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 (SP)—Arg‐Pro‐Lys‐Pro‐Gln‐Gln‐Phe‐Phe‐Gly‐Leu‐Met‐NH 2 —for analyzing the binding pockets of Phe 7 (S 7 ) and Phe 8 (S 8 ), in the neurokinin‐1 receptor. These analogues of phenylalanine are (2S, 3R)‐ and (2S, 3S)‐indanylglycines, E‐ and Z‐α, β‐dehydrophenylalanines, and 2(S)‐α, β‐cyclopropylphenylalanines [Δ E Phe, Δ Z Phe, ▿ E 2(S)Phe, and ▿ Z 2(S)Phe]. Binding data obtained with either conformationally (Ing diastereoisomers) or configurationally (Δ E Phe, Δ Z Phe) probes have unveiled large differences in the binding potencies of these rotameric probes. With the support of nmr data and energy calculations done on these SP‐substituted analogues, we attempt to answer questions inherent to such study. First, none of these six probes prevents the formation of bioactive conformation(s) of the backbone of SP. Second, both diastereoisomers (S, S) and (S, R) of indanylglycine preferentially adopt, in the sequence of SP, the gauche (−) and trans side‐chain orientations, respectively, as previously postulated from energy calculations with model peptides. However, in solution, the difference in energy between these rotamers included in the sequence of SP, compared to model peptides, is smaller since the other rotamer can be detected in [(2S, 3R) Ing 7 ]SP. Finally, from this study we can hypothesize that the large variations observed in the affinities of Phe 7 substituted analogues of SP must come from steric hindrance in the S 7 binding site, which drastically restricts the space filling around the C α (SINGLE BOND) C β bond of residue 7. © 1996 John Wiley & Sons, Inc.