MK-677, Ghrelin, and GHRP-6 Share Overlapping TM-III/TM-VI Binding Site on GHSR
Background
The ghrelin receptor (GHSR1a) is a constitutively active class-A GPCR that mediates growth hormone release, appetite, and metabolic signaling. Despite the clinical development of small-molecule GHSR agonists — including the spiroindoline MK-677 and benzolactam L-692,429 — the structural basis of how these nonpeptide agonists physically engage the binding pocket had not been rigorously compared to the endogenous 28-amino-acid octanoylated ligand ghrelin or the hexapeptide GHRP-6. A key unresolved question was whether small-molecule agonists that also act as ago-allosteric modulators (boosting ghrelin's maximal efficacy beyond its own ceiling) do so via the same receptor contacts used for their direct agonist activity, or through distinct surfaces. Resolving these overlapping versus divergent contacts is critical for rational drug design targeting GHSR.
Study Design
A library of 22 single-substitution GHSR mutants covering residues in the main ligand-binding pocket was constructed and expressed. All six agonists — ghrelin, GHRP-6, L-692,429, MK-677, SM-130686, and SM-157740 — were screened against each mutant using inositol phosphate turnover as the primary functional endpoint for agonist potency. Additionally, L-692,429 and GHRP-6, which possess ago-allosteric properties, were tested for their ability to increase ghrelin's maximal efficacy across the mutant panel. Molecular docking models were built over the inactive conformation of rhodopsin to place low-energy conformations of the nonpeptide agonists against their mutational hit maps.
Results
Across all six chemically diverse agonists, the single most impactful substitution was GluIII:09-to-Gln in the extracellular segment of transmembrane domain III, which decreased potency for inositol phosphate turnover for every ligand tested — making it the universal binding hotspot of GHSR. Residues PheVI:16, ArgVI:20, and PheVI:23 on the opposing face of transmembrane domain VI were also shared mutational hits across all six agonists, defining a common TM-III/TM-VI binding cleft. Each ligand additionally showed selective sensitivity to specific mutations outside this shared core, indicating individualized peripheral contacts. Rhodopsin-based molecular models successfully docked nonpeptide agonists in low-energy conformations that satisfied the majority of their experimentally derived mutational constraints.
The mutational map for ago-allosteric modulation by L-692,429 and GHRP-6 overlapped with — but was not identical to — the map for their direct agonist activity, suggesting partially distinct receptor contacts underlie the two functional modes.
Key Findings
- GluIII:09-to-Gln substitution decreased potency for all six agonists tested — the single strongest universal mutational hit.
- PheVI:16, ArgVI:20, and PheVI:23 on TM-VI were shared mutational hits across all six structurally diverse GHSR agonists.
- 22 single-substitution GHSR mutants mapped binding contacts for ghrelin, GHRP-6, MK-677, L-692,429, SM-130686, and SM-157740.
- Ago-allosteric mutational map for L-692,429 and GHRP-6 overlapped with but was not identical to their direct agonist map.
- Nonpeptide agonist docking on rhodopsin-based models satisfied the majority of experimentally identified mutational constraints.
Why It Matters
Identifying a shared TM-III/TM-VI hotspot across peptide and nonpeptide GHSR agonists means that medicinal chemistry efforts can converge on this core cleft rather than treating each chemotype as starting from scratch. For researchers and clinicians evaluating MK-677 or similar secretagogues, this mechanistic map explains why structurally diverse compounds converge on similar efficacy profiles at GHSR. The partial divergence between agonist and ago-allosteric mutational maps for L-692,429 and GHRP-6 opens a rational path to designing molecules that preferentially boost endogenous ghrelin ceiling rather than acting as simple agonists — a potentially important distinction for therapeutic contexts where preserving pulsatile GH secretion is preferred. This is foundational structural pharmacology; clinical translation requires further work in native tissues and in vivo models.
TitrateLab articles covering MK-677
Written by TitrateLab, separately from the study summarized above.
