The Structure of Ghrelin
The structure of ghrelin, a 28-residue octanoylated peptide hormone, is only known up to the level of primary structure identifying an active core of residues 1-5 or 1-4 including octanoyl-Ser3 as necessary to elicit receptor response. This chapter reviews the results and limitations of experimental and computer modelling studies, which have appeared in the literature. The 1H-NMR spectroscopy experimental studies revealed an unstructured and/or fast interconverting peptide at acidic pH, while molecular dynamics (MD) simulation studies at neutral pH pointed to a stable conformation over a time period of 25 ns in water and in the presence of a lipid bilayer. The significance of these findings is discussed with regards to the pH difference, the timescales accessible to simulation and NMR spectroscopy and the limitations of computational modelling. MD simulations of ghrelin in the presence of a lipid membrane revealed that the octanoyl side-chain did not insert into the lipid bilayer, but instead the peptide bound to the lipid headgroups with residues Arg15, Lys16, Glu17 and Ser18, which are located in a hairpin-like bend in the structure. The implications of these findings with regards to a recently obtained homology model of the ghrelin receptor are discussed.