Design of analogues of parathyroid hormone: A conformational approach

Mono- and bicyclic analogs of parathyroid hormone-related protein. 1. Synthesis and biological studies

The bioactive conformation of parathyroid hormone-related protein (PTHrP), a single-chain linear peptide structurally similar to parathyroid hormone (PTH), is of considerable interest because PTH and PTHrP both recognize and bind to a shared G-protein-coupled receptor. Both hormones are thought to present a bioactive conformation to the receptor which is substantially alpha-helical in nature. To better characterize this putative biologically relevant conformation, we prepared a series of conformationally constrained analogs of PTHrP with enhanced alpha-helical stability. A combination of structural constraint and helix stabilization was achieved through side chain-to-side chain lactam ring formation between Lys(i) and Asp(i+4) residues (13-to-17 and 26-to-30) along the PTHrP sequence. Mono- and bicyclic analogs derived from the agonist PTHrP-(1-34)NH2 and the antagonist PTHrP-(7-34)NH2 were prepared and characterized in terms of receptor binding and stimulation (or antagonism) of PTH-stimulated adenylyl cyclase activity in osteoblast-like cells. The binding affinity of monocyclic [Lys13,Asp17]-(I) and bicyclic [Lys13,Asp17,Lys26,Asp30]PTHrP-(1-34)NH2 (III) agonists was in the low nanomolar range and similar to that of the parent linear peptide. Furthermore, their efficacy was in the sub-nanomolar range and about 10-fold higher than that of the corresponding linear parent peptide. Analogs I and III are the first cyclic PTH/PTHrP receptor agonists and amongst the most potent PTHrP analogs yet designed. The rank-order of potency in the cyclic antagonist series does not correlate with the binding affinities. In light of the positional dependence and the differential effects of lactam bridge formation on the biological activities of agonist vs antagonists, these analogs may provide insight regarding the biologically relevant conformations of PTHrP-derived ligands [Maretto et al. (1997) Biochemistry 36, 3300-3307].

Structure of human parathyroid hormone(1-34) in the presence of solvents and micelles

The structure of the N-terminal 34-residue fragment of human parathyroid hormone was determined in 40% trifluoroethanol employing two-dimensional 1H nuclear magnetic resonance spectroscopy. The proton chemical shifts were assigned from magnitude and phase-sensitive COSY, relayed COSY, and NOESY spectra. Distance constraints, estimated from NOESY spectra, were used to create a set of structures by distance geometry (DGEOM) which were subsequently refined by restrained energy minimization and restrained molecular dynamics (CHARMm). The resulting structures contained two helices spanning residues 3-12 and residues 17-26. The NOE constraints for residues 13-16 did not provide a single structural solution; however, their conformations were not disordered. The structures prepared by DGEOM and refined with CHARMm contained either an irregular turn or a helical structure at residues 13-16. The secondary structure of human parathyroid hormone(1-34) was also assessed by circular dichroism in the presence of methanol, trifluoroethanol, and dodecylphosphocholine micelles. Under all three conditions, the peptide formed structures containing various amounts of helical content. The formation of helical secondary structure in the presence of micelles supports the proposal that the trifluoroethanol-induced structure of human parathyroid(1-34) was not an artifact of its environment but perhaps was an indication of the conformation that the molecule adopts when in close proximity to a membrane surface and possibly when bound to the parathyroid receptor.

Effects of hydrophobic substitutions at position 18 on the potency of parathyroid hormone antagonists

Position 18 in a parathyroid hormone (PTH) antagonist, [Nle8,18,Tyr34]bPTH(7-34)NH2 (ii), was shown to tolerate substitutions by a range of amino acids with retention of inhibitory activity. The effects of hydrophobic substitutions at this position as a means of enhancing binding interactions with the receptor were evaluated. Substitution of Nle at position 18 with either D-Ala, D-Trp, or L-Trp in analog ii or with Trp (D or L) in the recently reported, highly potent antagonist, [Nle8,18,D-Trp12,Tyr34]bPTH(7-34)NH2 (in vitro activities; Kb = 15 nM and Ki = 125 nM), was performed. In terms of activity on renal receptors, one antagonist, [Nle8,D-Trp12,18,Tyr34]bPTH(7-34)NH2, is the most active in vitro PTH antagonist yet reported (Kb = 4 nM; Ki = 30 nM). The rationale for design of this antagonist and the conclusions regarding PTH-receptor interactions are discussed.

Phosphorylation loops in synthetic peptides of the human neurofilament protein middle-sized subunit

Peptides containing 13 and 39 amino acid residues and serine-side-chain-phosphorylated (P) analogues thereof, corresponding to human neurofilament protein middle-sized subunit (NF-M), have been synthesized in order to localize the phosphorylation site of this protein. The secondary structure of the nonphosphorylated peptides, determined by circular dichroism (CD) measurements, predicted secondary structural calculations and energy conformational calculations, was suggested to be a series of alternating type I (III). beta-turns and 3(10) or alpha-helices. By contrast, the phosphorylated peptides exhibit a unique conformation, probably due to salt bridges between the phosphoserine and the lysine residues. This has provided the first clear evidence that phosphorylation induces conformational changes among these synthetic peptides and presumably, in NF proteins as well. These phosphorylation loops might be the major recognition sites of the neurofilament protein-directed kinases.