A Map of Peptide Mechanisms: What Acts Where

Five families, not fifty compounds

A catalogue of forty peptides looks unmanageable until you notice that almost all of them fall into a small number of mechanistic families. Learn the five and most of the catalogue becomes predictable.

The families are: incretin and metabolic receptor agonists; growth hormone axis modulators; tissue repair and matrix signalling; mitochondrial and cellular maintenance; and neuropeptides. A handful of compounds sit outside these — immune modulators such as thymosin alpha-1, and melanocortin ligands such as PT-141 — but the five cover the bulk.

Organising by mechanism rather than by hoped-for outcome is not merely tidier. It tells you which compounds are genuinely redundant with each other, which are complementary, and which comparisons are meaningful. Two compounds acting on the same receptor are alternatives; two acting on different arms of the same axis are not.

Two compounds that hit the same receptor are alternatives. Two that hit different arms of the same axis are not. That distinction is invisible if you organise by outcome.

Incretin and metabolic receptors

The incretin family acts on three G-protein-coupled receptors: GLP-1, GIP, and glucagon. What separates the compounds is simply which of the three they engage.

Semaglutide is selective for GLP-1. Tirzepatide engages GIP and GLP-1 — and is built on a GIP backbone, which is why it is more accurately described as a GIP analog with added GLP-1 activity than as a GLP-1 drug with a bonus receptor. Retatrutide adds glucagon receptor agonism to both.

Glucagon receptor agonism is the mechanistically interesting one, because glucagon raises blood glucose. The design premise is that concurrent GLP-1 agonism offsets that, leaving the hepatic lipid handling and energy expenditure effects available for study without the glycaemic excursion.

Cagrilintide sits adjacent to this family rather than within it. It is an amylin analog acting at calcitonin and amylin receptors — a satiety pathway entirely independent of incretin signalling, which is precisely why it appears in combination studies alongside semaglutide rather than as an alternative to it.

AOD-9604 belongs to neither. It is a C-terminal fragment of growth hormone, studied for lipolytic activity in adipocytes without the IGF-1-mediated growth signalling of the full hormone.

The growth hormone axis

This family splits cleanly in two, and conflating the halves is the most common error in the category.

GHRH analogs act on the GHRH receptor on pituitary somatotrophs. Sermorelin, Modified GRF (1-29) — commonly sold as CJC-1295 without DAC — and tesamorelin are all in this group. They differ mainly in sequence length and stabilisation strategy: the first two are truncated to the first 29 residues, tesamorelin retains the full 44.

Ghrelin receptor agonists act on GHS-R1a, a completely different receptor. Ipamorelin is the cleanest example, and its research value lies in selectivity — unlike earlier secretagogues it shows minimal effect on ACTH, cortisol, and prolactin in animal models, which makes it a useful tool for isolating GH-axis effects.

Because the two act on different receptors, they are studied together rather than as substitutes. That is the mechanistic reason for the combination products, not a marketing convenience.

IGF-1 LR3 sits at the far end of the same axis. Rather than stimulating release upstream, it is an IGF-1 analog acting directly at the IGF-1 receptor, engineered with an Arg3 substitution that sharply reduces binding to IGF binding proteins.

Tissue repair and matrix signalling

Three distinct mechanisms sit under the repair heading, and they are frequently treated as interchangeable when they are not.

BPC-157's best-supported mechanism is angiogenic: upregulation of VEGFR2 expression and activation of the VEGFR2-Akt-eNOS axis in endothelial cells. A second strand implicates modulation of the nitric oxide system.

Thymosin beta-4 works through a completely different route — it binds monomeric G-actin and regulates polymerisation. Because actin dynamics underlie cell motility, the downstream relevance is to migration and wound-closure models rather than to vascular signalling.

GHK-Cu is different again: a copper-binding tripeptide studied for extracellular matrix remodelling, collagen and glycosaminoglycan synthesis, and the balance between matrix metalloproteinases and their inhibitors. Copper delivery is likely inseparable from its activity, since copper is a cofactor for lysyl oxidase — which is why a copper-only control arm matters and is usually absent.

KPV is the outlier: the C-terminal tripeptide of alpha-MSH, studied for NF-κB pathway inhibition. Its interest is in retaining the anti-inflammatory activity of the parent hormone while lacking the melanocortin receptor binding responsible for pigmentation.

Angiogenic signalling, actin sequestration, and matrix remodelling are three different mechanisms. Filing them together under 'repair' obscures the only thing that would let you choose between them.

Mitochondrial and cellular maintenance

MOTS-c is a mitochondrial-derived peptide — encoded within the mitochondrial 12S rRNA gene, which upended the assumption that the mitochondrial genome encodes only respiratory chain components. Research focuses on its translocation to the nucleus under metabolic stress and AMPK-dependent signalling.

SS-31 targets the inner mitochondrial membrane through interaction with cardiolipin, a phospholipid essential to cristae architecture. Its alternating aromatic-cationic motif drives membrane targeting independent of membrane potential, which distinguishes it from earlier strategies such as TPP+ conjugation.

NAD+ is not a peptide at all but a redox coenzyme, and its relevance here is as a consumed substrate for sirtuins and PARPs. 5-Amino-1MQ is also not a peptide — it is a small-molecule NNMT inhibitor, and it approaches the same NAD+ pool from the opposite direction by blocking the enzyme that diverts nicotinamide away from the salvage pathway.

Glutathione belongs here as the principal intracellular redox buffer. Its unusual γ-peptide bond confers resistance to standard peptidase cleavage, and the GSH/GSSG ratio is a standard readout of cellular oxidative state.

Epitalon is the weakest-supported member. Published work examines telomerase expression and pineal regulation, but the foundational literature comes largely from a single institute and is not widely replicated.

Neuropeptides

Semax and Selank share a design strategy rather than a mechanism. Both are short peptides extended with a Pro-Gly-Pro sequence that markedly increases stability against peptidase degradation — but Semax derives from ACTH(4-10) and Selank from the immunomodulatory tetrapeptide tuftsin.

Semax has been studied for rapid upregulation of BDNF and NGF expression in hippocampal tissue. Selank's published work centres on BDNF, GABAergic and serotonergic interactions, and enkephalin degradation.

DSIP is the honest anomaly in this family. Isolated in the 1970s from rabbit cerebral venous blood, it still has no identified receptor after five decades, and reported effects span sleep architecture, stress hormone modulation, and thermoregulation with no unifying pathway. Its mechanism is genuinely unresolved rather than merely incomplete.

For all three, the primary literature is largely Russian-language and requires translation, which is a practical constraint worth knowing before designing around it.

How to use this map

When comparing two compounds, first ask whether they act on the same receptor. If they do, they are alternatives and the comparison is about potency, selectivity, and half-life. If they do not, they are not substitutes and a head-to-head comparison may not be a meaningful question at all.

Second, ask what the evidence base actually is. Mechanism and evidence are different axes: BPC-157 has a well-articulated proposed mechanism and a literature dominated by one research group. A clear mechanism is not the same as a confirmed one.

Our per-compound research references state both — the pathway, and an explicit assessment of how strong the underlying literature is, including when that is inconvenient for us.