apparent Kds: 8.4 and 2.7 nM, respectively). 2) Using the same cells, Gly-hPTH showed 27-fold reduced potency compared to hPTH(1-84) in stimulating intracellular cAMP production (EC50: 32 and 1.2 nM, respectively). 3) Gly-hPTH demonstrated antagonist activity by reducing hPTH-induced cAMP production by 33 +/- 5% (mean +/- SD) when tested at a 1:1 molar ratio. In these studies the recombinant authentic hPTH(1-84) was used as standard for comparisons, and it showed an equal receptor binding affinity and cAMP production as the chemically synthesized peptide [Nle8,18,Tyr34]bovinePTH(1-34)-NH2.

Stabilized NMR structure of the hypercalcemia of malignancy peptide PTHrP[Ala-26](1-34)amide

The structure of the biologically active mutant PTHrP[Ala-26](1-34)amide in 10% trifluoroethanol was studied by two-dimensional proton NMR spectroscopy. Complete assignments of all backbone and side chain hydrogens were made with the aid of totally correlated and nuclear Overhauser effect spectroscopy. The NMR data were utilized in the distance geometry algorithm (DIANA) and the resulting family of structures further refined using dynamic simulated annealing (X-PLOR). The major structural features include two segments of alpha-helix extending from Glu-4 to Lys-13 and from Phe-21/Phe-22 to Ala-34, with a turn from Gln-16 to Arg-19 and a hinge around Ser-14/Ile-15. A close resemblance to the structure of PTH(1-34)amide in water was noted. A comparison of the structural features common to PTH and PTHrP in different solvents was made which enabled the key structural features likely to be involved in PTH receptor binding to be identified.

Stabilized structure of the presynaptic (Y2) receptor-specific neuropeptide Y analog N-acetyl[Leu-28,Leu-31]NPY(24-36)

Neuropeptide Y analog N-acetyl[Leu-28,Leu-31]NPY(24-36)-amide binds specifically to prejunctional or Y2 receptors acting to inhibit neurotransmitter release. The structure of this biologically active mutant was studied by two-dimensional proton nuclear magnetic resonance spectroscopy. Assignments of all backbone and side chain hydrogens were made by using totally correlated spectroscopy (TOCSY) experiments providing through-bond 1H-1H connectivities, and nuclear Overhauser effect spectroscopy (NOESY), which provided through-space and sequential backbone connectivities. Structure analysis of the peptide was performed using distance geometry and dynamic simulated annealing revealing the presence of a helical structure exhibiting an amphiphilic character and slight constriction in the segment 24-29.

The structure of human parathyroid hormone from a study of fragments in solution using 1H NMR spectroscopy and its biological implications

In order to gain insight into the structure of human parathyroid hormone (hPTH), four fragments [hPTH(1-34), hPTH(18-48), hPTH(28-48), and hPTH(53-84)], which cover all regions of the intact hormone, have been investigated by CD and NMR spectroscopy in combination with distance geometry, and restrained molecular dynamics and energy minimization calculations, under a variety of solution conditions. Significantly, all fragments showed little propensity to form stable structures in aqueous solution alone, and it was only on the addition of trifluoroethanol (TFE) that defined structural features were observed. In an extension of earlier work [Klaus et al. (1991) Biochemistry 30, 6936-6942], hPTH(1-34) in 70% trifluoroethanol (TFE) showed two helices that were longer than in 10% TFE, but essentially showed the same characteristics. Although overlap in the 1H NMR spectra prevented the determination of quantitative NOE data for residues 26-30, the combination of the alpha-proton chemical shift data and quantitative NOE data indicated the helices extend from residues 3 to 13 and 15 to 29. No evidence was found for interaction of the two helical regions. The nature and extent of this second helix in the intact hormone were better defined from the data for hPTH(18-48). Under limiting solution conditions, where the fragment assumed its maximum helical content, a well-defined helix was observed between residues 21 and 38 with a possible discontinuity between Leu-28 and Gln-29. There was little evidence of any form of secondary structure between Gly-38 and the terminus of this fragment, Ser-48. In keeping with this result, the shorter fragment, hPTH(28-48), showed little evidence of stable secondary structure on addition of TFE. From the alpha-proton chemical shifts residues 23-27 appeared to sustain helical structure more readily than the rest of molecule under all solution regimes in both hPTH(1-34) and hPTH(18-48). In contrast to the other two longer fragments hPTH(53-84) showed little propensity for helical secondary structure even at the highest TFE concentrations. However, there was evidence that the molecule did adopt a defined three-dimensional structure. Various long-range NOE's were observed in 10% TFE that allowed the calculation of an open tertiary structure consisting of an initial series of turns surrounded by a loop structure of several loose turns.(ABSTRACT TRUNCATED AT 400 WORDS)