–Esha Sircar & Dr. Rajib Sengupta, Amity Institute of Biotechnology, Amity University Kolkata
The thioredoxin (Trx) system is a major antioxidant system that maintains the intracellular redox state. It contains Trx, a redox-active protein, which regulates the activity of various enzymes including those that function to counteract oxidative stress within the cell. Trx, NADPH, and thioredoxin reductase (TrxR) represent the complete thioredoxin system existing in nearly all living cells. It functions in thiol-disulfide exchange reactions essential to control the reduced intracellular redox environment, cellular growth, defense against oxidative/nitrosative stress, apoptosis and has many roles in mammalian cells including implications in cancer and neurodegenerative diseases. Numerous scientific reports describe the higher expression of Trx and TrxR in some, but not all tumors. Recent reports suggest that TrxR activity is essential for the carcinogenic process. Thus, both Trx and TrxR have been regarded as interesting targets for chemotherapy.
Trxs are a family of small ubiquitous dimeric proteins catalyzing thiol-disulfide oxidoreductions by using redox-active thiols that are present in a CXXC sequence motif. In oxidized Trx, the active site is a disulfide and in reduced Trx, it is a corresponding dithiol. Reduced Trx reduces protein disulfides (and also S-nitrosylated proteins) and forms oxidized Trx, which, in turn, is reduced by TrxR and NADPH. Trxs exist in all living cells and in multiple forms (19 isoforms in plants). Humans have two Trxs- cytosolic/nuclear (Trx1) and mitochondrial (Trx2), both of which are essential and embryonically lethal. The Trx fold is found in a large number of proteins, including glutaredoxins, glutathione peroxidases, glutathione transferases, protein disulfideisomerases (PDIs), the thioredoxin-related protein of 14 (TRP14), and thioredoxin-related protein of 32 (TRP32).
TrxR from bacteria, fungi, and plants are homodimers of 70 kDa with active site cysteine residues. In contrast, mammalian TrxRs are larger dimeric selenoenzymes (114 kDa or larger) with a catalytically active selenocysteine residue located in the C-terminus. The catalytic mechanism of mammalian TrxR involves the transfer of electrons from NADPH to the enzyme-bound FAD, which reduces the disulfide within the CVNVGC motif in the N-terminal domain that subsequently reduces the selenyl sulfide motif to a selenothiol at the C-terminal Sec- containing GCUG motif of the neighboring subunit. The selenothiol is the proper active site that can reduce many of the potential substrates of the enzyme. TrxR has three isoforms: TrxR1 (cytosol), TrxR2 (mitochondrial), and TrxR3 (present primarily in testis). The selenothiol is the proper active site that can reduce many of the potential substrates of the enzyme, and the essential role SeCys residue makes it the target of several drugs in cancer treatment.
There are a number of inhibitors with chemotherapy applications that target either Trx or TrxR to induce apoptosis in cancer cells. Many compounds target the SeCys-containing active site of TrxR. These include gold/platinum compounds, arsenic trioxide, nitrous compounds, and flavonoids. Inhibition of TrxR leads to an accumulation of oxidized Trx resulting in cellular conditions that cause programmed cell death (apoptosis). There are many more Trx-related proteins present in cells controlling numerous different cellular processes. Trx-interacting protein (Txnip), another ubiquitous protein, binds directly to the active site Cys of Trx and blocks the reductase activity. Many different studies signify major physiological roles for Txnip in glucose metabolism and cell differentiation. The expression of Txnip was induced by a variety of stresses, including UV light, heat shock, and H2O2. Overexpression of Txnip makes cells more susceptible to oxidative/nitrosative stress which promotes apoptosis. However, the cellular roles of Txnip, TRP14, and TRP32 have not yet been fully explored.
References:
- Physiological functions of thioredoxin and thioredoxin reductase, ESJ Arnér, A Holmgren, European Journal of Biochemistry 267 (20), 6102-6109
- Focus on mammalian thioredoxin reductases—important selenoproteins with versatile functions, ESJ Arnér, Biochimica et Biophysica Acta (BBA)-General Subjects 1790 (6), 495-526
- Thioredoxin and Thioredoxin Reductase in Relation to Reversible S-Nitrosylation, R Sengupta, A Holmgren, Antioxidants & redox signaling 18 (3), 259-269
Mankind is a click away from being in a clinical trial. Here’s How.
-Sashreek Ganguli, Amity University Kolkata The questions raised on the efficiency of a clinical trial in this global pandemic are deeply concerning. Operational short-comings in the current clinical trial procedures relating to the identification, recruitment, data acquisition, etc. increase costs and stretches the already lengthy clinical trial, leading to minimized patient participation. Here, the concept […]