Thioredoxins enzyme, role

Protein structure determinations have identified several examples of one domain inserted within another. One example is the E. coli DsbA protein, which catalyzes the formation of disulfide bonds in the periplasm. The enzyme consists of two domains a thioredoxin-like domain that contains the active site, and an inserted helical domain similar to the C-terminal domain of thermolysins (Martin et al., 1993). The inserted domain forms a cap over the active site, suggesting that it plays a role in binding to partially folded polypeptide chains before oxidation of... 

FTR plays the central role in light regulation of the activity of enzymes involved in oxygenic photosynthesis. The light signal is converted into reducing equivalents in the form of reduced 2Fe Fd by photosystem I and FTR catalyzes the reduction of disulfides in thioredoxin (Trx) / and m using the... 

Fig. 4. Enzymes regulated by the ferredoxin/thioredoxin system. The role of an FTR S-S group in the reduction of thioredoxins is based on unpublished findings of Droux, Miginiac-Maslow, Jacquot, Gadal and Buchanan. The role of thioredoxins in regulating phosphoglycerate kinase of C4 mesophyll cells is not indicated.

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

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+. 

As a component of glutathione peroxidase and the iodothyronine 5 -deiodinases, selenium is an essential micronutrient for humans. Its role in the deiodinase enzymes may be one reason that children require more selenium for growth than adults. Selenium is also a component of the enzyme thioredoxin reductase, which catalyses the NADPH-dependent reduction of the redox protein thioredoxin. Other selenium-containing proteins of unknown functions, including selenoprotein P found in the plasma, have also been identified. Excess selenium administered as selenite and selenate has been shown to be metabolized to methylated compounds and excreted. 

A stromal protein called thioredoxin (Tx) also plays a role in controlling some Calvin cycle enzymes. In the dark, thioredoxin contains a disulfide bond in the light, electrons are transferred from PSI, via ferredoxin, to thioredoxin, reducing its disulfide bond ... 

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. ... 

---