Fertility hormone fragment that switches on the FSH receptor (FSH β 33: 53)
A small piece of the natural fertility hormone FSH that can partly switch on its own receptor by itself; used only as a lab research tool.
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
FSH β-chain (33–53) is a 21-amino-acid fragment taken from positions 33 to 53 of the beta subunit of follicle-stimulating hormone (FSH), a pituitary hormone that drives egg and sperm development. The fragment was synthesised and studied to understand which part of FSH physically contacts its receptor, the FSH receptor (FSHR). It turned out to be one of the two principal receptor-binding segments of the FSH beta subunit — and it can activate that receptor partially on its own, without the rest of the FSH molecule (Santa Coloma, Dattatreyamurty, and Reichert, Biochemistry, 1990).
History
FSH had been known for decades to drive follicle development in women and spermatogenesis in men, but the specific surface of the hormone that docks onto FSHR was unclear. Early receptor-mapping work in the 1980s identified a short tetrapeptide, Thr-Arg-Asp-Leu (TRDL), within the beta subunit as a receptor-contact motif (Dattatreyamurty and colleagues, 1987). Expanding that lead, Santa Coloma, Dattatreyamurty, and Reichert synthesised the full 21-residue sequence spanning positions 33–53 of the FSH beta subunit and published the first characterisation of its receptor binding and biological activity in 1990. Their study confirmed the fragment binds FSHR and produces measurable biological effects — establishing it as a research tool for mapping hormone–receptor recognition. Subsequent work in 1992 showed that replacing the cysteine at position 51 with serine did not abolish binding inhibition, clarifying that the cysteine is not essential for receptor contact (Santa-Coloma, Crabb, and Reichert, Biochem Biophys Res Commun, 1992).
What it does
The fragment binds the FSH receptor and produces two opposing effects simultaneously: it weakly stimulates estradiol production in rat Sertoli cells at baseline, but reduces the response when full FSH is also present. In pharmacological terms this is partial agonism — the peptide activates the receptor, but not as strongly as intact FSH — combined with competitive partial antagonism (Santa Coloma and colleagues, Biochemistry, 1990). The Kd measured in that study was approximately 5.5 × 10⁻⁵ M, orders of magnitude weaker than native FSH, which reflects that the isolated fragment lacks the structural context provided by the full hormone. Researchers later showed that chemically linking this fragment with other FSH receptor-binding regions (from the alpha subunit and beta region 81–95) in a way that mimics their three-dimensional arrangement on the intact hormone raises potency substantially — demonstrating that the 33–53 segment is one component of a multi-contact recognition surface (Hage-van Noort and colleagues, PNAS, 1992).
Because FSHR is expressed on granulosa cells of the ovary and Sertoli cells of the testis in normal tissue, the fragment has also been used as a targeting vector for drug delivery. Zhang and colleagues (Cancer Research, 2009) conjugated the FSH β(33–53) peptide to paclitaxel-loaded nanoparticles and demonstrated improved anti-tumour effects in ovarian carcinoma models in vivo compared with naked nanoparticles. A subsequent study fused the fragment with a cationic antimicrobial peptide (IIKK) to create FSH33-53-IIKK, which selectively killed FSHR-expressing prostate, cervical, and breast cancer cells in vitro and reduced tumour growth in xenograft models (Chen and colleagues, J Cancer, 2016). The logic of these applications rests on the finding that FSHR, normally restricted to gonadal cells, is also expressed on the endothelium of tumour blood vessels across at least 11 cancer types — while being absent in normal tissue more than 10 mm from the tumour (Radu and colleagues, New England Journal of Medicine, 2010).
Evidence
- Human: No clinical trials. The peptide is a biochemical research tool and preclinical targeting vector; it has not entered human studies.
- Animal: Paclitaxel nanoparticles conjugated to the FSH β(33–53) targeting sequence outperformed unconjugated paclitaxel nanoparticles in mouse ovarian carcinoma xenograft models (Zhang and colleagues, 2009). FSH33-53-IIKK fusion peptide reduced prostate cancer xenograft tumour volume in vivo (Chen and colleagues, 2016). Related FSHβ peptide fragments spanning overlapping regions promoted spermatogenesis and follicle development in prepubertal mice after a short treatment course (Fan and colleagues, Frontiers in Endocrinology, 2022).
- In vitro: The fragment stimulates basal estradiol production and partially inhibits FSH-induced estradiol synthesis in rat Sertoli cell cultures (Santa Coloma and colleagues, 1990). FSHR-expressing cancer cell lines (prostate PC3, cervical HeLa, breast MCF7) are preferentially killed by FSH33-53-IIKK relative to FSHR-negative lines (Chen and colleagues, 2016).
Mechanism
FSH β(33–53) contacts the leucine-rich repeat (LRR) domain of the FSHR ectodomain. The core recognition motif appears to centre on the TRDL subdomain (residues 34–37), which was independently identified as capable of inhibiting FSH binding in the 1980s. The full 33–53 fragment encompasses this motif within a broader contact surface; circular dichroism and NMR analysis of the isolated peptide found approximately equal contributions from antiparallel beta-sheet and turn structures, with a small alpha-helical content, suggesting the fragment adopts a partially ordered conformation in solution relevant to receptor engagement. Receptor activation by the fragment — partial agonism — proceeds through Gαs-coupled cAMP signalling, the canonical FSHR pathway documented in Sertoli and granulosa cells (Gloaguen, Frontiers in Endocrinology, 2011; Ulloa-Aguirre and colleagues, Frontiers in Endocrinology, 2018). The crystal structure of FSH in complex with the entire FSHR ectodomain, including the hinge region, was solved in 2012 and showed that the hinge region forms an integral part of the receptor's extracellular domain rather than a separate structural unit — providing structural context for how peptide fragments of FSH can engage a subset of receptor contacts (Jiang and colleagues, PNAS, 2012).
Known effects
- FSHR binding — Documented; Kd ~5.5 × 10⁻⁵ M in radioligand assay (Santa Coloma and colleagues, 1990)
- Partial agonist of basal estradiol synthesis — Preclinical (rat Sertoli cell model)
- Partial antagonist of FSH-stimulated estradiol synthesis — Preclinical (rat Sertoli cell model)
- Tumour-targeting ligand — Preclinical; validated in ovarian and prostate xenograft models as a nanoparticle/fusion-peptide guiding component
Open questions
- No structural data on the fragment bound to FSHR in an atomic-resolution co-crystal; whether the solution conformation matches the receptor-bound conformation is unknown.
- Potency as an isolated peptide is very low (micromolar Kd); no optimised analogue with improved affinity has been characterised in published literature.
- Selectivity of FSHR-targeting payloads for tumour vasculature versus normal gonadal FSHR has not been fully characterised in humans.
- The role of biased FSHR signalling — distinct Gαs, β-arrestin, and Gαi/o pathways recently described at this receptor (Landomiel and colleagues, Frontiers in Endocrinology, 2019) — has not been explored for this fragment specifically.
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.
Does the four-residue sequence TRDL physically shift the FSH receptor into a partly active shape when it binds?
If true, it could lead to miniature drug molecules that gently activate the FSH receptor, potentially offering a safer alternative to injected FSH in fertility treatments where the current drugs can cause dangerous over-stimulation of the ovaries.
Do the final six residues of the fragment help it stick to the FSH receptor without contributing to actually activating it?
If the fragment has separable holding and activating parts, chemists could design stripped-down molecules that block the FSH receptor without switching it on, which could be useful in conditions like hormone-sensitive cancers where FSH receptor activation is harmful.
Can this partial FSH activator keep follicles alive at doses too low to cause overstimulation?
If true, it could offer women undergoing chemotherapy a gentler way to preserve their fertility, avoiding the ovarian hyperstimulation syndrome risk that limits use of standard FSH-based fertility drugs in this vulnerable group.
If the fragment is chemically locked into a ring shape at the natural turn, does it become a stronger FSH receptor activator?
A more potent, longer-lasting version of this natural fragment could one day replace the complex injected FSH preparations used in fertility clinics, potentially as a simpler, more stable drug.
Does this fragment exclusively activate the FSH receptor without accidentally triggering its close relatives, the LH and TSH receptors?
If the fragment is naturally selective, it could become a template for fertility drugs that do not cause thyroid or LH-pathway side effects, which is a known concern with some current fertility medications.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.7417252659797668 | boltz-2 |
| ranking score | 0.7386017441749573 | boltz-2 |
▸structural qualityopenfold3
| metric | value | note |
|---|---|---|
| gpde | 1.264 | 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{pep10788,
sequence = {YTRDLVYKDPARPNTQIVYNP},
target = {fshr},
author = {peptidemodel},
year = {2026},
status = {synthesized}
}