The conformation of orexin-A, an orphan G-protein coupled receptor agonist (the human sequence is:
has been determined when bound to sodium dodecylsulphate-d₂₅ (SDS) micelles by ¹H and ¹³C NMR and molecular modeling. Orexin-A has been implicated in sleep-wakefulness and feeding regulation. The conformational preference of orexin-A consists of a short helical section, involving Asp⁵ to Gln⁹ that makes up helix I, followed by a bend from Lys¹⁰ to Ser¹³. Residues Leu¹⁶ to Gly²² make up helix II. The conformation of orexin-A can now be used to explain the results of earlier Ala substitution mutagenesis experiments (J. G. Darker, et al. Bioorg. Med. Chem. Lett. 11, 737-740 (2001); S. Ammoun, et al. J. Pharmacol. Expt. Ther. 305, 507-514 (2003)). Darker et al., working with orexin-A (15-33) amide, observed a significant drop in functional potency at the OX₁R receptor when Leu¹⁶, Leu¹⁹, Leu²⁰, His²⁶, Gly²⁹, Ile³⁰, Leu³¹, Thr³², and Leu³³ were replaced by Ala. Ammoun et al. identified three areas of interest, which were the same for OX₁R and OX₂R receptors, as amino acids 15-17, 20 and 25-26 with the most marked reduction in activity being produced by the replacement of Leu²⁰ by Ala. We suggest that Leu¹⁶, Leu¹⁹, and Leu²⁰, which are in helix II, are likely responsible for binding orexin-A to the surface of the micelle.