Bone-signaling peptide fragment: parathyroid hormone (1, 13)
A tiny piece of the natural hormone that controls calcium and bone growth, used in labs to study how the bone-building pathway switches on; 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
Parathyroid hormone (1–13), or PTH(1–13), is a short 13-amino-acid fragment cut from the front end of human parathyroid hormone — the natural hormone the body uses to manage calcium and to drive new bone formation. The full hormone is 84 residues long; this fragment is just the first thirteen, the very tip that contacts the receptor. It exists mainly as a laboratory tool for probing exactly how the parathyroid hormone receptor (PTH1R) switches on, rather than as a drug in its own right. Its clinically familiar cousins are the longer N-terminal fragments — teriparatide (PTH 1–34), which is the approved bone-building osteoporosis drug (Marcus 2011), and abaloparatide, a synthetic PTHrP 1–34 analog (Brent 2021).
History
Parathyroid hormone has been on the radar of bone and calcium researchers for over a century, but the modern story of using its N-terminal piece as a therapy began with PTH(1–34) / teriparatide. Marcus (2011), one of the principal investigators, recounts how the recognition that intermittent low-dose PTH builds bone rather than dissolves it led to teriparatide's development and eventual approval as the first anabolic osteoporosis agent. Shorter fragments such as PTH(1–13), PTH(1–14) and PTH(1–15) emerged later as research tools to dissect which parts of the hormone are needed for binding versus activation: the N-terminal residues drive receptor activation, while the C-terminal half of PTH(1–34) is what anchors the hormone to the receptor's extracellular domain (Gardella 2015; Dean 2006).
What it does
PTH(1–13) binds to and activates the parathyroid hormone type 1 receptor (PTH1R), a G-protein-coupled receptor expressed on bone-forming osteoblasts and on kidney cells that handle calcium and phosphate (Gardella 2015). Compared to the full hormone or to PTH(1–34), the 1–13 fragment is a much weaker binder because it lacks the C-terminal residues that normally dock into PTH1R's extracellular domain — so on its own it only activates the receptor at relatively high concentrations. In structure-function work it is used precisely because of this weakness: it isolates the "activation" half of the ligand from the "affinity-providing" half, letting researchers test how the N-terminus alone engages the receptor's transmembrane core (Zhao 2016; Dean 2006).
Mechanism
PTH1R is a class B (secretin-family) G-protein-coupled receptor. Class B GPCRs use a two-domain binding mode: the C-terminal portion of the peptide ligand first docks into the receptor's large extracellular domain (ECD), and then the N-terminal portion of the ligand inserts into the transmembrane bundle (the so-called juxtamembrane or J domain) to trigger activation and Gαs coupling (Gardella 2015; Sutkeviciute 2019). The cryo-EM structure of the active human PTH1R in complex with a long-acting PTH analog and stimulatory G protein, solved by Zhao and colleagues (Science 2019), visualizes both ends of this interaction directly.
PTH(1–13) corresponds to that activation-providing N-terminal segment. Dean and colleagues (2006) developed a modified ¹²⁵I-PTH(1–15) radioligand specifically designed to bind only the juxtamembrane portion of PTH1R — the extracellular loops and transmembrane helices — and used it to probe Gαs-coupled receptor conformations independently of the ECD. Zhao and colleagues (2016) extended this comparison across class B GPCRs, showing that the extracellular domain contributes very differently to activation depending on the receptor and confirming that short N-terminal peptides like PTH(1–13/14/15) selectively engage the transmembrane core.
The stored sequence here is the bare 13 amino acids (SVSEIQLMHNLGK). Many of the published "PTH(1–13)" or "PTH(1–14/15)" tool ligands are actually modified analogs — for example with Aib (α-aminoisobutyric acid) substitutions at positions 1 and 3, an N-methylated or Nle residue at position 8, a C-terminal amide cap, or radiolabel-suitable substitutions at positions 14 and 15 — used to stabilize the helical conformation and boost potency at the truncated J-domain site (Dean 2006). The plain unmodified 1–13 sequence shown on this card does not include those modifications.
Evidence
- Human: No clinical trials of PTH(1–13) as a therapeutic. The clinically tested N-terminal PTH analogs are the longer PTH(1–34) (teriparatide) and the PTHrP-based abaloparatide (Marcus 2011; Brent 2021).
- In vitro / structural: PTH(1–13) and the closely related 1–14 / 1–15 fragments (often in Aib-substituted form) are documented in PTH1R pharmacology as N-terminal "activation domain" ligands that selectively probe the juxtamembrane site of the receptor (Dean 2006; Zhao 2016; Gardella 2015).
- Analytical chemistry: Mass-spectrometry-based PTH immunoassays have used the N-terminal region of PTH (including residues spanning the 1–13 segment) to distinguish full-length PTH(1–84) from circulating N-terminally truncated fragments such as PTH(7–84) — relevant to the diagnosis of endocrine and bone disease (Lopez 2010; Kumar 2010).
Known effects
- PTH1R activation (Gαs / cAMP) — confirmed in receptor pharmacology assays for N-terminal PTH fragments including the 1–13 / 1–14 / 1–15 series, generally requiring stabilizing modifications for usable potency (Dean 2006; Gardella 2015).
- No standalone bone-anabolic effect documented — bone-forming activity in vivo is established for the longer PTH(1–34) and PTHrP(1–34) analogs, not for the bare PTH(1–13) sequence (Marcus 2011; Brent 2021; Hattersley 2016).
Regulatory status
PTH(1–13) itself is not an approved drug and is not in clinical development as a therapeutic. It is used as a research peptide. The related clinically approved N-terminal PTH-family analogs are:
- Teriparatide — recombinant human PTH(1–34), an approved anabolic agent for osteoporosis (Marcus 2011).
- Abaloparatide — a synthetic analog of human PTHrP(1–34) with selective binding to a specific PTH1R conformation, also approved for osteoporosis (Brent 2021; Hattersley 2016).
Related peptides
- Teriparatide / PTH(1–34) — the clinically used N-terminal PTH fragment; PTH(1–13) is its truncated activation-domain core.
- Abaloparatide — synthetic PTHrP(1–34) analog with conformational selectivity for PTH1R (Hattersley 2016; Brent 2021).
- PTH(1–14), PTH(1–15) — closely related N-terminal tool fragments used alongside PTH(1–13) to dissect class B GPCR activation (Dean 2006; Zhao 2016).
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 the same inflammation or aging that damages bone also chemically alter the hormone meant to protect it, making the hormone less effective?
If this holds, it would mean oxidative stress, common in aging and inflammatory bone loss, could sabotage the parathyroid hormone signal before it even reaches bone cells. Researchers could then look for ways to protect or mimic the hormone in people whose bones are most at risk.
Could a very small piece of a bone hormone slow the cartilage damage that drives osteoarthritis?
If this pans out, it could open a new approach to osteoarthritis, a condition with no good disease-modifying treatments today. A minimal fragment that targets cartilage cells specifically might slow joint breakdown without the side effects tied to full-length bone therapies.
If a peptide is disordered on its own but snaps into shape only when it reaches its target, does that make it weaker as a drug candidate, and can small tweaks fix that?
Understanding this folding cost could tell researchers exactly how much potency is gained by pre-shaping the peptide with chemical modifications. That kind of roadmap would make designing compact, effective bone-signaling drugs faster and less of a guessing game.
Could this short hormone fragment be specific enough to activate the bone-building receptor without also triggering a related receptor linked to pain and the nervous system?
If confirmed, this fragment could serve as a cleaner starting point for bone-building drugs, ones that might avoid side effects tied to the second receptor. For people needing long-term osteoporosis treatment, a more targeted option could matter.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.9008890986442566 | boltz-2 |
| ranking score | 0.6274662017822266 | boltz-2 |
▸3-letter notation
▸recipeboltz-2 2.2.1
| parameter | value |
|---|---|
| model | boltz-2 2.2.1 |
| weights | — |
| hardware | vast_v100_32gb |
| mlx version | — |
| python | — |
| random seed | 1 |
| msa strategy | colabfold_local |
| runtime | — |
| predicted by | — |
| predicted at | 2026-05-22 |
▸citationbibtex
@peptide{pep10661,
sequence = {SVSEIQLMHNLGK},
target = {pth1r},
author = {peptidemodel},
year = {2026},
status = {synthesized}
}