Redox-dependent structural switching converts Escherichia coli thioredoxin 2 into a molecular chaperone.

Thioredoxins (Trxs) are ubiquitous oxidoreductases that maintain cellular redox homeostasis through thiol-disulfide exchange reactions. Escherichia coli thioredoxin 2 (EcTrx2) possesses a unique N-terminal zinc-binding domain absent from the canonical thioredoxin EcTrx1, but the physiological significance of this domain has remained unclear. Here we show that EcTrx2 undergoes reversible, redox-dependent structural switching accompanied by a functional conversion under oxidative stress. Oxidative conditions promoted the formation of high-molecular-weight (HMW) oligomeric complexes, whereas reducing conditions favored low-molecular-weight (LMW) species. Increased surface hydrophobicity of oxidized EcTrx2 correlated with a marked enhancement of holdase chaperone activity and a concomitant reduction in disulfide reductase activity. Size-exclusion chromatography coupled with transmission electron microscopy further revealed that the HMW oligomers were the predominant chaperone-active species, whereas the LMW form primarily retained reductase activity. Deletion of the N-terminal zinc-binding domain abolished the redox-dependent structural transition and impaired chaperone activation, demonstrating that this domain is required for stress-responsive functional switching. These findings identify EcTrx2 as a redox-regulated molecular chaperone and provide a mechanistic basis for reversible structural and functional switching in a bacterial thioredoxin during oxidative stress.