Protein disulfide reductase

Glutathione helps to maintain the sulfhydryl groups of proteins in a reduced state. An enzyme, protein-disulfide reductase, catalyzes sulfhydryl disulfide interchanges between glutathione and proteins. The reductase is important in insulin breakdown and may catalyze the reassortment of disulfide bonds during polypeptide chain folding. 

TrxRs are homodimeric flavoproteins [80] that catalyze the NADPH-dependent reduction of thioredoxin (Trx), a ubiquitous 12 kDa protein that is the major protein disulfide reductase in cells [81], and belongs to the pyridine nucleotide-disulfide oxidoreductase family [82]. Each monomer includes an FAD prosthetic group, a NADPH binding site and an active site containing a redox-active selenol group. Electrons are transferred from NADPH via FAD to the active-site selenol of TrxR, which then reduces the substrate Trx [83]. The crystal structure of TrxR is shown in Fig. 13 [84],... 

The discovery of non-specific disulfide reductases which are labile in aerobic cellular extracts suggests that kinetic constraints of thiol/disulfide exchange in vivo are very complex. One of such proteins is thioredoxin which behaves as a non-specific protein-disulfide reductase. Thioredoxin also works as a cofactor of sulfoxide reductases. The dithiol active site of thioredoxin sits on a protrusion of the protein surface [274], Thioredoxin is an ubiquitous protein whose molecular weight is about 12 KDa [274,275], It has been found in cytosolic and mitochondrial [276] compartments of animal cells, and it is partly bound to membranes. High contents in thioredoxin have been found in neurons, secretory and epithelial cells. Redox recycling of thioredoxin is insured by thioredoxin reductase, which has been identified in a variety of mammalian cells as a symmetrical dimer with a molecular weight of 116KDa[274]. Thioredoxin reductase is NADPH-specific, but it exhibits a very wide disulfide substrate specificity. 

Regeneration of the ribonucleotide reductase is accomplished in Escherichia coli and in mammals by thioredoxin, a dithiol polypeptide (M.W. 12,000) coenzyme, which also plays a role in other protein disulfide reductase reactions. In thioredoxin, two cysteine residues in the sequence -Cys-Gly-Pro-Cys are converted to cystine. Reduced thioredoxin is regenerated by thioredoxin reductase, a flavoprotein enzyme that uses NADPH + H+. 

A second type of enzyme system which reduces disulfide bonds in proteins has been described. Protein-disulfide reductase (NAD (P)H) [NAD(P)H protein-disulfide oxidoreductase EC 1.6.4.4], purified from pea seeds (51), catalyzes the Reaction 2, shown on p. 111. 

Protein-disulfide reductase (glutathione) [ glutathione protein-disulfide oxidoreductase EC 1.8.4.2) has been reported in hepatic tissue (52). The enzyme rapidly cleaved the three disulfide bonds of insulin and the disulfide bonds of other proteins. The Km for reduced glutathione was 8.9 X 10 3M, a very high value. The enzyme catalyzed the reaction (Equation 3) from either direction. 

Recently, a heat-labile compound in meat extract which decreases the heat stability of soybean trypsin inhibitors was reported (53). It is tempting to speculate that this is some type of protein-disulfide reductase, but further work is needed on this point... 

Both cyanobacterial thioredoxins can function as protein disulfide reductases in a model reaction using insulin as substrate (5). Five micrograms of either protein will catalyze the precipitation of 1 mg of insulin by DTT in approximately 15 min. Spinach chloroplast thioredoxins were equally effective in this reaction. 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

Disulfides can be reduced to two thiols (Fig. 5.14). The best example is the reduction of oxidized glutathione (GSSG) back to the reduced form (GSH) (Fig. 5.14), which is mediated by glutathione reductase. In addition, exchange can occur with other thiols mediated by protein disulfide isomerase. In principle, sulfenic acids can probably also be reduced back to thiols, but because of the reactivity of the sulfenic acid, this is not generally observed. 

NADPH <3> (NADPH-dependent activation system is composed of at least two protein factors one is heat-stable and the other is indistinguishable from NADPH-dependent disulfide reductase [8]) [8]... 

The reaction [Eq. (7)] requires a disulfide-reducing system such as dithiothreitol or disulfide reductase and a reducing agent such as NADPH or reduced ferredoxin. It is proposed [Eq. (5)] that carbon monoxide oxidoreductase binds CO as a one-carbon intermediate [C,], which can be either oxidized to C02 or condensed with the methyl group of a methylated corrinoid protein and CoA in the final step of acetyl-CoA synthesis. 

Thioredoxins are small ubiquitous enzymes present in many species from Archaebacteria to man. They serve as general protein disulfide oxido-reductases interacting with a broad range of proteins. Allergenic thioredoxins have been identified in fungi as well as in plants. The thioredoxins from wheat (Tri a 25) and maize (Zea m 25) are related to baker s asthma, an occupational disease affecting 4%-10% of bakery workers in European countries (Holmgren 1995). 

The stmcture of PdR (Figure 9) is significantly different from that of AdR although both proteins are related to disulfide reductases and other flavin-dependent enzymes. The... 

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