Research digest · Thymosin beta-4 (Ac-LKKTETQ)
TB-500 is the actin-binding fragment of thymosin beta-4 — here is what the research actually shows.
Most of the efficacy data belong to the full-length parent protein, the isolated seven-amino-acid fragment has no completed controlled human trial, and this digest holds that distinction on every page.
TB-500 in one paragraph
TB-500 is the synthetic, N-acetylated heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), corresponding to residues 17–23 of the 43-amino-acid protein thymosin beta-4 [1]. Those seven residues are the conserved actin-binding motif of the beta-thymosins — the part of the parent protein that grips monomeric actin. That single structural fact governs the whole record. Thymosin beta-4 binds globular (G-) actin one-to-one and caps both ends of the monomer to hold a buffered pool of unpolymerized actin, controlling cytoskeletal dynamics and cell migration [1]. TB-500 carries that motif and nothing else.
Here is the distinction this site keeps in front of you on every page: in commerce and in the anti-doping literature, "TB-500" means the ~889-dalton heptapeptide, but the overwhelming majority of published efficacy studies were run with full-length recombinant or synthetic thymosin beta-4 (~4963 daltons), not the seven-mer [1]. Whether the isolated fragment reproduces the parent protein's effects at the doses used in peptide research is not established in controlled human trials. So this is a digest of two related but separable things — a well-studied parent protein and a much-less-studied fragment that borrows its name — and it labels which is which every time a number appears.
TB-500 as a research peptide: the Ac-LKKTETQ fragment
The TB-500 peptide is a laboratory and veterinary-context construct, not an endogenous molecule. The parent thymosin beta-4 is ubiquitous — present in nearly all human cells and released by platelets and macrophages at sites of injury — but the Ac-LKKTETQ fragment itself is synthesized, supplied as a lyophilized powder, and reconstituted for research use [1]. Its molecular formula is C38H68N10O14 at roughly 889 daltons; the parent protein, by contrast, is a 43-residue chain near 4963 daltons.
That size gap matters for interpretation. The N-terminal region of full-length thymosin beta-4 can be cleaved to Ac-SDKP, a separate peptide with its own anti-fibrotic and angiogenic activity — but Ac-SDKP comes from the N-terminus, not the C-terminal-region LKKTETQ segment that constitutes TB-500, so the fragment does not generate it. When a study reports a thymosin beta-4 finding, the first question this digest asks is which molecule was actually in the syringe. For the structural and biochemical backbone of that story, see the thymosin beta-4 research base.
What the literature establishes — and where it stops
X-ray crystallography of a gelsolin-domain-1–thymosin beta-4 hybrid bound to actin, resolved to 2 ångström, established that the protein forms a one-to-one complex with G-actin and sequesters the monomer by capping both ends, preventing polymerization [1]. That is the firmest fact in the file: a solved structure, not an inference. In a rat full-thickness wound model, topical or intraperitoneal thymosin beta-4 increased re-epithelialization by 42% at four days and up to 61% at seven days versus saline [2]. In mice, the protein activated the PINCH–ILK–Akt survival pathway and improved cardiac function after coronary artery ligation [3]. And in a randomized, placebo-controlled Phase 1 study, intravenous synthetic thymosin beta-4 was well tolerated in healthy volunteers up to 1260 mg with no dose-limiting toxicities [4].
Now the honest edge. Every one of those four findings used full-length thymosin beta-4, not the TB-500 fragment. There are no completed controlled clinical trials of the heptapeptide for any indication [4]. There is no validated human pharmacokinetic half-life for it. And the same pro-migratory, pro-angiogenic biology that aids repair is the basis of a recognized tumor/angiogenesis safety signal [12]. This digest organizes the record by confidence: a solved structure and a reproduced animal figure are bright signals; a fragment with no human efficacy trial is a clearly labeled gap. For the recovery-specific evidence, see TB-500 and muscle recovery.
Definitional questions, answered
The short answers below resolve the most common confusions before you go deeper. Each is expanded in the relevant page, and every quantitative claim is cited.
What is TB-500?
TB-500 is the synthetic N-acetylated heptapeptide Ac-LKKTETQ, corresponding to residues 17–23 — the actin-binding motif — of the 43-amino-acid protein thymosin beta-4 [1]. It is a research and veterinary-context substance with no approved human indication. It is not an endogenous molecule itself; the parent protein is.
What does TB-500 stand for?
"TB" references thymosin beta-4 (Tβ4), the parent protein; "TB-500" is the commercial and veterinary research designation for its synthetic Ac-LKKTETQ actin-binding fragment [1]. The number is a product-line label, not a chemical descriptor. The same fragment also circulates under designations such as TB1000.
What is TB-500 used for in research?
In animal and cell models, thymosin beta-4 and its actin-binding region are studied for wound healing, muscle and ligament repair, angiogenesis, and cardiac and neurological recovery [5][7][8]. The efficacy of the isolated seven-mer is unproven in controlled human trials [4]. Studied outcomes are properties of the parent protein far more often than of the fragment.
How does TB-500 work?
TB-500 carries the actin-binding LKKTETQ motif of thymosin beta-4, the body's main G-actin sequestering peptide [1]. Full-length thymosin beta-4 binds monomeric actin one-to-one to regulate cytoskeletal dynamics, cell migration, angiogenesis, and anti-inflammatory signaling [5]. Whether the isolated heptapeptide reproduces these effects at research doses is not established in humans.