Conformational flexibility of the peptide hormone ghrelin in solution and lipid membrane bound: a molecular dynamics study.

Human ghrelin is a peptide hormone of 28 aminoacid residues, in which the Ser3 is modified by an octanoyl group. Ghrelin has a major role in the energy metabolism of the human body stimulating growth hormone release as well as food intake. Here we perform molecular dynamics simulations in explicit water and in a DMPC-lipid bilayer/water system in order to structurally characterize this highly flexible peptide and its lipid binding properties. We find a loop structure with residues Glu17 to Lys 20 in the bending region and a short alpha-helix from residues Pro7 to Glu13. The presence of a lipid membrane does not influence these structural features, but reduces the overall flexibility of the molecule as revealed by reduced root mean square fluctuations of the atom coordinates. The octanoyl-side chain does not insert into the lipid membrane but points into the water phase. The peptide binds to the lipid membrane with its bending region involving residues Arg15, Lys16, Glu17, and Ser18. The implications of these results for the binding pocket of the ghrelin receptor are discussed.