Thioredoxin reductases (TR, TrxR) are the only known enzymes to reduce
thioredoxin (Trx). Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. Both classes are
flavoproteins which function as homodimers. Each monomer contains a
FAD prosthetic group, a
NADPH binding domain, and an active site containing a redox-active
disulfide bond.