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 parathyroid hormone-related protein (PTHrP) bind and activate the same membrane receptor in spite of major differences in their amino acid sequence. The hypothesis was made that they share the same bioactive conformation when bound to the receptor. A common structural motif in all bioactive fragments of the hormone in water/trifluoroethanol mixtures or in aqueous solution containing 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, we have recently synthesized and studied the PTHrP(1-34) analog [(Lys13-Asp17, Lys26-Asp30)]PTHrP(1-34)NH2, which contains lactam-constrained Lys-Asp side chains at positions i, i+4. This very potent agonist exhibits enhanced helix stability with respect to the corresponding linear peptide and also two flexible sites at positions 12 and 19 in 1:1 trifluoroethanol/water. These structural elements have been suggested to play a critical role in bioactivity. In the present work we have extended our conformational studies on the bicyclic lactam-constrained analog to aqueous solution. By CD, 2D-NMR and structure calculations we have shown that in water two helical segments are present in the region of the lactam bridges (13-18, and 26-31) with high flexibility around Gly12 and Arg19. Thus, the essential structural features observed in the aqueous-organic medium are maintained in water even if, in this solvent, the overall structure is more flexible. Our findings confirm the stabilizing effect of side-chain lactam constraints on the alpha-helical structure.