Glutaredoxin

Details of the incubation and assay are described in the legend to Figure 8. Reproduced by permission of Raven Press, New York from Brot and coworkers in Holmgren (ed.) Thioredoxin and Glutaredoxin Systems. 1986. 

N. Brot, H. Fliss and H. Weissbach, in Thioredoxin and Glutaredoxin Systems Structure and Function (Ed. A. Holmgren, C.-I. Branden, H. Jbrnvall and B.-M. Sjoberg), Raven Press, New York, 1986, pp. 141-153. 

Thioredoxin glutathione reductase (TGR) - capable of reducing GSSG due 143 to glutaredoxin domain... 

Alves R, Herrero E, Sorribas A (2004) Predictive reconstruction of the mitochondrial iron-sulfur cluster assembly metabolism. II. Role of glutaredoxin Grx5. Proteins Struct Funct Bioinf 57 481-492... 

Aslund, E, Bemdt, K.D., Holmgren, A. Redox potentials of glutaredoxins and other thiol-disulfide oxidoreductases of the thioredoxin superfamily determined by direct protein-protein redox equilibria. J Biol Chem 272(49), 30780-30786 (1997)... 

Thus, oxidation of SH groups in proteins to produce P-SS-P or P-SO, for example, can be repaired by reduction using the enzyme thioredoxin, which in turn is reduced by NADPH. As indicated above, GSH and thioltransferase can also be used. P-SS-G (GSH protein adducts) can be repaired by reduction, using the enzyme glutaredoxin. GSH and NADPH are required. 

A likely mechanism for the ribonucleotide reductase reaction is illustrated in Figure 22-41. The 3 -ribonu-cleotide radical formed in step (T) helps stabilize the cation formed at the 2 carbon after the loss of H20 (steps and (3)). Two one-electron transfers accompanied by oxidation of the dithiol reduce the radical cation (step ). Step (5) is the reverse of step ( ) regenerating the active site radical (ultimately, the tyrosyl radical) and forming the deoxy product. The oxidized dithiol is reduced to complete the cycle (step ). Ini , coli, likely sources of the required reducing equivalents for this reaction are thioredoxin and glutaredoxin, as noted above. 

If there are three or more -SH groups in a chain some incorrect pairing may, and often does, occur. Tire protein disulfide isomerases break these bonds and allow new ones to form.92 The active sites of these isomerases contain pairs of -SH groups which can be oxidized to internal -S-S- bridges by NAD+-dependent enzymes. These enzymes and their relatives thioredoxin and glutaredoxin are discussed further in Box 15-C. Glutathione and oxidation-reduction buffering are considered in Box 11-B. 

The small proteins thioredoxin and glutaredoxin are present in relatively high concentrations in bacteria, plants, and animals. For example, thioredoxin has a concentration of 15 pM in E. coli. Both proteins were discovered by their role as reducing agents in conversion of the ribonucleotides AMP, GMP, CMP, and UMP to the corresponding 2-deoxyribonucleo-tides which are needed for synthesis of DNA a/b... 

It was a surprise to discover that a mutant of E. coli lacking thioredoxin can still reduce ribonucleotides. In the mutant cells thioredoxin is replaced by glutaredoxin, whose active site disulfide linkage is reduced by glutathione rather than directly by NADPH. Oxidized glutathione is, in turn, reduced by NADPH and glutathione reductase. Thus, the end result is the same with respect to ribonucleotide reduction. 

However, the two proteins have significantly different specificities and functions. The disulfide loop in glutaredoxin, whose eukaryotic forms are often called thioltransferases/ has the sequence CPYC. Although glutaredoxins are weaker reductants of mixed disulfides of proteins with glutathione than are thioredoxins/1 sthey are more specific. 

Both thioredoxin and glutaredoxin are members of a larger group of thiol disulfide oxidoreductases which are found in all known organisms. In E. coli there are one thioredoxin, three different glutaredoxins,h/t and the periplasmic protein disulfide... 

Ribonucleotides are reduced to the 2 -deoxyribo-nucleotides (Eq. 16-21) that are needed for DNA synthesis by enzymes that act on either the di- or triphosphates of the purine and pyrimidine nucleosides348-351 (Chapter 25). These ribonucleotide reductases utilize either thioredoxin or glutaredoxin (Box 15-C) as the immediate hydrogen donors (Eq. 16-22). The pair of closely spaced -SH groups in the reduced thioredoxin or glutaredoxin are converted into a disulfide bridge at the same time that the 2 -OH of the ribonucleotide di- (or tri-) phosphate is converted to H20. While some organisms employ a vitamin B12-... 

The reduction of the 2 -hydroxyl group in the ribose ring is coupled to the cysteine-to-cystine oxidation occurring in the Bt subunit. This process cannot occur directly and other intermediates are necessary, with NADPH as the final reductant. Reduced thioredoxin (or glutaredoxin) transfers electrons to an enzyme disulfide bridge, which in turn reduces the tyrosine radical which then interacts with the 2 -OH group. Little is known about the way in which the 2 -OH group is activated... 

Chrestensen CA, Starke DW, Mieyal JJ. 2000. Acute cadmium exposure inactivates thioltransferase (Glutaredoxin), inhibits intracellular reduction of protein-glutathio-nyl-mixed disulfides, and initiates apoptosis. J Biol Chem 275 26556-26565. 

Johansson C, Lillig CH, Elolmgren A. 2004. Human mitochondrial glutaredoxin reduces S-glutathionylated proteins with high affinity accepting electrons from either glutathione or thioredoxin reductase. J Biol Chem 279 7537-7543. 

Michelet L, Zaffagnini M, Massot V, Keryer E, Vanacker H, Miginiac-Maslow M, Issakidis-Bourguet E, Lemaire SD. 2006. Thioredoxins, glutaredoxins, and glutathio-nylation New crosstalks to explore. Photosynth Res 89 225-245. 

Shelton MD, Chock PB, Mieyal JJ. 2005. Glutaredoxin Role in reversible protein S-glutathionylation and regulation of redox signal transduction and protein translocation. Antioxid Redox Signal 7 348-366. 

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