The Bioactive Conformation of Human Parathyroid Hormone. Structural Evidence for the Extended Helix Postulate

Conformational restrictions in the form of [i, i + 4] lactam bridges were sequentially incorporated into the shortest fragment of hPTH with recognized efficacy in the OVX rat model of osteoporosis, hPTH(1−31)NH₂ (1). Cyclo(Lys¹⁸−Asp²²)[Ala¹,Nle⁸,Lys¹⁸,Asp²²,Leu²⁷]hPTH(1−31)NH₂ (2) is a potent agonist of the PTH/PTHrP receptor located on the surface of ROS 17/2.8 cells as measured by its ability to stimulate adenylyl cyclase activity (EC₅₀ = 0.29 nM). A second analogue, which constrains the entire C-terminal receptor binding domain, bicyclo(Lys¹⁸−Asp²²,Lys²⁶−Asp³⁰)[Ala¹,Nle⁸,Lys¹⁸,Asp²²,Leu²⁷] hPTH(1−31)NH₂ (6), is also shown to be a potent agonist (EC₅₀ = 0.13 nM), thus providing further evidence for an extended helix as the relevant bioactive conformation in this region of the hormone. Adjacent lactam bridges were incorporated into the analogue bicyclo(Lys¹³−Asp¹⁷,Lys¹⁸−Asp²²)[Ala¹,Nle⁸,Lys¹⁸,Asp^17,22,Leu²⁷]hPTH(1−31)NH₂ (7) to evaluate the receptor's tolerance to conformational restriction in the midregion of the peptide. In fact, peptide 7 is also a highly potent agonist (EC₅₀ = 0.43 nM) in the cAMP-based assay, which suggests that at least one bioactive form of the hormone requires a helical conformation extending from residue 13 to residue 22. Incorporation of all three lactam bridges afforded the most conformationally constrained PTH peptide agonist yet reported, tricyclo(Lys¹³−Asp¹⁷,Lys¹⁸−Asp²²,Lys²⁶−Asp³⁰)[Ala¹,Nle⁸,Lys¹⁸,Asp^17,22,Leu²⁷]hPTH(1−31)NH₂ (9). Peptide 9 (EC₅₀ = 0.14 nM) forces residues 13−30 into an extended helical conformation and is a more potent PTH receptor agonist than the parent linear hPTH(1−31)NH₂ (1, EC₅₀ = 4.7 nM). Comparative circular dichroism studies indicate that peptide 9 is highly helical even in the absence of TFE, indicating that residues 1−12 are also likely to be helical in the bioactive conformation. Taken together, these results provide strong structural evidence that hPTH binds to its receptor in an extended helical conformation.