Biochemical properties of poplar thioredoxin z

Trx‐z is a chloroplastic thioredoxin, exhibiting a usual WCGPC active site, but whose biochemical properties are unknown. We demonstrate here that Trx‐z supports the activity of several plastidial antioxidant enzymes, such as thiol‐peroxidases and methionine sulfoxide reductases, using electrons provided by ferredoxin–thioredoxin reductase. Its disulfide reductase activity requires the presence of both active site cysteines forming a catalytic disulfide bridge with a midpoint redox potential of −251 mV at pH 7. These in vitro biochemical data suggest that, besides its decisive role in the regulation of plastidial transcription, Trx‐z might also be involved in stress response.