Thioredoxin (Trx) and glutathione/glutaredoxin (GSH/Grx) systems play the dominant role in cellular redox homeostasis. Recently the Trx system has been shown to be responsible to control the balance of GSH/GSSG once the glutathione reductase system is not available. To decipher the structural basis of electron transfer from the Trx system to GSSG, we solved the crystal structures of oxidized Trx1 and glutathionylated Trx1Cys33Ser mutant at 1.76 and 1.80 A, respectively. Comparative structural analysis revealed a key residue Met35 involved in the Trx-GSSG recognition. Subsequent mutagenesis and kinetic studies proved that Met35Arg mutation could alter the apparent K(m) and V(max) values of the reaction. These findings gave us the structural insights into GSSG reduction catalyzed by the Trx system.