Structure-activity studies on cysteine-substituted neurokinin A analogs.

A complete series of analogs of tyrosine modified neurokinin A ([Tyr1]-NKA or [Tyr0]-NKA) has been synthesized by substituting each natural residue with 1-Cys. These analogs were tested for their ability to bind recombinant neurokinin-2 (NK-2) receptor. Substitution of Phe6 with Cys completely abolished binding of the analog to the receptor. Substitution of residues in the carboxyl-terminal region of the peptide (Met10, Leu9, Gly8, Val7) and Asp4 with Cys gave reductions in binding affinity of between 23- and 250-fold. Molecular dynamics simulations of these analogs suggest that changes in peptide structure and flexibility are not large contributors to the losses in receptor binding affinity. Reductions in binding affinity are therefore more confidently ascribed to losses of peptide-receptor interactions.