Peptide Mechanism of Action Reference

How to Read a Mechanism

A peptide's mechanism of action is the answer to "what does this molecule do at the molecular level, and how does that produce the effect being studied?" For nearly every research peptide, the answer has three parts:

1. The target — the specific receptor, enzyme, or cellular structure the peptide binds.
2. The signaling event — what the target does once activated (typically a cascade of intracellular signals).
3. The downstream effect — the physiological or tissue-level outcome the signal produces.

A canonical example, in shorthand:

The reason to organize this page by target rather than by peptide is that targets are the natural clusters. Once you group peptides by receptor, you see why Ipamorelin and MK-677 belong together despite one being a peptide and one being a small molecule (both hit the ghrelin receptor). You see why Semaglutide and Tirzepatide overlap but are not identical (Semaglutide is a single GLP-1 agonist; Tirzepatide is dual GIP/GLP-1). You see why PT-141 and Melanotan II are so similar (both melanocortin agonists), and why they still differ (different receptor subtype selectivity). Target-first is how pharmacologists actually think about this — and it is how AI search engines organize the underlying literature.

Master Mechanism Table

All 45+ compounds mapped to their molecular targets. Grouped by receptor family; within each family, ordered by how canonically they represent that mechanism.

"Not fully established" reflects the honest state of the literature for that compound. Where mechanism is disputed or under active study, this table says so rather than picking a single hypothesis to present as settled.

GHRH Receptor

The GHRH receptor sits on the somatotroph cells of the anterior pituitary — the cells that manufacture and release growth hormone. Its natural ligand is GHRH, a 44-amino-acid hypothalamic hormone. The pharmacologically active region is the first 29 amino acids; the remaining 15 are structural.

Every peptide in the GHRH analog class is, one way or another, a variant of GHRH 1-29:

• Sermorelin is literally GHRH 1-29 — the identical fragment. Half-life is short (~10–20 min) because it is unmodified.
• CJC-1295 (no DAC / Mod GRF 1-29) is GHRH 1-29 with four amino acid substitutions that resist enzymatic degradation. Same receptor, longer half-life (~30 min).
• CJC-1295 (with DAC) adds a Drug Affinity Complex that binds albumin. Same receptor, dramatically longer half-life (~6–8 days).
• Tesamorelin is a stabilized GHRH analog with a specific research focus on visceral adipose tissue.

All four bind the same receptor and produce the same immediate signaling event. What differentiates them is their pharmacokinetic profile — see the Half-Life Reference for how these differences translate into dosing rhythms.

Ghrelin Receptor (GHS-R)

The ghrelin receptor is the second major route to growth hormone release. This is the mechanistic cousin of the GHRH receptor — same downstream outcome (GH release), completely different upstream molecule.

The compounds in this class differ in selectivity — how cleanly they hit only the GH-releasing effect versus how much collateral activity they produce:

• Ipamorelin is the most selective — it produces GH release without significantly elevating cortisol or prolactin. That selectivity is why it is often preferred in research.
• GHRP-2 and GHRP-6 are less selective, with some effect on cortisol, prolactin, and (for GHRP-6) appetite.
• Hexarelin is the most potent GH releaser in this class but with the least selectivity.
• MK-677 (Ibutamoren) is not a peptide — it is a small-molecule ghrelin receptor agonist, which is why it is orally bioavailable and has a much longer half-life (~24 hours) than the peptide GHRPs.

Incretin Receptors — GLP-1, GIP, Glucagon

The incretin class is the clearest example of how peptide design has evolved. All three of the marquee compounds are lipidated for albumin binding and multi-day half-lives (see the Half-Life Reference for how). What distinguishes them is how many receptors in the incretin family they hit:

Each added receptor extends the compound's effect profile. GLP-1 activation drives insulin release and appetite modulation. GIP adds further insulin-related effects. Glucagon receptor activation adds energy expenditure signaling. The three compounds are a graded series — one receptor, two receptors, three — which explains why each successive generation produces different research outcomes despite hitting the same broad system.

Cagrilintide is often grouped near this class but hits a different target — the amylin and calcitonin receptors — with a similar lipidation strategy for extended half-life.

Melanocortin Receptors

PT-141 (Bremelanotide) and Melanotan II are structural cousins — both cyclic heptapeptides derived from α-MSH. Their sequences are almost identical. Their research profiles diverge because of receptor subtype selectivity:

• MC1R — skin melanocytes; drives pigmentation (melanogenesis).
• MC3R and MC4R — CNS; involved in sexual response signaling, energy balance.
• MC5R — exocrine, other tissues.

PT-141 preferentially activates MC3R and MC4R, producing its sexual response research profile with minimal pigmentation effects. Melanotan II is a broader agonist, hitting MC1R (hence pigmentation effects) along with MC3R and MC4R.

Two peptides, near-identical structures, different receptor selectivity — different outcomes. This is a clean illustration of why mechanism-first thinking matters: sequence similarity alone would suggest these peptides are interchangeable, but they are not.

Mitochondrial Targets

The mitochondrial-derived peptides break the pattern of the rest of this page: they do not primarily act on cell-surface receptors. They act inside the cell, on the mitochondria themselves.

• MOTS-c is a 16-amino-acid peptide encoded within the mitochondrial genome (not the nuclear genome — unusual). It appears to act through AMPK signaling to influence cellular metabolism.
• SS-31 (Elamipretide) is a tetrapeptide that binds cardiolipin on the inner mitochondrial membrane. Cardiolipin is critical for the structural organization of the electron transport chain; SS-31 protects it and preserves mitochondrial function under stress.
• Humanin is a 24-amino-acid mitochondrial-derived peptide with cytoprotective activity, apparently signaling through multiple receptor systems.

Healing Peptides — Multi-Pathway

The healing peptides — BPC-157, Thymosin β4 (TB-500), GHK-Cu — are grouped here because they share an honest feature: their mechanisms are multi-pathway and, in some cases, not fully established. Unlike the incretin class, where you can point to a specific receptor and be done, the healing peptides act through several partially-mapped routes at once.

BPC-157

The most-studied peptide with the least-nailed-down single mechanism. Published research points to:

• Angiogenesis — promotion of new blood vessel formation, tied to VEGF signaling
• Nitric oxide signaling — modulation of NO pathways relevant to vascular and gut function
• Growth factor modulation — effects on multiple growth factor systems

No single receptor has been identified as BPC-157's primary target. This is a real feature of the current literature — not a gap in this summary. The peptide clearly does things; the exact molecular route is still being characterized.

Thymosin β4 (sold as TB-500)

Thymosin β4 has a well-established primary function: it binds and sequesters G-actin, the monomeric form of the actin cytoskeleton. That role in actin dynamics underlies its effects on cell migration, wound closure, and tissue repair. This is one of the cleaner mechanistic stories in the healing peptide class.

GHK-Cu

GHK-Cu operates through two intertwined roles: it delivers copper to cells (copper is a cofactor for enzymes critical to collagen synthesis and other remodeling processes), and it modulates gene expression in ways that appear to promote a tissue-remodeling and regenerative phenotype. Different from a receptor agonist, and different from BPC-157's unmapped multi-pathway pattern.

The others

Pentadeca Arginate is a newer BPC-157 analog; its mechanism is presumed similar but not independently characterized in depth. KPV may act through the melanocortin pathway (anti-inflammatory branch), though this is not fully established. LL-37 is both antimicrobial (via direct membrane disruption of bacteria) and immunomodulatory (via multiple receptor systems).

Neuroactive Peptides

Semax and Selank — related but different

Both are seven-amino-acid peptides ending in the -Pro-Gly-Pro stabilizing motif (see Sequence Database). Both are Russian-developed nootropics delivered intranasally. Their mechanisms are related but not identical.

• Semax is derived from ACTH 4-7 and has reported activity on the melanocortin system in addition to inducing BDNF (brain-derived neurotrophic factor). Neuroprotection and cognitive-enhancement research is largely built on this BDNF-inducing effect.
• Selank is derived from the immunomodulator tuftsin and has reported effects on GABA-related anxiolytic signaling.

Same peptide class, same delivery route, similar structure — different neurochemical mechanisms. This is why they are researched in different contexts (Semax for cognitive/neuroprotective; Selank for anxiolytic).

DSIP

Delta Sleep-Inducing Peptide, a nine-amino-acid peptide. Studied in the context of sleep architecture, but its precise molecular mechanism is not firmly established.

Cerebrolysin

Not a single molecule but a standardized mixture of low-molecular-weight neuropeptides and amino acids. Its neurotrophic effects are attributed to the mixture as a whole; individual component mechanisms are only partially characterized.

Reproductive Axis

Two peptides in this class, both acting on the hypothalamic-pituitary-gonadal (HPG) axis at different points.

• Gonadorelin is synthetic GnRH itself — a 10-amino-acid peptide that binds the GnRH receptor on pituitary gonadotroph cells, triggering release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• Kisspeptin-10 acts one step upstream. It binds the kisspeptin receptor (KISS1R) on hypothalamic neurons, causing them to release GnRH. It is the pathway that turns on the GnRH → pituitary axis in the first place.

Both have very short half-lives (2–4 minutes), reflecting how tightly the reproductive axis is regulated.

Growth Factor Receptors

The IGF family binds the IGF-1 receptor (or, in the case of IGF-2, the IGF-2 receptor and the IGF-1 receptor). All are related to insulin structurally, and their receptors are structurally related to the insulin receptor.

• IGF-1 LR3 is an engineered variant — the "LR3" refers to structural modifications that reduce binding to circulating IGF-binding proteins, effectively extending free-fraction half-life to ~20–30 hours.
• IGF-1 DES is a variant lacking the first three N-terminal amino acids, producing different binding characteristics.
• IGF-2 binds both its own receptor (IGF-2R) and IGF-1R.
• MGF (Mechano Growth Factor) is a splice variant of IGF-1 produced locally in muscle in response to mechanical loading. Same receptor family, different context.

The IGF receptor is fundamentally different from the GPCRs above — it is a receptor tyrosine kinase, meaning it directly phosphorylates intracellular targets rather than triggering a G-protein-mediated cascade. This is one reason IGF signaling drives such different downstream effects from the GH-releasing peptides that ultimately elevate IGF-1 indirectly.