Thioredoxin system

Obviously the redox poise in biological systems is very important and the movement of selenium through this process has been investigated for denitrifiers such as Paracoccus denitrificans,159 a specialized selenate-respiring bacterium Thauera selenatis which used selenate as the sole electron acceptor,160,161 and phototrophic bacteria which produced different reduced forms of selenium when amended with either selenite or selenate and even added insoluble elemental Se.162 As noted above, Andreesen has commented on the importance of redox active selenocysteines135 and Jacob et al.136 note the importance of the thioredoxin system to redox poise. 

Nikitovic, D., Holmgren, A., S-Nitrosoglutathione is cleaved by the thioredoxin system with liberation of glutathione and redox regulating nitric oxide,/. Biol. Chem. 27 (1996),... 

Dietrichs D, Meyer M, Rieth M, Andreesen JR. 1991. Interaction of selenoprotein PA and the thioredoxin system, components of the NADPH-dependent reduction of glycine in Eubacterium acidaminophilum and Clostridium litorale. J Bacteriol 173 5983-91. 

Ribonucleotide reductase and the thioredoxin system. In some lactobacilli, vitamin B12 is involved in the reduction of ribonucleotide triphosphate to deoxyribonucleotide. 

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. 

Fig. 7.4 Reactions associated with the thioredoxin system. Thioredoxin is a redox-regulating protein with a redox-active disulfide/dithiol within the conserved active site sequence -Cys-Gly-pro-Cys-. Thioredoxin reductase, a 55 kDa flavoprotein that catalyzes the NADPH-dependent reduction of thioredoxin (1) and thioredoxin oxidase (2), a flavin-dependent sulfhydry 1 oxidase that catalyzes the oxidative protein folding with the generation of disulfides...

Hagg, D., Englund, M.C., Jemas, M., Schmidt, C., Wiklund, O., Hulten, L.M., Ohlsson, B.G., Carlsson, L.M., Carlsson, B.,andSvensson, P.A. 2006. Oxidized LDL induces a coordinated up-regulation of the glutathione and thioredoxin systems in human macrophages. Atherosclerosis 185 282-289. 

Bjornstedt, M., Kumar, S., and Holgren, A. 1995. Selenite and selenodiglutathione. Reactions with thioredoxin systems. Methods Enzymol. 252 209-219. 

Nordberg, J. and Amer, E.S.J. 2001. Reactive oxygen species, antioxidants, and the mammalian thioredoxin system. Free Rad Biol Med 31(11) 1287-1312. 

Goswami and Rosenberg have suggested that liver and kidney microsomes contain in addition to the type I deiodinase multiple low-Mm enzymes for the ORD of T4 and rT3 that differ from the type II enzyme [60,64,65]. This was mainly based on different susceptibilities to iopanoic acid and PTU if reactions were carried out at low substrate concentrations in the presence of various cofactors, i.e., DTT, GSH, glutaredoxin and thioredoxin [60,64,65]. It was even reported that the deiodinase activity stimulated by the thioredoxin system accepted rT3 but not T4 as substrate [64]. The uncertainty in the estimation of the low conversion rates in the nM substrate range which are not accounted for by residual activity of the type I deiodinase, however, questions the validity of the above conclusions. The possible exist-... 

A relatively simple and quick procedure for the isolation of Photosystem I-enriched particles from the thermophilic cyanobacterium Phormidium laminosum, without the use of detergents for solubilization, is described. The procedure involves sonication of cells, centrifugation and DEAE-cellulose chromatography. The particles had an 02 uptake activity of up to 200 pmol 02. mg chlorophyll h 1 and appeared as vesicles of 200 100 nm diameter when observed under electron microscopy. The analysis of the chlorophyll-protein complexes by polyacrylamide gel electrophoresis showed that these particles are enriched in the complexes associated with Photosystem I and partially depleted in those associated with Photosystem II. The particles did not contain ferredoxin and were active in NADP-photoreduction only in the presence of added ferredox in. They were also able to photoreduce externally added electron mediators using ascorbate as electron donor, the reduced mediators can be coupled to hydrogenase for the production of H2 or for the activation of cyanobacterial phosphoribulokinase using a ferredoxin/thioredoxin system. 

Buchanan, B.B. 1991. Regulation of CO2 assimilation in oxygenic photosynthesis the ferre-doxin/thioredoxin system. Perspective on its discovery, present status and future development. Arch. Biochem. Biophys.289, 1-9. 

The metabolic function of the thioredoxin reductase-thioredoxin system is to supply reducing equivalents to a wide variety of acceptors. By far the best characterized of these is the E. coli ribonucleotide reductase system (23, 261) the reductase consists of two subunits, proteins B1 and B2 (262, 263), The B1 protein contains three reactive dithiol-disulfide pairs and appears to be the immediate acceptor of reducing equivalents from thioredoxin. As isolated, the three pairs are in the reduced state and, in the presence of the B2 protein, three molecules of ribonucleotide can be reduced prior to any input of reducing equivalents from thiore-... 

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.

Another mechanism of light-dependent enzyme activation has been proposed in which a membrane-bound dithiol-containing factor (light-effect mediator or LEM) reduced by the photosynthetic electron transport system reductively activates regulated enzymes in the chloroplast [28]. Certain facets of this mechanism may be identical to the ferredoxin/thioredoxin system while other aspects are still the subject of debate [18,33],... 

In summary, current evidence [39-41] is thus consistent with the view that the ferredoxin/thioredoxin system functions in photosynthetically diverse types of plants as a master switch to restrict the activity of degradatory enzymes and activate biosynthetic enzymes in the light. It is significant that enzymes controlled by the ferredoxin/thioredoxin system (FBPase, SBPase, NADP-G3PDH, and PRK) function in the regenerative phase of the reductive pentose phosphate cycle that is needed to sustain its continued operation - i.e, to regenerate the carbon dioxide acceptor, Rbu-1,5-P2, from newly formed 3-PGA. It seems likely that one of these thioredoxin-linked enzymes limits the regeneration of Rbu-1,5-P2. 

It is likely that the regulatory mechanisms discussed in Section 5 apply to the regulation of CO2 fixation in C4 plants. In particular it is known that the ferre-doxin/thioredoxin system of light-linked enzyme activation (see Section 5.2.2) is present in C4 plants. NADP-malate dehydrogenase, FBPase and SBPase from maize leaves are regulated in this way [33]. 

Both potato tuber and potato leaf ADP-Glc PPases are plas-tidic the leaf enzyme is in the chloroplast, and the tuber enzyme is in the amyloplast (74). The ferredoxin-thioredoxin system is located in the chloroplast and thus, with photosynthesis, reduced thioredoxin is formed and activated within the leaf ADP-Glc PPase. At night, oxidized thioredoxin is formed it oxidizes and inactivates the ADP-Glc PPase. This activation/inactivation process during the light/dark cycle allows a fine tuning and dynamic regulation of starch synthesis in the chloroplasts. Thioredoxin isoforms are present in many different subcellular locations of plant tissues cytosol, mitochondria, chloroplasts, and even nuclei (75) and are also present in amyloplasts (76). 

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. 

Kistner A, Habermann E (1992) Reductive cleavage of tetanus toxin and botulinum neurotoxin A by the thioredoxin system from brain. Evidence for two redox isomers of tetanus toxin. Naunyn. Schmiedebergs Arch. Pharmacol. 345 227-34... 

One possibihty for minimizing oxidized protein damage is the thiol repair (Fig. 3). This repair system requires either glutathione or the thioredoxin system. The thioredoxin/thioredoxin reductase repair system [10] is able to reduce disulfide bonds. It can dethiolate protein disulfides and thus is an extremely important regulator for redox homeostasis in the cells. Thioredoxin is a smaU ubiquitous protein that contains a pair of cysteines that undergo reversible oxidation and are re-reduced by the enzyme thioredoxine reductase. The thioredoxin reductase transfers electrons from NADPH to thioredoxin via a flavin. 

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