Coenzyme glutathione reduction

In lipoic acid (6), an intramolecular disulfide bond functions as a redox-active structure. As a result of reduction, it is converted into the corresponding dithiol. As a prosthetic group, lipoic acid is usually covalently bound to a lysine residue (R) of the enzyme, and it is then referred to as lipoamide. Lipoamide is mainly involved in oxidative decarboxylation of 2-0X0 acids (see p. 134). The peptide coenzyme glutathione is a similar disulfide/ dithiol system (not shown see p. 284). 

This thiol-disulfide interconversion is a key part of numerous biological processes. WeTJ see in Chapter 26, for instance, that disulfide formation is involved in defining the structure and three-dimensional conformations of proteins, where disulfide "bridges" often form cross-links between q steine amino acid units in the protein chains. Disulfide formation is also involved in the process by which cells protect themselves from oxidative degradation. A cellular component called glutathione removes potentially harmful oxidants and is itself oxidized to glutathione disulfide in the process. Reduction back to the thiol requires the coenzyme flavin adenine dinucleotide (reduced), abbreviated FADH2. 

The degradation of vinyl chloride and ethene has been examined in Mycobacterium sp. strain JS 60 (Coleman and Spain 2003) and in Nocardioides sp. strain JS614 (Mattes et al. 2005). For both substrates, the initially formed epoxides underwent reaction with reduced coenzyme M and, after dehydrogenation and formation of the coenzyme A esters, reductive loss of coenzyme M acetate resulted in the production of 5-acetyl-coenzyme A. The reductive fission is formally analogous to that in the glutathione-mediated reaction. 

NADPH formed in the oxidative phase is used to reduce glutathione, GSSG (see Box 14-3) and to support reductive biosynthesis. The other product of the oxidative phase is ribose 5-phosphate, which serves as precursor for nucleotides, coenzymes, and nucleic acids. In cells that are not using ribose 5-phosphate for biosynthesis, the nonoxidative phase recycles six molecules of the pentose into five molecules of the hexose glucose 6-phosphate, allowing continued production of NADPH and converting glucose 6-phosphate (in six cycles) to C02. 

One example is the known interference by reducing compounds that affect the chemical conversion of substrate to a colored indicator. This is especially true for the tetrazolium assays (Ulukaya, Colakogullari, and Wood 2004 Chakrabarti et al. 2000 Pagliacci et al. 1993 Collier and Pritsos 2003). The growing list of interfering compounds includes ascorbic acid and sulfhydryl reagents such as glutathione, coenzyme A, dithiothreitol, etc. Similar interferences by compounds that affect the oxidation and reduction chemistry of cells are likely to cause artifacts with the resazurin reduction assay. Assays that measure markers of metabolism also can be influenced by the pH of the culture medium and other factors that may stimulate or stress the metabolic rates of cells. 

Glutathione reductase amino acid composition, 102,104,105 cystine residues, 104 kinetic studies, 138-141 mechanism, 94, 97-98,134 metabolic functions, 129-133 reaction catalyzed, 92 reduction of, 112, 113 specificity of, 92-93 coenzymes and, 94 thiol groups, 141-142 two-electron-reduced enzyme, properties, 133-138... 

Biochemical Function. The best way to describe tocopherol s role is that of a lipid-soluble antioxidant. It protects unsaturated lipids from o gen-induced peroxide formation. There Is evidence for both free-radical one-electron chemistry (Fig. 8.20) and two-electron qui-none-hydroquinone chemistry (Fig. 8.21) (35). The oxidized/reduced glutathione system may he part of the system that regenerates reduced a-tocopherol. At one time it was thought that the preference for the 2-R stereoisomers indicated that the vitamin was part of a biochemical oaddation/reduction system, possibly as a coenzyme. So far that role for a-tocopherol has not been found. The current evidence points to the... 

E. coli mutants unable to synthesize thioredoxin are still able to form deoxyribonucleotides. In these bacteria, a related substance, glutaredoxin, and two molecules of glutathione carry out the reduction. In Lactobacillus, the triphosphate is reduced and vitamin is an essential coenzyme. Another example of this use of vitamin Bi2 is in Euglena, where the diphosphates are reduced. The mammalian system is nearly identical to that of E. coli. 

Sulfur is a constituent of the amino acids, methionine, cystine and cysteine, which are essential in proteins supplied in animal diets. Sulfur is also a constituent of the vitamins, thiamine and biotin, and of glutathione and coenzyme A. It plays an important role in the action of proteolytic enzymes and in oxidation—reduction processes. As a constituent of proteins it is present in cell protoplasm and is therefore of vital importance in cell metabolic processes. 

Fig. 10. Hydrogen donor systems for ribonucleotide reduction. Enzyme reactions are I thioredoxin reductase (EC 1.6.4.5) II ribonucleotide reductase (EC 1.17.4) III glutathione reductase (EC 1.6.4.2). GSH, GSSG reduced and oxidized glutathione NADPH, NADP reduced and oxidized nicotinamide adenine dinucleotide phosphate coenzymes. The hydrogen transfer chain is continued in Fig. II...

While it is doubtful if cystine, the disulphide of cysteine, has any critical biological role as such, it is an ubiquitous constituent of aerobic systems resulting from the facile oxidation of cysteine. It also can arise from the digestion of protein disulphides. Cystine is relatively insoluble and if allowed to build up tends to form crystalline precipitates within the cell. There is normally little of the disulphide in cells, while in the blood the oxidized form dominates. One method for the reduction of cystine to cysteine is via a nicotinamide coenzyme-linked dehydrogenase. Glutathione... 

In Lactobacillus leischmanii, reduced thioredoxin is the natural reductant, but dithiols (dihydrolipoate, dithiothreitol) and monothiols (2-mercaptoethanol and glutathione) have been found to be active reduc-tants. In this reaction, coenzyme B12 is believed to act as a hydrogen carrier. 

Another coenzymic role of glutathione was described by Strittmatter and Ball in the DEN -linked fonnaldehyde dehydrogenase system of liver 128). The enzymic reduction of nitrate requires GSH 129) as a reducing agent. It has also been diown that GSH is the coenzyme of the enzymic isomerization of 4-maIeyl- to 4-fumaryl-acetoacetate 128a). 

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