Structure and activities of constrained analogues of human parathyroid hormone and parathyroid hormone-related peptide: Implications for receptor-activating conformations of the hormones

Parathyroid hormone (PTH) has a helix-bend-helix structure in solution. Par t of the C-terminal helix, residues 21-31, is amphiphilic and forms a criti cal receptor-binding region. Stabilization of this cr-helix by lactam forma tion between residues spaced i, i + 4 on the polar face was previously repo rted to increase adenylyl cyclase-stimulating (AC) activity if between resi dues 22 and 26 but to diminish it if between residues 26 and 30 [Barbier et al. (1997) J. Med. Chem. 40, 1373-1380]. This work reports the effects of other cyclizations on the polar face, differing in ring size or position, o n alpha -helix conformation, as measured by circular dichroism, and on AC-s timulating activity. All analogues cyclized between residues 22 and 26 had at least a 1.5-fold increase in activity, suggesting an ex-helical structur e between about residues 21 and 26. Cyclization between residues 25 and 29 or residues 26 and 30 diminished activity by 20-30%, despite stabilizing al pha -helix, suggesting that residues 25-31 bind to the receptor in a helica l, but not classical alpha -helical, conformation. Analogues cyclized betwe en residues 13 and 17 had slightly increased activity. A bicyclic analogue, with lactams between residues 13 and 17 and residues 22 and 26, had about the same activity as that cyclized only between 22 and 26. Parathyroid horm one-related peptide (PTHrP) may bind in a manner similar to the common rece ptor, but hydrophobic moment calculations suggest that it must bind as a ti ghter helix in order to optimally present its hydrophobic residues to the r eceptor. Both PTHrP analogues cyclized between either residues 22 and 26 or residues 26 and 30 had more stable alpha -helices but reduced AC activitie s, consistent with this hypothesis.