Thioredoxin reductase mechanism

NF-kB inhibition. Aqueous extract of mainstream smoke (smoke-bubbled phosphate-buffered saline), in Swiss 3T3 cells, decreased DNA binding of NF-kB during the first 2 hours of exposure and increased more than twofold over controls after 4-6 hours of exposure. There was lack of phosphorylation and degradation of iKB-a and a significant increase in thioredoxin reductase mRNA after 2-6 hours of exposure. Results indicated that the activity of NF-kB in smoke-treated cells was subject mainly to a redox-controlled mechanism dependent on the availability of reduced thioredoxin rather than being controlled by its normal regulator, iKB-a " ". 

A variation is observed for E. coli thioredoxin reductase. The reducible disulfide and the NADPH binding site are both on the same side of the flavin rather than on opposite sides as in Fig. 15-12.190/259 Mercuric reductase also uses NADPH as the reductant transferring the 4S hydrogen. The Hg2+ presumably binds to a sulfur atom of the reduced disulfide loop and there undergoes reduction. The observed geometry of the active site is correct for this mechanism. 

In neural cells, the redox status is controlled by the thioredoxin (Trx) and glutathione (GSH) systems that scavenge harmful intracellular ROS. Thioredoxins are antioxidants that serve as a general protein disulphide oxidoreductase (Saitoh et al., 1998). They interact with a broad range of proteins by a redox mechanism based on the reversible oxidation of 2 cysteine thiol groups to a disulphide, accompanied by the transfer of 2 electrons and 2 protons. These proteins maintain their reduced state through the thioredoxin system, which consists of NADPH, thioredoxin reductase (TR), and thioredoxin (Trx) (Williams, Jr. et al., 2000 Saitoh et al., 1998). The thioredoxin system is a system inducible by oxidative stress that reduces the disulfide bond in proteins (Fig. 7.4). It is a major cellular redox system that maintains cysteine residues in the reduced state in numerous proteins. 

H. E. Ganther, Selenium metabolism, selenoproteins and mechanisms of cancer prevention complexities with thioredoxin reductase, Carcinogenesis, 20 (1999), 1657D1666. 

Glutaredoxin is another small ubiquitous protein with a different dithiol-active center which catalyzes GSH-disulfide transhydrogenase reactions. It is GSH-specific and cannot be reduced by thioredoxin reductase. It uses GSH and an NADPH-coupled glutaredoxin reductase to catalyze the reduction of a variety of disulfide substrates, including 2-hydroxyethyl-disulfide and ribonucleotide reductase [281]. Since GSSG inhibits the latter reaction, a high ratio of GSH to GSSG will promote the synthesis of deoxyribonucleotides, which is a likely control mechanism of DNA synthesis. 

In this section on mechanism and in the section to follow on structure, the comparisons will show that the relationship between lipoamide dehydrogenase and glutathione reductase is more marked than is the relationship of either to thioredoxin reductase. Thus, in catalysis, lipoamide dehydrogenase and glutathione reductase cycle between the oxidized state and a spectrally characteristic state in which the enzyme has accepted two electrons and these are shared between the FAD and the active center disulfide. This intermediate does not seem to be operative in thioredoxin reductase, and in this enzyme the FAD and disulfide interact in a different way. The oxidized forms of these enzsrmes can then be represented as... 

The scheme shown in Fig. 14 represents a working hypothesis for the reaction mechanism of thioredoxin reductase. This constitutes a gross oversimplification. Thioredoxin reductase may he more complex than are... 

Figure 8 Proposed catalytic mechanism for ferredoxin-thioredoxin reductase...

GSH reductase (Styblo et ah, 1997) and thioredoxin reductase (Lin eta/., 1999). The inhibition may be due to the interaction of trivalent arsenic with critical thiol groups in these molecules. A mechanism of toxicity of pentavalent inorganic arsenic, such as arsenate, is its reduction to a trivalent form, such as arsenite. The reduction of arsenate to arsenite occurs in vivo. Another potential mechanism is the replacement of phosphate with arsenate. 

Thioredoxin reductase (TrxR) acts in the reverse direction and shows a somewhat different mechanism, which is dependent on the protein source. Prokaryotes, plants, and lower eukaryotes contain a 35-kDa TrxR with one redox-active disulfide. Higher eukaryotes produce a 55-kDa TrxR that has either an additional redox-active disulfide or a selenenylsulfide in the flexible C-terminal part of the neighboring subunit (15). In low Mr TrxR, a large conformational change is required to move reducing equivalents from the apolar flavin site to the surface of the protein where the thioredoxin redox partner binds. In high Mr TrxR, this transfer is mediated by the second disulfide or selenylsulfide, and the conformational changes required are comparatively small (17). 

Huang HH, Arscott LD, Ballou DP, Williams CH Jr. Acid-base 26. catalysis in the mechanism of thioredoxin reductase from Drosophila melanogaster. Biochemistry 2008 47 1721-1731. 

Little is known about the specific biochemical mechanism(s) by which selenium and selenium compounds exert their acute toxic effects. Long-term effects on the hair, skin, nails, liver, and nervous system are also well documented, and a general theory has been developed to explain the toxicity of exposure to excess selenium, as discussed below. Generally, water-soluble forms are more easily absorbed and are generally of greater acute toxicity. Mechanisms of absorption and distribution for dermal and pulmonary uptake are unknown and subject to speculation, but an active transport mechanism for selenomethionine absorption in the intestine has been described (Spencer and Blau 1962). The mechanisms by which selenium exerts positive effects as a component of glutathione peroxidase, thioredoxin reductase, and the iodothyronine 5 -deiodinases are better understood, but the roles of other selenium-containing proteins in mammalian metabolism have not been clarified. 

The metabolism of selenium is now fairly well understood. To become incorporated into selenium-specific proteins (e.g., glutathione peroxidase, thioredoxin reductase, iodothyronine 5 -deiodinase) through a cotranslational mechanism requires that selenium be in the form of selenide (Sunde 1990). All forms of selenium can be transformed to selenide, although the rates of transformation vary. For example, selenate is not converted to selenide as readily as selenite. The formation of selenide from selenocysteine requires a specific enzyme, selenocysteine (3-lyase, which catalyzes the decomposition of selenocysteine to alanine and hydrogen selenide. Excess selenium is methylated and exhaled or excreted in the urine in both humans and animals. Further research is required to determine which selenium metabolites or intermediates lead to toxicity. 

Gallegos A, Berggren M, Gasdaska JR, et al. 1997. Mechanisms of the regulation of thioredoxin reductase activity in cancer cells by the chemopreventive agent selenium. Cancer Res 57 4965-4970. 

The normal catalytic process involves the sequence I- -II->-III->-IV- -I, while the steps IV- V- -VI- -III may also be part of the overall reaction. Thelander (131) has pointed out that the reaction mechanism of thioredoxin reductase differs from those of glutathione reductase and lipoyldehydrogenase because during the anaerobic titration of thioredoxin reductase no flavin semiquinone intermediate could be detected. Although thioredoxin reductase contains two subunits only one octapeptide... 

Mechanism-based inactivation of thioredoxin reductase from Plasmodium falciparum by Mannich bases. Implication for cytotoxicity. Biochemistry 42,13319-13330. 

Scheme 15 Kinetic mechanism of high molecuiar weight thioredoxin reductases.

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