Mixed thioredoxin

Figure 2.7 (a) Illustration of the twist of (3 sheefs. Befa sfrands are drawn as arrows from the amino end to the carboxy end of the p strand in this schematic drawing of fhe protein thioredoxin from E. coli, fhe sfrucfure of which was defermined in the laboratory of Carl Branden, Uppsala, Sweden, fo 2.8 A resolution. The mixed p sheet is viewed from one of ifs ends, (b) The hydrogen bonds between the P strands in the mixed p sheet of fhe same profein. [(a) Adapfed from B. Furugren.]... 

The C-terminal domain of phosducin is a five-stranded mixed p sheet with a helices on both sides, similar to the thioredoxin fold of disulfide iso-merase DsbA described in Chapter 6. Despite significant sequence homology to thioredoxin, the phosducin domain, unlike other members of this family. 

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. 

The third system is that of thioltransferase, which may not be distinct from glutaredoxin in some tissues. The thioltransferase system is composed of thioltransferase, GSH, glutathione reductase and NADPH. All thioltransferases which have been found in mammalian cells have molecular weights which are close to 11 KDa. Sub-millimolar concentrations of either cystamine or cystine inactivate GST-IT from human placenta with concomitant formation of enzyme mixed-disulfides [282]. Thioltransferase from human placenta specifically reactivates GST-IT in the presence of 10-100 pM GSH while thioredoxin is inactive [282], Thioltransferase-mediated cleavage of mixed disulfides was also shown to restore enzyme activities in phosphofructokinase [283] and pyruvate kinase [284]. 

FIGURE 3.11 The hydrogen bonds between the p-strands in the mixed p-sheet of thioredoxin from E. coli. From Branden and Tooze, 1991. 

A thioredoxin has also been partially purified from L. leichtnannii (39). Although this thioredoxin is similar in size (MW approximately 12,000) to the E. coli thioredoxin, it is not able to function as a substrate for the E. coli thioredoxin reductase. More recently thioredoxins have been purified to homogeneity from rat Novikoff ascites hepatoma (127) and from calf liver (45). The properties of these two thioredoxins are quite similar (MW 11,400 and 12,000 respectively). The amino acid compositions however, are different. For instance the Novikoff tumor thioredoxin contains six half-cysteine residues, whereas calf liver thioredoxin has only four. Both these thioredoxins have a tendency to aggregate in the oxidized form. This aggregation is probably due to mixed sulfide formation between the additional sulfhydryl groups. Like yeast thioredoxin II, calf liver thioredoxin contains only one tryptophan residue unfortunately the tryptophan content of Novikoff tumor thioredoxin was not determined. 

A series of disulfide exchanges occurs in the reductive half-reaction of QSOX. A dithiol substrate such as a denatured protein is oxidized by the disulfide of the thioredoxin domain. The initial substrate-enzyme mixed disulfide likely occurs with the N-terminal solvent-exposed cysteine (Cys80 in the avian enzyme). The reducing equivalents are then passed to the disulfides in the C-terminal ERV 1 /ALR domain. The roles of the two disulfides in this domain are unclear. Regardless, the electrons are eventually passed to the oxidized flavin, which then reacts with molecular oxygen. ... 

T-1 is reduced by NADPH and the flavoprotein thioredoxin reductase from E. coli. T-2 is not reduced by the bacterial reductase, nor does it inhibit reduction of T-1 in a mixed system, indicating that it does not bind to the E. coli protein. 

Patwari P, Higgins LJ, Chutkow WA, Yoshioka J, Lee RT. 2006. The interaction of thioredoxin with Txnip Evidence for formation of a mixed disulfide by disulfide exehange. J Biol C/jcm 281 21884-21891. 

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