Overlapping binding site for the endogenous agonist, small-molecule agonists, and ago-allosteric modulators on the ghrelin receptor.
Level 5 - mechanism / opinion, no new human data
In vitro receptor mutagenesis and molecular modeling study
PubMed 18923064 · doi:10.1124/mol.108.049189
What was done
Single amino acid substitutions were introduced at 22 positions within the ligand-binding pocket of the ghrelin receptor. Researchers evaluated receptor activation by measuring inositol phosphate turnover in response to six agonists: two peptides (endogenous ghrelin and GHRP-6) and four synthetic nonpeptide agonists (L-692,429, MK-677, SM-130686, and SM-157740). They also evaluated the allosteric modulation of ghrelin efficacy by L-692,429 and GHRP-6, and computationally docked nonpeptide ligands into a rhodopsin-based receptor homology model.
What was found
The GluIII:09-to-Gln mutation in transmembrane domain III (TM-III) caused the strongest decrease in potency across all six agonists. Substitutions at PheVI:16, ArgVI:20, and PheVI:23 on TM-VI also reduced potency for all six ligands, while individual agonists showed additional selective mutational hits. The mutational map for ago-allosteric modulation by L-692,429 and GHRP-6 overlapped with, but was not identical to, their agonism map. No numerical potency values or fold-changes were reported in the abstract.
Why it matters
This study shows that endogenous peptides, small-molecule agonists, and ago-allosteric modulators share a common core binding pocket spanning TM-III and TM-VI on the ghrelin receptor. This maps key contact points to guide the rational design of synthetic ghrelin receptor ligands.
Limits
The study is restricted to in vitro mutational functional assays and molecular modeling without in vivo validation. The abstract reports no numerical data such as EC50 values, binding affinities, or exact sample sizes. Molecular docking relied on an inactive rhodopsin template rather than an experimental ghrelin receptor structure.
Cited by
- supports MK-677 (ibutamoren) is an orally available small molecule that binds to the ghrelin receptor and stimulates growth hormone release and hunger.