Fig. 5. The insurmountable antagonist-bound NK₁R state prohibits SP binding.

(A) Superposition of agonist-bound NK₁R:SP:G_q (green) with the surmountable antagonist CP-99,994–bound NK₁R (PDB ID: 6HLL) (blue) reveals a slightly outward-moved but similar ECL2 conformation for the antagonist conformation. Note the small movement of R177 ^ECL2. (B) Superposition of agonist-bound NK₁R:SP:G_q with the insurmountable antagonist aprepitant-bound NK₁R (PDB ID: 6HLO) reveals an inward shifted and twisted ECL2 in the insurmountable conformation that clashes at M174^ECL2 with P₄ of the endogenous agonist and also poses R177^ECL2 away from its central position in the polar network at the extracellular receptor surface. (C) Superposition of agonist-bound NK₁R:SP:G_q, CP-99,994- and aprepitant-bound NK₁R showing the hydrogen bonds (indicated as orange, dotted lines) that lock helices V and VI in the insurmountable conformation. The rotamer orientation of H197^5.39 in the insurmountable conformation prevents the necessary inward movement of helix VI and the formation of a new hydrogen bond between H197^5.39 with the backbone carbonyl oxygen of F268^6.55 (indicated as green, dotted line) in the active state.