Thioredoxin reductase

Thioredoxin-disulfide reductase
Crystal structure of human thioredoxin reductase 1; rendering based on PDB: 2OHV​.
Identifiers
EC number	1.8.1.9
CAS number	9074-14-0
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
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Thioredoxin reductases (TR, TrxR) (EC 1.8.1.9) are the only known enzymes to reduce thioredoxin (Trx).^[1] 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.^[2]

Cellular Role

Thioredoxin reductase is the only enzyme known to catalyze the reduction of thioredoxin^[1] and hence is a central component in the thioredoxin system. Together with thioredoxin (Trx) and NADPH this system's most general description is as a method of forming reduced disulfide bonds in cells. Electrons are taken from NADPH via TrxR and are transferred to the active site of Trx, which goes on to reduce protein disulfides or other substrates.^[3] The Trx system exists in all living cells and has an evolutionary history tied to DNA as a genetic material, defense against oxidative damage due to oxygen metabolism, and redox signaling using molecules like hydrogen peroxide and nitric oxide.^[4][5]

Schematic diagram of TrxR's cellular role Adapted from Holmgren et al.^[3]

Diversity

Two classes of thioredoxin reductase have evolved independently:

• A high molecular weight (MW = ~55,000) type containing a selenocysteine residue in its active site has been identified in higher eukaryotes including humans. This TxR is related to glutathione reductase, trypanothione reductase, mercuric reductase and lipoamide dehydrogenase.^[2]
• A low molecular weight (MW = ~ 35,000) type has been identified in archaea, bacteria and other eukarya.^[2]

These two classes of TrxR have only ~20% sequence identity in the section of primary sequence where they can be reliably aligned.^[2] The net reaction of both classes of TrxR is identical but the mechanism of action of each is distinct.^[6]

Humans express three thioredoxin reductase isozymes: TrxR1 (cytosolic), TrxR2 (mitochondrial), TrxR3 (testis specific).^[7] Each isozyme is encoded by a separate gene:

thioredoxin reductase 1 Identifiers Symbol TXNRD1 Entrez 7296 HUGO 12437 OMIM 601112 RefSeq NM_003330 UniProt Q16881 Other data EC number 1.8.1.9 Locus Chr. 12 q23-q24.1

thioredoxin reductase 2 Identifiers Symbol TXNRD2 Entrez 10587 HUGO 18155 OMIM 606448 RefSeq NM_006440 UniProt Q9NNW7 Other data EC number 1.8.1.9 Locus Chr. 22 q11.21

thioredoxin reductase 3 Identifiers Symbol TXNRD3 Entrez 114112 HUGO 20667 OMIM 606235 RefSeq XM_051264 UniProt Q86VQ6 Other data EC number 1.8.1.9 Locus Chr. 3 p13-q13.33

Structure

E. coli thioredoxin reductase structure: In E. coli ThxR there are two binding domains, one for FAD and another for NADPH. The connection between these two domains is a two-stranded anti-parallel f3-sheet.^[8] Each domain individually is very similar to the analogous domains in glutathione reductase, and lipoamide dehydrogenase but they relative orientation of these domains in ThxR is rotated by 66 degrees.^[8] This becomes significant in the enzyme mechanism of action which is described below. ThxR homo-dimerizes with the interface between the two monomers formed by three alpha-helicies and two loops.^[8] Each monomer can separately bind a molecule of thioredoxin.

• 

  Structure of E. coli ThxR dimer bound thioredoxin

• 

  Structure of E. coli ThxR with FAD and NADPH prosthetic groups labeled

Mammalian thioredoxin reductase structure: Mammalian TrxR structure is similar to E. coli. It contains a FAD and NADPH binding domain, and an interface between two monomer subunits. In mammalian ThxR there is an insertion in the FAD binding domain between two alpha helices which forms a small pair of beta strands.^[9] The active disulfide in the enzyme is located on one of these helices and thus the active disulfide bond is located in the FAD domain and not the NADPH domain as in E. coli and other prokaryotes.^[9]

• 

  Structure of human ThxR FAD and NADPH prosthetic groups

Mechanism

E. coli thioredoxin reductase mechanism: In E. coli ThxR the spatial orientation of the FAD and NADPH domains are such that the redox-active rings of FAD and NADPH are not in close proximity to each other.^[1] When the FAD domain of E. coli is rotated 66 degrees with the NADPH domain remaining fixed the two prosthetic groups move into close contact allowing electrons to pass from NADPH to FAD and then to the active site disulfide bond.^[1][10] The conserved active site residues in E. coli are -Cys-Ala-Thr-Cys-.^[1]

Mammalian thioredoxin reductase mechanism: Mammalian TrxRs have a much higher sequence homology with glutathione reductase than E. coli.^[1] The active-site Cys residues in the FAD domain and bound NADPH domain are in close proximity removing the necessity for a 66 degree rotation for electron transfer found in E. coli. An additional feature of the mammalian mechanism is the presence of a selenocystine residue at the C-terminal end of the protein which is required for catalytic activity. The conserved residues in mammalian active site are -Cys-Val-Asn-Val-Gly-Cys-.^[1]

Proposed mechanism in mammals and presumably humans: Starting from the completely oxidized form, the reaction begins with the reduction of the selenenylsulfide to the selenolate anion (Se(-1)) with electrons received from NADPH via FAD (Step A). Due to the low pKa value of the selenol the selenolate anion is the predominant form under physiological conditions. A second electron transfer from a second molecule of NADPH reduces the active site tihiol bonds with one Cys residue stabilized by an interaction with FAD (Step B). The selenolate anion then attacks the disulfide bonds of Trx and the resulting enzyme-Trx mixed selenenylsulfide (Step C), which is then subsequently attacked by the neighboring Cys residue to regenerate the selenenylsulfide (Step D). This selenenylsulfide is then reduced by the active-site thiolate from the other subunit (Step E). Adapted from Zhong et al.^[11] Consistent with findings that (2,2‘:6‘,2‘‘-terpyridine)platinum(II) complexes inhibit human TrxR.^[12]

Clinical significance

- In cancer treatment: Since the activity of this enzyme is essential for cell growth and survival, it is a good target for anti-tumor therapy. Furthermore, the enzyme is upregulated in several types of cancer, including malignant mesothelioma.^[13][14] For example, motexafin gadolinium (MGd) is a new chemotherapeutic agent that selectively targets tumor cells, leading to cell death and apoptosis via inhibition of thioredoxin reductase and ribonucleotide reductase.^[15]

- In cardiomyopathy: Dilated cardiomyopathy (DCM) is a common diagnosis in cases of congestive heart failure. Thioredoxin reductases are essential proteins for regulating cellular redox balance and mitigating the damage caused by reactive oxygen species generated via oxidative phosphorylation in the mitochondria. Inactivation of mitochondrial TrxR2 in mice results in thinning of the ventricular heart walls and neonatal death.^[7] Furthermore two mutations in the TrxR2 gene are found in patients diagnosed with DCM and not in a control population. It is hypothesized that the pathological impact of these mutations is an impaired ability to control oxidative damage in cardiac myocytes.^[16]

References

1. Mustacich D, Powis G (Feb 2000). "Thioredoxin reductase". The Biochemical Journal. 346 Pt 1 (Pt 1): 1–8. doi:10.1042/0264-6021:3460001. PMC 1220815. PMID 10657232.
2. Hirt RP, Müller S, Embley TM, Coombs GH (Jul 2002). "The diversity and evolution of thioredoxin reductase: new perspectives". Trends in Parasitology 18 (7): 302–8. doi:10.1016/S1471-4922(02)02293-6. PMID 12379950.
3. Holmgren A, Lu J (May 2010). "Thioredoxin and thioredoxin reductase: current research with special reference to human disease". Biochemical and Biophysical Research Communications 396 (1): 120–4. doi:10.1016/j.bbrc.2010.03.083. PMID 20494123.
4. Meyer Y, Buchanan BB, Vignols F, Reichheld JP (2009). "Thioredoxins and glutaredoxins: unifying elements in redox biology". Annual Review of Genetics 43: 335–67. doi:10.1146/annurev-genet-102108-134201. PMID 19691428.
5. Lillig CH, Holmgren A (Jan 2007). "Thioredoxin and related molecules--from biology to health and disease". Antioxidants & Redox Signaling 9 (1): 25–47. doi:10.1089/ars.2007.9.25. PMID 17115886.
6. Arscott LD, Gromer S, Schirmer RH, Becker K, Williams CH (Apr 1997). "The mechanism of thioredoxin reductase from human placenta is similar to the mechanisms of lipoamide dehydrogenase and glutathione reductase and is distinct from the mechanism of thioredoxin reductase from Escherichia coli". Proceedings of the National Academy of Sciences of the United States of America 94 (8): 3621–6. doi:10.1073/pnas.94.8.3621. PMC 20490. PMID 9108027.
7. Conrad M, Jakupoglu C, Moreno SG, Lippl S, Banjac A, Schneider M, Beck H, Hatzopoulos AK, Just U, Sinowatz F, Schmahl W, Chien KR, Wurst W, Bornkamm GW, Brielmeier M (Nov 2004). "Essential role for mitochondrial thioredoxin reductase in hematopoiesis, heart development, and heart function". Molecular and Cellular Biology 24 (21): 9414–23. doi:10.1128/MCB.24.21.9414-9423.2004. PMC 522221. PMID 15485910.
8. Williams CH (Oct 1995). "Mechanism and structure of thioredoxin reductase from Escherichia coli". FASEB Journal 9 (13): 1267–76. PMID 7557016.
9. Sandalova T, Zhong L, Lindqvist Y, Holmgren A, Schneider G (Aug 2001). "Three-dimensional structure of a mammalian thioredoxin reductase: implications for mechanism and evolution of a selenocysteine-dependent enzyme". Proceedings of the National Academy of Sciences of the United States of America 98 (17): 9533–8. doi:10.1073/pnas.171178698. PMC 55487. PMID 11481439.
10. Lennon BW, Williams CH (Aug 1997). "Reductive half-reaction of thioredoxin reductase from Escherichia coli". Biochemistry 36 (31): 9464–77. doi:10.1021/bi970307j. PMID 9235991.
11. Zhong L, Arnér ES, Holmgren A (May 2000). "Structure and mechanism of mammalian thioredoxin reductase: the active site is a redox-active selenolthiol/selenenylsulfide formed from the conserved cysteine-selenocysteine sequence". Proceedings of the National Academy of Sciences of the United States of America 97 (11): 5854–9. doi:10.1073/pnas.100114897. PMC 18523. PMID 10801974.
12. Becker K, Herold-Mende C, Park JJ, Lowe G, Schirmer RH (Aug 2001). "Human thioredoxin reductase is efficiently inhibited by (2,2':6',2' '-terpyridine)platinum(II) complexes. Possible implications for a novel antitumor strategy". Journal of Medicinal Chemistry 44 (17): 2784–92. doi:10.1021/jm001014i. PMID 11495589.
13. Nilsonne G, Sun X, Nyström C, Rundlöf AK, Potamitou Fernandes A, Björnstedt M, Dobra K (Sep 2006). "Selenite induces apoptosis in sarcomatoid malignant mesothelioma cells through oxidative stress". Free Radical Biology & Medicine 41 (6): 874–85. doi:10.1016/j.freeradbiomed.2006.04.031. PMID 16934670.
14. Kahlos K, Soini Y, Säily M, Koistinen P, Kakko S, Pääkkö P, Holmgren A, Kinnula VL (May 2001). "Up-regulation of thioredoxin and thioredoxin reductase in human malignant pleural mesothelioma". International Journal of Cancer. Journal International Du Cancer 95 (3): 198–204. doi:10.1002/1097-0215(20010520)95:3<198::AID-IJC1034>3.0.CO;2-F. PMID 11307155.
15. Hashemy SI, Ungerstedt JS, Zahedi Avval F, Holmgren A (Apr 2006). "Motexafin gadolinium, a tumor-selective drug targeting thioredoxin reductase and ribonucleotide reductase". The Journal of Biological Chemistry 281 (16): 10691–7. doi:10.1074/jbc.M511373200. PMID 16481328.
16. Sibbing D, Pfeufer A, Perisic T, Mannes AM, Fritz-Wolf K, Unwin S, Sinner MF, Gieger C, Gloeckner CJ, Wichmann HE, Kremmer E, Schäfer Z, Walch A, Hinterseer M, Näbauer M, Kääb S, Kastrati A, Schömig A, Meitinger T, Bornkamm GW, Conrad M, von Beckerath N (May 2011). "Mutations in the mitochondrial thioredoxin reductase gene TXNRD2 cause dilated cardiomyopathy". European Heart Journal 32 (9): 1121–33. doi:10.1093/eurheartj/ehq507. PMID 21247928.

External links

• Thioredoxin Reductase (NADPH) at the US National Library of Medicine Medical Subject Headings (MeSH)