TOPOGRAPHIC ANALYSIS OF THE S-7 BINDING SUBSITE OF THE TACHYKININ NEUROKININ-1 RECEPTOR

Conformationally and configurationally restricted rotameric probes of phenylalanine have been incorporated in the sequence of substance P rg -Puo-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2-for analyzing the binding pockets of Phe(7) (S-7) and Phe(8) (S-8), in the neurokinin-1 recepto r. These analogues of phenylalanine ave (2S, 3R)- and (2S, 3S)-indanyl glycines, E- and Z-alpha, beta-dehydrophenylalanines, and 2(S)-alpha, beta-cyclopropylphenylalanines [Delta(E)Phe, Delta(Z)Phe, del(E)2(S)Ph e, and del(Z)2(S)Phe]. Binding data obtained with either conformationa lly (Ing diastereoisomers) or configurationally (Delta(E)Phe, Delta(Z) Phe) probes have unveiled large differences in the binding potencies o f these rotameric probes. With the support of nmr data and energy calc ulations done on these SP-substituted analogues, we attempt to answer questions inherent to such study. First, none of these six probes prev ents the formation of bioactive conformation(s) of the backbone of SP. Second, both diastereoisomers (S, Si and (S, R) of indanylglycine pre ferentially adopt, in the sequence of SP, the gauche(-) and trans side -chain orientations, respectively, as previously postulated from energ y calculations with model peptides. However, in solution, the differen ce in energy between these rotamers included in the sequence of SP, co mpared to model peptides, is smaller since the other rotamer can be de tected in [(2S, 3R)Ing(7)]SP. Finally, from this study we can hypothes ize that the large variations observed in the affinities of Phe(7) sub stituted analogues of SP must come from steric hindrance in the S-7 bi nding site, which drastically restricts the space filling around the C -alpha-C-beta bond of residue 7. (C) 1996 John Wiley & Sons, Inc.