Brain pain-relief peptide, extended form (Met-enkephalin + arginine)
A lab-made version of one of the brain's own natural pain-relief molecules (met-enkephalin), with one extra building block added. Used only as a research tool, not a medicine.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
[Met⁵,Arg⁶] enkephalin (YGGFMR) is a six-residue synthetic peptide that extends the natural brain signaling molecule met-enkephalin by one amino acid — arginine added at the C-terminus. Met-enkephalin (Tyr-Gly-Gly-Phe-Met) is one of the body's own opioid peptides, produced in the central nervous system and peripheral tissues including the adrenal glands. The added arginine at position 6 makes this an analog used in research to explore how small structural changes at the C-terminus affect activity and selectivity at opioid receptors. It is a research compound, not a drug.
History
Met-enkephalin belongs to a large family of mammalian opioid peptides, all sharing the N-terminal Tyr-Gly-Gly-Phe-Met or Tyr-Gly-Gly-Phe-Leu core and all derived from three biosynthetic precursor proteins: proenkephalin (PENK), proopiomelanocortin (POMC), and prodynorphin (PDYN). Pasternak and colleagues (Pharmacological Reviews 2013) trace the conceptual evolution of mu opioid pharmacology from the earliest enkephalin binding studies through to modern receptor subtype and biased-agonism frameworks. The four opioid receptor types — delta, kappa, mu, and NOP — were found to be broadly conserved across vertebrate species in phylogenetic analyses by Dreborg and colleagues (PNAS 2008) and Stevens (Frontiers in Bioscience 2009). Wang and colleagues (2002) characterized enkephalin-family peptides in bovine adrenal medulla using fast HPLC coupled to electrospray ionization mass spectrometry (Peptides 2002). The [Met⁵,Arg⁶] extension variant has been studied alongside other C-terminally extended enkephalin analogs as part of efforts to map which residues govern receptor subtype selectivity and proteolytic stability.
What it does
[Met⁵,Arg⁶] enkephalin binds to opioid receptors — principally the mu-opioid receptor (MOR, gene OPRM1) — and activates them. The mu-opioid receptor is a class A G protein-coupled receptor (GPCR) that mediates analgesia, reward, and a range of modulatory effects in both the central nervous system and peripheral tissues (Kim and colleagues, Experimental & Molecular Medicine 2025). Met-enkephalin itself has documented protective effects in experimental models of autoimmune and inflammatory conditions, an activity abolished by the opioid receptor antagonist naltrexone, confirming opioid receptor mediation (Turčić and colleagues, Acta Pharmaceutica 2025). Adding arginine at position 6 changes the peptide's charge, C-terminal geometry, and susceptibility to carboxypeptidases relative to the parent pentapeptide, making it a useful probe for structure–activity studies at the mu receptor.
Evidence
- Human: No human clinical trials published for [Met⁵,Arg⁶] enkephalin specifically. Research use is confined to biochemical and pharmacological studies.
- Animal: L-Met-enkephalin has shown protective effects in animal models of autoimmune and inflammatory disease; these effects are opioid receptor-dependent (Turčić and colleagues 2025). D-amino acid substitutions in the enkephalin scaffold, including at the Met⁵ position, produce analogs with altered hepatoprotective and receptor-modulatory profiles compared with the native L-form (Turčić and colleagues 2025).
- In vitro: Binding studies of enkephalin variants at mu- and delta-opioid receptors have been used to map receptor selectivity across the analog series; the [Met⁵]enkephalin parent has been benchmarked alongside cyclized and D-amino acid variants in comparative pharmacology research. Turčić and colleagues (2025) examined how D-amino acid substitutions in the enkephalin scaffold alter receptor modulation profiles relative to the native L-form.
Known effects
- Mu-opioid receptor agonism — Mechanistic / in vitro; primary pharmacological activity of the enkephalin scaffold at OPRM1
- Anti-inflammatory / immunomodulatory — Preclinical (animal models); demonstrated for L-Met-enkephalin; opioid receptor-dependent (Turčić and colleagues 2025)
- Analgesic signaling — Mechanistic; consistent with mu-opioid receptor activation as reviewed in Pasternak and colleagues (Pharmacological Reviews 2013)
Mechanism
[Met⁵,Arg⁶] enkephalin acts as an agonist at the mu-opioid receptor (OPRM1), a class A GPCR whose molecular activation mechanism and structural biology have been detailed in recent cryo-EM studies reviewed by Kim and colleagues (Experimental & Molecular Medicine 2025). The broader evolution of mu-opioid receptor concepts — from the first enkephalin binding assays through to receptor subtype multiplicity and signaling-biased agonism — is surveyed in Pasternak and colleagues (Pharmacological Reviews 2013). Modifications at the C-terminus of the enkephalin scaffold, such as the Arg⁶ extension in this analog, alter the peptide's charge and C-terminal geometry relative to the parent pentapeptide YGGFM, influencing both receptor engagement and susceptibility to peptidase degradation. Research into C-terminally extended enkephalin variants has been part of the broader program of mapping which positions beyond the conserved Tyr-Gly-Gly-Phe-Met core govern selectivity between mu and delta opioid receptor subtypes.
Related peptides
The enkephalin family includes [Leu⁵]enkephalin (Tyr-Gly-Gly-Phe-Leu), the leucine counterpart sharing the same N-terminal tetrapeptide. Both derive from endogenous precursor processing and act at delta and mu opioid receptors. Beta-endorphin, a longer opioid peptide also derived from POMC, contains the [Met⁵]enkephalin sequence at its N-terminus. Within the platform, the mu-opioid receptor (OPRM1) is shared as the primary target with other opioid peptide analogs — see the OPRM1 target page for related cards.
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Could making a mirror-image version of the methionine in [Met5,Arg6]-enkephalin produce a longer-lasting peptide that protects the liver from injury, similar to what has been shown for the shorter parent peptide?
If true, a simple chemical modification of this peptide could yield a new approach to treating or preventing acute liver failure, one of the most serious and under-treated conditions in emergency medicine, using a molecule derived from the body's own opioid system.
Does the arginine added to the end of [Met5,Arg6]-enkephalin make this peptide bind the pain-relieving mu-opioid receptor more selectively than the natural version, which hits both mu and delta opioid receptors?
If true, this naturally occurring peptide extension could be used to isolate the contribution of mu-opioid receptors specifically in experiments, improving the accuracy of research into pain, mood disorders, and addiction.
Could this arginine-extended enkephalin, released from the adrenal gland alongside adrenaline during stress, normally dampen inflammation in the gut and liver, and do patients who lose their adrenal glands miss this anti-inflammatory signal?
If true, patients who have had their adrenal glands removed or who have adrenal insufficiency might benefit from replacement of this opioid peptide signal alongside standard hormone therapy, potentially reducing their unusually high susceptibility to inflammatory complications.
Does the positively charged arginine at the end of [Met5,Arg6]-enkephalin interact with the beginning of the peptide to create a folded shape that fits the mu-opioid receptor better than the flexible straight-chain parent peptide?
If true, this self-folding behavior would explain why the extended peptide is such a good receptor binder and could guide the design of locked, stable versions of enkephalin that work more consistently as research tools or drug leads.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.9609720706939697 | boltz-2 |
| ranking score | 0.8381711840629578 | boltz-2 |
▸structural qualityopenfold3
| metric | value | note |
|---|---|---|
| gpde | 0.850 | global PDE — lower = better |
| disorder | NaN | fraction disordered |
▸3-letter notation
▸recipeboltz-2 1.0
| parameter | value |
|---|---|
| model | boltz-2 1.0 |
| weights | — |
| hardware | nvidia_nim_api |
| mlx version | — |
| python | — |
| random seed | — |
| msa strategy | none |
| diffusion samples | 1 |
| runtime | — |
| predicted by | mlx@peptide |
| predicted at | 2026-04-24 |
▸citationbibtex
@peptide{pep10705,
sequence = {YGGFMR},
target = {oprm1},
author = {peptidemodel},
year = {2026},
status = {synthesized}
}