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 parathyr oid hormone-related protein (PTHrP) bind and activate the same membrane rec eptor in spite of major differences in their amino acid sequence. The hypot hesis was made that they share the same bioactive conformation when bound t o the receptor. A common structural motif in all bioactive fragments of the hormone in water/trifluoroethanol mixtures or in aqueous solution containi ng 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, w e have recently synthesized and studied the PTHrP(1-34) analog [(Lys(13)-As p(17), Lys(26)-Asp(30))]PTHrP(1-34)NH2, which contains lactam-constrained L ys-Asp side chains at positions i, i+4. This very potent agonist exhibits e nhanced helix stability with respect to the corresponding linear peptide an d also two flexible sites at positions 12 and 19 in 1:1 trifluoroethanol/wa ter. These structural elements have been suggested to play a critical role in bioactivity. In the present work we have extended our conformational stu dies on the bicyclic lactam-constrained analog to aqueous solution. By CD, PD-NMR and structure calculations we have shown that in water two helical s egments are present in the region of the lactam bridges (13-18, and 26-31) with high flexibility around Gly(12) and Arg(19). Thus, the essential struc tural features observed in the aqueous-organic medium are maintained in wat er even if in this solvent, the overall structure is more flexible. Our fin dings confirm the stabilizing effect of side-chain lactam constraints on th e alpha-helical structure. Copyright (C) 1999 European Peptide Society and John Wiley & Sons. Ltd.