Two thioredoxins have been described in Escherichia coli, TrxA and Trx2. Both thioredoxins are capable of reducing disulfide bonds using a conserved pair of cysteine residues present in a WCGPC motif. A number of unique structural and regulatory features distinguish the Trx2 subfamily from the much larger TrxA family. The Trx2 subfamily has an additional N-terminal domain of +/- 30 residues, which contains two additional conserved CXXC motifs. Moreover, the gene coding for Trx2 is under control of the oxidative stress transcription factor OxyR in E. coli. This suggests that Trx2 may play a role in the cellular defense against oxidative stress. We show here that Trx2 contains zinc in a 1:1 stoichiometry, making it the first identified zinc-binding thioredoxin. The zinc atom is coordinated by the four cysteines of the two N-terminal CXXC motifs. The zinc center of Trx2 binds zinc with a very high affinity (Ka of >1018 m-1). We show that in vitro oxidation of the zinc binding cysteines by H2O2 releases the zinc and induces a conformational change. The zinc-free protein conserves its reductase activity. Altogether, our results suggest that the zinc center might play the role of a redox switch, changing a yet to be identified activity.