Thiol-disulfide redox reaction

In 1996, Tamura and Stadtman showed that thioredoxin reductase from mammalia-in contrast to the bacterial homologs-is a selenoprotein, and that selenocysteine is the penultimate C-terminal amino acid residue. Selenium-containing isoenzymes, which are tissue specific, have later on been characterized biochemically, or the existence of such isoforms was suggested by in-sihco methods. The substrate of thioredoxin reductase is thioredoxin, which is the key regulator of the redox status within cells. Thioredoxin catalyzes thiol-disulfide exchange reactions in proteins and peptides it is involved as redox mediator in the... 

Figure 1.10 Sulfhydryl groups may undergo a number of additional reactions, including acylation and alkylation. Thiols also may participate in redox reactions, which generate reversible disulfide linkages.

Cys(StBu) with phosphines only a slight excess of tributylphosphine is required for this purpose. Under these conditions reduction of the diselenide does not occur at all, and subsequent air oxidation of the two cysteine residues at high dilution leads in a highly selective manner to the diselenide- and disulfide-bridged peptides. The selectivity of the disulfide bridging is assured by the complete absence of thiol/diselenide exchange reactions even at alkaline pH values due to the very low reactivity of the diselenide toward mono-thiols as a result of their highly differentiated redox potentials. ... 

The range of functionality provided by the 20 amino acids found in proteins consists of weak acids and bases, nucleophiles, hydrogen bond donors and acceptors, and the redox active thiol/disulfide. This limited range of chemistry is inadequate for the catalysis of many reactions found to occur in biological systems. Therefore, a variety of small organic molecules, called cofactors, coenzymes, or vitamins, have evolved to broaden the limited range of chemistry that can be catalyzed by simple proteins. 

Besides their affinity for metallic ions, thiol groups have other characteristics such as ability to interconvert to disulfides through redox reactions, to add to conjugated double bonds and to form complexes with the pyridine nucleotides by nucleophilic attack at the 4-position of the pyridine ring. 

An illustrative experimental example of a cell exposed to an oxidant was carried out by Kirlin et al. (1999). When they used HT29 cells (colon adenocarcinoma) and exposed them to sodium butyrate, it was found that there was an oxidant reaction that caused a drop from 260 to —200 mV in h. The 60 mV decrease resulted in a 100-fold change in protein dithiols disulfide ratio. A correlation was noted between h, GST, and NADPH quinone reductase Kirlin further indicated that the h provides two additional pieces of information (1) in reactions that use GSH as a reductant to maintain protein thiol/disulfides in their reduced form, it is an indicator of the reducing power quantitatively and (2) if the redox state is controlled by a GSH redox couple, it is an indication of the functional state of the protein (Kirlin et al., 1999). 

The relationship between NADPH deficiency and anemia is an indirect one. NADPH is required to reduce the peptide glutathione from the disulfide to the free thiol form. Mammalian red blood cells lack mitochondria, which host many redox reactions. 

The thiol group of Cys is the most reactive side residue. The thiolate anion is a potent nucleophile and the thiol is a week acid with pKj = 8.37. Cys serves as the active site residues of many oxidoreductases. Cys residues form complexes of varying stability with a variety of metal ions. It reacts with organic mercurials stoichiometiically. Thiol residues of Cys cross link to form disulfide bonds (cystine) in proteins. Thiols and disulfides undergo rapid exchange and redox reactions. 

Electron Exchange with Disulfide-Thiol Groups In principle, a reversible redox reaction occurs... 

The chemiluminescence-redox detector (CRD) is based on specific redox reactions coupled with chemiluminescence measurement. An attractive feature of this detector is that it responds to compounds such as ammonia, hydrogen sulfide, carbon disulfide, and sulfur dioxide. Hydrogen peroxide, hydrogen, carbon monoxide, sulfides, and thiols that are not sensitively detected by flame ionization detection can be detected with the CRD detector. Compounds that typically constitute a large portion of the matrix of many industrial samples are not detected, thus simplifying matrix effects and sample cleanup procedures for some applications. 

Reactions take place in the presence of redox systems, initiators, and suitable pH medium. The nucleophilic sulphur atom from cysteines may allow a broad range of chemical modifications, including redox reactions [50] like thiol/disulfide exchange, oxygen transfer redox couples, and thiol/thlyl radical transfer reactions, all of which occur during steady-state cellular conditions [51],... 

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