The thioredoxin redox system is composed of the NADPH-dependent homodimeric flavoprotein thioredoxin reductase (TrxR) and the 12-kDa protein thioredoxin. It is responsible for the reduction of disulfide bridges in proteins such as ribonucleotide reductase and several transcription factors. Furthermore, thioredoxin is involved in the detoxification of hydrogen peroxide and protects the cell against oxidative damage. There exist two classes of TrxRs: the high M(r) and the low M(r) proteins. The well characterized Escherichia coli TrxR represents a member of the low M(r) class of proteins, whereas the mammalian, Caenorhabditis elegans, and Plasmodium falciparum proteins belong to the family of high M(r) proteins. The primary structure of these proteins is very similar to that of glutathione reductase and lipoamide dehydrogenase. However, the high M(r) TrxRs possess, in addition to their redox active N-terminal pair of cysteines, a pair of cysteine residues or a selenenylsulfide motif at their C terminus. These residues have been shown to be crucial for the reduction of thioredoxin. In this study we address the question whether the active site residues of P. falciparum TrxR are provided by one or both subunits. Differentially tagged wild-type and PfTrxR mutants were co-expressed in E. coli and the recombinant protein species were purified by affinity chromatography specific for the respective tags of the recombinant proteins. Co-expression of PfTrxR wild-type and mutant proteins resulted in the formation of three different protein species: homodimeric PfTrxR wild-type proteins, homodimeric mutant proteins, and heterodimers composed of one PfTrxR wild-type subunit and one PfTrxR mutant subunit. Co-expression of the double mutant PfTrxRC88AC535A with PfTrxR wild-type generated an inactive heterodimer, which indicates that PfTrxR possesses intersubunit active sites. In addition, the data presented possibly imply a coopertive interaction between both active sites of PfTrxR.