Conformational studies of parathyroid hormone (PTH)/PTH-related protein (PTHrP) point-mutated hybrids

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 between the two hormones in their amino acid sequence. Recently, it was show vn that in (1-34)PTH/PTHrP segmental hybrid peptides, the N-terminal 1-14 segment of PTHrP is incompatible with the C-terminal 15-34 region of PTH leading to substantial reduction in pote ncy. The sires of incompatibility were identified as positions 5 in PTH and 19 in PTHrP. In the present paper we describe the synthesis, biological ev aluation, and conformational characterization of two point-mutated PTH/PTHr P 1-34 hybrids in which the arginine residues at positions 19 and 21 of the native sequence of PTHrP have been replaced by valine (hybrid V-21) and gl utamic acid (hybrid E-19), respectively, taken from the PTH sequence. Hybri d V-21 exhibits both high receptor affinity and biological potency, while h ybrid E-19 binds weakly and is poorly active. The conformational properties of the two hybrids were studied in aqueous solution containing dodecylphos phocholine (DPC) micelles and in water/2,2,2-trifluoroethanol (TFE) mixture s. Upon addition of TFE or DPC micelles to the aqueous solution, both hybri ds undergo a coil-helix transition. The maximum helix content in 1 : 1 wate r/TFE, obtained by CD data for both hybrids, is similar to 80%. In the pres ence of DPC micelles, the maximum helix content is similar to 40%. The conf ormational properties of the two hybrids in the micellar system were furthe r investigated by combined 2D-nmr, distance geometry (DG), and molecular dy namics (MD) calculations. The common structural motif: consisting of two he lical segments located at N- and C-termini, was observed in both hybrids. H owever, the biologically potent hybrid V-21 exhibits two flexible sites, ce ntered at residues 12 and 19 and connecting helical segments, while the fle xibility sites in the weakly active hybrid E-19 are located at position 11 and in the sequence 20-26. Our findings support the hypothesis that the pre sence and location of flexibility points between helical segments are essen tial for enabling the active analogs to fold into the bioactive conformatio n upon interaction with the receptor. (C) 1999 John Wiley & Sons, Inc.