Zinc Coordination by Thymosin β4: Structural Determinants and Functional Implications

Thymosin β4 (Tβ4) is a highly acidic, intrinsically disordered 43-amino-acid peptide with diverse biological functions, yet its interactions with metal ions remain poorly understood. In this study, we provide the first experimental demonstration that Tβ4 forms discrete Zn²⁺-bound adducts and undergoes Zn²⁺-induced aggregation under physiological pH conditions. Combining zeta potential analysis, dynamic light scattering (DLS), electrospray ionization mass spectrometry (ESI-MS), nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscopy with elemental mapping (SEM/EDS), we show that Zn(II) binding progressively neutralizes Tβ4's negative surface charge and triggers a sharp aggregation transition. ESI-MS unambiguously identifies Tβ4/Zn(II) complexes of peptide-to-zinc molar ratio 1:3, while DLS and SEM reveal the formation of compact, low-solubility supramolecular assemblies. NMR measurements support a metal-induced aggregation, confirming the absence of folding upon Zn(II) binding. By quantitatively comparing the experimentally determined critical aggregation concentration with physiologically observed extracellular Zn(II) ranges, we demonstrate that aggregation is unlikely in plasma or basal interstitial environments but may become feasible in Zn-rich microdomains, such as the synaptic cleft, where transient Zn(II) levels can exceed 1 μM. These findings introduce a previously unrecognized dimension of Tβ4 chemistry and suggest that a Zn(II)-mediated supramolecular assembly of Tβ4 could influence peptide behavior in neurological or inflammatory conditions characterized by elevated extracellular Zn(II). This work establishes a foundational biochemical framework for future studies aimed at elucidating the biological implications of Tβ4/Zn(II) complexation and aggregation in vivo.