Mono- and bicyclic analogs of parathyroid hormone-related protein. 2. Conformational analysis of antagonists by CD, NMR, and distance geometry calculations

The conformation of the three cyclic antagonist analogs of parathyroid hormone-related protein [PTHrP)-(7-34) {[Lys¹³, Asp¹⁷)PTHrP-(7-34)NH₂, [Lys²⁶, Asp³⁰]PTHrP-(7-34)NH₂, [Lys¹³, Asp¹⁷, Lys²⁶, Asp³⁰]PTHrP-(7-34)NH₂} is investigated by CD, NMR, and extensive computer simulations in aqueous solution and a TFE:water mixture. The structural analysis of these peptides, designed to stabilize different regions of the sequence in α-helical conformations, is an important step in addressing the correlation between helical content and binding affinity and bioactivity in this hormone-receptor system. Results from CD and NMR spectroscopy of all three analogues in aqueous solution indicate the presence of α-helix only in regions containing a 20-membered lactam ring. Upon addition of TFE, the three analogues display differences in the anticipated increase in helical content. The high-resolution structures produced at 50:50 TFE:water indicate specific differences in the extent and location of the helical regions. These conformations provide insight into the biological profiles of these analogues, reported in the previous manuscript [Bisello et al. (1997) Biochemistry 36, 3293-3299]. Since all three analogues are α-helical in the C-terminal region (residues 25-34 have been previously identified as containing the binding domain) and display similar binding affinities, we conclude that this conformational feature is important for the interaction between the peptide and the receptor. The extent of the helix (toward the N-terminus) and the presence of a hinge in the central region of the peptide play roles in the observed efficacy as measured by antagonism of PTH-stimulated adenylyl cyclase activity. The most active analogue consists of helical segments from residues 13-18 and 20-34, separated by a kink centered at Arg¹⁹.