TB-500 Fragment 17-23 (Ac-LKKTETQ): What It Is and How It Works
TB-500 fragment 17-23 is the Ac-LKKTETQ sequence from thymosin beta-4 — the actin-binding region. A research guide to the sequence, its real molecular weight, how actin sequestration actually works, and why vendor labeling is inconsistent.
What Is TB-500 Fragment 17-23?
TB-500 fragment 17-23 is a short synthetic peptide with the sequence Ac-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ) — seven amino acids taken from positions 17 to 23 of the 43-amino-acid protein thymosin beta-4 (Tβ4). This region contains the LKKTET motif, the part of Tβ4 that binds actin, and it is the sequence most commonly sold under the research name "TB-500."
That said, one thing worth knowing up front: "TB-500" is a product name, not a standardized chemical designation. Some suppliers use it for this 7-amino-acid fragment; others use it for full-length Tβ4. The sequence on the certificate of analysis, not the label on the vial, is what tells you which one you actually have.
> Research disclaimer: This article describes peptide chemistry and published mechanistic research for informational and research purposes only. TB-500 is sold as a research chemical and is not for human consumption. Nothing here is medical advice, and none of the mechanisms described below have been demonstrated as a clinical treatment in humans.
What Does "Fragment 17-23" Actually Mean?
Thymosin beta-4 is a small, intrinsically disordered protein of 43 amino acids found in nearly every mammalian cell type, and it is one of the most abundant proteins in many cells. Not all 43 residues do the same job — the protein has distinct regions with distinct functions.
"Fragment 17-23" is simply positional notation: amino acids 17 through 23 of that sequence. Researchers isolated this stretch because it carries the LKKTET motif, which mediates Tβ4's interaction with actin. The idea behind synthesizing it separately is straightforward — if a short sequence carries the activity you care about, it is cheaper to make, easier to characterize, and simpler to work with than the full protein.
The important caveat is that isolating a motif does not automatically reproduce everything the parent protein does. Full-length Tβ4 has functions that the fragment does not carry, and the two should not be treated as interchangeable.
How Much Does the Fragment Actually Weigh?
This is where a persistent piece of misinformation circulates, including in older versions of this page. The "500" in TB-500 is not a molecular weight.
The acetylated 7-amino-acid fragment Ac-LKKTETQ has a molecular weight of roughly 890 daltons — not 500. For comparison, full-length thymosin beta-4 is about 4,921 daltons. So the fragment is roughly one-fifth the mass of the full protein, but the number in the product name does not correspond to either figure.
| Property | Fragment 17-23 (Ac-LKKTETQ) | Full Thymosin Beta-4 |
|----------|------------------------------|----------------------|
| Amino acids | 7 | 43 |
| Molecular weight | ~890 Da | ~4,921 Da |
| Contains LKKTET actin-binding motif | Yes | Yes |
| Additional non-actin functions | Not retained | Present |
| Body of published research | Smaller | Larger |
If you see "TB4-Frag 500" and "TB-500" listed as separate catalog items, compare the stated sequences. If both read Ac-LKKTETQ, they are the same compound under two names.
How Does the Fragment Work at the Molecular Level?
This is the part most summaries get backwards, so it is worth being precise.
Thymosin beta-4 is the major G-actin sequestering peptide in mammalian cells. Actin exists in two forms: G-actin, the free globular monomer, and F-actin, the polymerized filament that gives cells structure and drives movement. Tβ4 binds G-actin monomers and holds them in an unpolymerized reserve.
That is the opposite of "forcing polymerization." Tβ4 acts as a buffer: it maintains a large, ready pool of monomeric actin that the cell can rapidly release and polymerize into filaments at the leading edge exactly when and where it needs to move. Cell migration depends not on maximizing F-actin, but on controlling assembly and disassembly with tight spatial and temporal precision — and a monomer reservoir is what makes that control possible.
Downstream of this actin regulation, published research on Tβ4 has examined:
1. Cell migration — keratinocytes, endothelial cells, and fibroblasts migrating toward injury sites
2. Angiogenesis — endothelial cell migration and new blood vessel formation
3. Wound healing — faster re-epithelialization in animal and cell-culture models
4. Inflammation modulation — effects on inflammatory signaling, studied mainly with the full protein
A fair reading of the literature: the actin-sequestering role of the LKKTET motif is well characterized, and the fragment retains measurable repair-related activity in animal and in vitro models. But most formal mechanistic research has been conducted with full-length Tβ4, not the 7-amino-acid fragment, and results from one should not be silently attributed to the other. Our mechanism of action article covers these pathways in more depth.
Is the Fragment More Bioavailable Than Full Tβ4?
You will frequently read that the fragment's smaller size makes it dramatically more bioavailable. Treat this claim carefully — it is a plausible inference, not a well-documented finding.
Smaller peptides do generally diffuse through tissue more readily, and that reasoning is not unreasonable on its face. But comparative pharmacokinetic studies putting Ac-LKKTETQ head-to-head against full-length Tβ4 in humans do not exist in the published literature. Short unstructured peptides are also rapidly degraded by peptidases, which cuts against any simple "smaller equals better exposure" story.
The honest position is that the fragment's absorption, distribution, and clearance are not well mapped, and confident numerical claims about its bioavailability are not supported by data. Our half-life and timing guide discusses what is and is not known about the kinetics.
What Research Actually Exists on This Sequence?
The evidence base is best understood in tiers:
That last tier is the one that matters most for anyone trying to reason about what this compound does in people. A well-characterized molecular mechanism is a starting point for research, not evidence of a clinical effect. For a fuller comparison of the fragment against the parent protein, see our TB-500 vs TB4 fragment comparison, and for how the fragment compares to a different repair peptide entirely, our TB-500 vs BPC-157 comparison.
Frequently Asked Questions
Is TB-500 the same as fragment 17-23?
Usually, but not always. TB-500 is a commercial name most often applied to the synthetic peptide Ac-LKKTETQ, corresponding to thymosin beta-4 amino acids 17-23. However, some suppliers apply the same name to full-length Tβ4. Because "TB-500" is a product name rather than a standardized chemical designation, the only reliable way to know what is in a vial is to check the sequence on the certificate of analysis.
What is the molecular weight of TB-500 fragment 17-23?
The acetylated fragment Ac-LKKTETQ weighs approximately 890 daltons. The "500" in the name TB-500 is not a molecular weight and does not correspond to the fragment's mass. Full-length thymosin beta-4, by contrast, is approximately 4,921 daltons.
Does TB-500 increase actin polymerization?
Not in the straightforward way it is often described. Thymosin beta-4 is the major G-actin sequestering peptide — it binds monomeric actin and holds it in an unpolymerized reserve rather than driving filament assembly. That buffered monomer pool is what allows a cell to rapidly polymerize actin at a specific location when it migrates. The effect on cell movement is regulatory, not simply additive.
Is thymosin beta-4 the same as TB-500?
No. Thymosin beta-4 is the complete 43-amino-acid protein produced naturally in the body. The fragment sold as TB-500 is a 7-amino-acid sequence drawn from it. The fragment contains the actin-binding motif but does not carry the full protein's other functions, and the two have different molecular weights and different bodies of research behind them.
Is the fragment better than full thymosin beta-4?
There is no evidence establishing that it is. The fragment is smaller and cheaper to synthesize, and it retains the actin-binding motif, but comparative studies in humans do not exist. Most published mechanistic research used the full protein, so claims that the fragment performs equally well or better are inferences rather than demonstrated findings.
Sourcing Quality Research Peptides
For laboratory research applications, sequence identity is the whole question with this compound — as covered above, the name on the vial does not reliably tell you whether you have the 7-amino-acid fragment or the full protein. Look for vendors that publish third-party testing and certificates of analysis stating the actual sequence alongside HPLC purity above 98%. Apollo Peptide Sciences provides independent third-party testing and full COAs on its TB-500, which matters whenever your research depends on knowing exactly what is in the vial.