ABSTRACT
The thioredoxin system, which is an oxidoreductase system, occurs in virtually all living cells and consists of three vital components, thioredoxin (Trx), thioredoxin reductase (TrxR), and NADPH (Arnér, 2009; Arnér and Holmgren, 2000; Lu and Holmgren, 2014). In most organisms, this flavoprotein system performs thioldependent/thiol-disulfide exchange reactions to maintain cellular redox homeostasis that play essential roles in many cellular processes. These cellular processes encompass a wide range of roles that include cell growth, transcription, DNA synthesis and repair, redox regulation of signal transduction, protection against oxidative damage, and apoptosis (Arnér, 2009; Gromer et al., 2004; Labunskyy et al., 2014; Lu and Holmgren, 2014). Thioredoxin reductases (TrxRs) occur in two known classes, wherein one class has been characterized in prokaryotes and lower eukaryotes with proteins of ~35,000 molecular weight, and another class has been characterized in animals with proteins of ~55,000 molecular weight that share a common ancestry with glutathione reductase (Arnér, 2009; Arnér and Holmgren, 2000; Lu and Holmgren, 2014). These two classes evolved independently of each other, but all TrxRs are flavoproteins and function as homodimers although the specific details of their reaction mechanisms differ.
