Why do fragments of one hormone differ so much?

Because function is not spread evenly along a sequence. Which residues a fragment keeps determines which activities it can retain.

Why does Semax have no identified receptor?

It retains only part of the core — His and Phe — and replaces the rest, so it is neither clearly a receptor ligand nor clearly not one.

Semax (ACTH Fragment Peptide)

What a Fragment Keeps Decides What It Does

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-234 cited sources

Fragments of one hormone can have entirely different properties because a hormone's functions are not distributed evenly along its sequence. Which residues a fragment keeps determines which of the parent's activities it can retain.

Key facts

Parent: alpha-MSH, 13 residues
Receptor-binding core: His-Phe-Arg-Trp, positions 6–9
Kept by: MT-2, which engages melanocortin receptors
Discarded by: KPV and, in part, Semax
KPV activity: Anti-inflammatory, C-terminal
Consequence: Same parent, incompatible profiles

Why function is not spread evenly

A receptor recognises a specific surface, and that surface is formed by particular residues rather than by the molecule as a whole. Other regions may contribute stability, solubility or a separate activity entirely. Cutting a hormone into pieces therefore separates functions rather than diluting them uniformly.

The receptor-binding core

For the melanocortin receptors the recognised core is His-Phe-Arg-Trp, positions 6 to 9 of alpha-MSH. A fragment retaining it can engage those receptors; one that does not, cannot. That single fact explains most of the divergence between the three compounds derived from this hormone.

Research material referenced

Semax 10mg — third-party HPLC tested

View — £24.99

How each fragment lands

MT-2 is built around that core, cyclised and stabilised, and engages melanocortin receptors — which is the point of it. KPV is positions 11 to 13 and contains none of the core, which is precisely why it was isolated: to keep the anti-inflammatory activity while discarding pigmentation. Semax retains positions 4 to 7, so it holds part of the core region and replaces the rest with Pro-Gly-Pro.

Why Semax is the ambiguous case

It keeps His and Phe, two of the four core residues, and replaces Arg-Trp-Gly. A partial core is neither clearly retained nor clearly discarded, which may be part of why Semax has no identified receptor while MT-2's target is well defined. Its described mechanisms are downstream and indirect rather than receptor-level.

The general lesson for fragment compounds

This site carries several — TB-500 from thymosin beta-4, modified GRF from GHRH, KPV and Semax from the POMC family. The recurring question is the same each time: which part of the parent's function did the fragment inherit, and which did it leave behind? Assuming a fragment carries the parent's properties is the standard error, and it is checkable against the sequence.

Regulatory position

None of these fragments holds a marketing authorisation anywhere. All are supplied for laboratory research only, and no claim is made about pigmentation, inflammation, cognition or any condition in any person.

Extended research context

The Semax (ACTH Fragment Peptide) deep dive

Deep dive: what the Pro-Gly-Pro extension actually accomplishes

Semax is built from a four-residue fragment of ACTH with Pro-Gly-Pro appended, and that appendix does two separate jobs at once. Proline is the only proteinogenic amino acid whose side chain bonds back to its own backbone nitrogen, forming a ring that removes the amide hydrogen and locks rotation. Proteolytic enzymes generally need an extended, rotatable backbone at their active site, so bonds near proline are poor substrates for most of them — which is why proline-rich motifs recur throughout stabilised peptide design. The second job is subtractive: Pro-Gly-Pro occupies the positions where ACTH carries Arg-Trp-Gly, and those are the residues contributing to the parent hormone's adrenal-stimulating activity. One substitution therefore buys protease resistance and removes an unwanted pharmacology, which is unusually economical design.

Deep dive: the naming point worth getting right

Semax is near-universally described as an ACTH(4-10) analogue, and peer-reviewed paper titles use that phrase. Structurally it is not quite that. ACTH residues 4 to 10 are Met-Glu-His-Phe-Arg-Trp-Gly; Semax is Met-Glu-His-Phe-Pro-Gly-Pro. The first four residues match and the last three are wholly different — replacement rather than modification. PubChem, indexing by structure rather than design lineage, records it as ACTH(4-7) plus Pro-Gly-Pro. Neither name is wrong, but the loose one obscures the fact that the substituted segment is precisely the functionally consequential part.

Deep dive: reading an unevenly distributed evidence base

Semax's roughly 231 indexed records split along an unusual line. Mechanistic work — BDNF and trkB expression in rat hippocampus, transcriptomics in focal cerebral ischaemia, neurotrophin dynamics across brain regions — appears in international journals in English and can be assessed directly. Clinical work is concentrated in Russian-language publications, principally the Korsakov Journal of Neurology and Psychiatry, indexed by translated title and often without accessible English full text, much of it predating current standards for pre-registration and reporting. The honest position is that this is evidence which is difficult to verify independently, which is a different thing from evidence that is absent, and a different thing again from evidence that is established.

Research applications

▸Neurotrophin expression research (BDNF, NGF, TrkB, TrkA)
▸Neuroprotection and cerebral ischaemia models
▸Study of proline-stabilised peptide design
▸Comparative work on ACTH fragments without steroidogenic activity
▸Transcriptomic profiling in rodent brain models

Handling checklist

✓Store lyophilised material cold, dry and protected from light
✓Expect methionine oxidation as the degradation route (+16 Da; only one Met, so not +32)
✓No reducing agent needed — the sequence contains no cysteine
✓Introduce diluent gently against the vial wall; swirl rather than shake
✓Aliquot to avoid repeated freeze-thaw cycles
✓Check whether a certificate reports net peptide content or gross salt weight

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Describing Semax as ACTH(4-10) without qualification

Fix: It shares only residues 4-7; Pro-Gly-Pro replaces Arg-Trp-Gly, and that replacement is the design's whole point.

✗ Treating Russian registration as equivalent to MHRA approval

Fix: Authorisations are jurisdictional and do not transfer. Semax has never been assessed by the MHRA, EMA or FDA.

✗ Reading a BDNF expression change as a demonstrated outcome

Fix: The studies measured gene and protein expression in rats. Expression is upstream of function and upstream again of any clinical claim.

✗ Assuming a named receptor exists

Fix: No primary receptor has been definitively established; the literature characterises downstream effects more confidently than the initiating event.

✗ Expecting ACTH-like adrenal effects

Fix: Semax does not stimulate adrenal steroidogenesis — the responsible residues were substituted out.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Why do fragments of one hormone differ so much?: Because function is not spread evenly along a sequence. Which residues a fragment keeps determines which activities it can retain.
What is the melanocortin receptor-binding core?: His-Phe-Arg-Trp, positions 6 to 9 of alpha-MSH. A fragment retaining it can engage those receptors; one without it cannot.
Why does Semax have no identified receptor?: It retains only part of the core — His and Phe — and replaces the rest, so it is neither clearly a receptor ligand nor clearly not one.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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