Oxidation-reduction pathways

Distinct coenzymes are required in biological systems because both catabolic and anabolic pathways may exist within a single compartment of a cell. The nicotinamide coenzymes catalyze direct hydride transfer (from NAD(P)H or to NAD(P)+) to or from a substrate or other cofactors active in oxidation-reduction pathways, thus acting as two-electron carriers. Chemical models have provided... 

VANADIUM COMPOUNDS ON BIOLOGICAL SYSTEMS CELLULAR GROWTH, OXIDATION-REDUCTION PATHWAYS, AND ENZYMES... 

The beginning of the polymerization reaction can be followed by NMR spectroscopy. Thus the nadimide polymerization advancement is easily monitored by the disappearance of the ethylenic proton and carbon resonances. However these criteria should be handled with care. For instance, the ethylene proton disappearance was particularly misleading in the case of the 2,3,4,6-tetrahydroph-thalimide curing study [38]. An isomerization stage took place with the formation of a tetrasubstituted double bond. Moreover, an oxidation reduction pathway was also evidenced, which gave saturated and aromatic derivatives. 

Other mechanisms are also involved in the reticulocyte-mediated release of iron from transferrin, and its subsequent incorporation into heme. Intact oxidation-reduction pathways are essential and seem to operate at a step following the attack on the anion (90). Heme exerts an inhibitory influence on iron transfer from protein to cell, seemingly by interfering at an early stage in the release of the metal from transferrin (89). Non-transferrin carriers for iron have also been identified in the reticulocyte and may participate in its incorporation into heme (81). 

Once absorbed, metallic and inorganic mercury enter an oxidation-reduction cycle. Metallic mercury is oxidized to the divalent inorganic cation in the red blood cells and lungs of humans and animals. Evidence from animal studies suggests that the liver is an additional site of oxidation. Absorbed divalent cation from exposure to mercuric compounds can, in turn, be reduced to the metallic or monovalent form and released as exhaled metallic mercury vapor. In the presence of protein sulfhydryl groups, mercurous mercury (Hg+) disproportionates to one divalent cation (Hg+2) and one molecule at the zero oxidation state (Hg°). The conversion of methylmercury or phenylmercury into divalent inorganic mercury can probably occur soon after absorption, also feeding into the oxidation-reduction pathway. 

Scheme 1 Tyrosine oxidation/reduction pathways in photosystem II...

The mechanism for the formation of siUca is complex because oxidation, reduction, and hydrolysis pathways are all possible. 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

Still has also carried out mechanistic experiments9 3 from which he could deduce that the major reduction pathway is by attack of hydride ion at the sulphur atom. This conclusion was deduced from the fact that reduction with sodium borodeuteride-aluminium oxide gave a sulphoxide that had only incorporated about 25% mole equivalent of deuterium on to a methyl carbon atom bound to the sulphur atom. The mechanistic pathway for direct reduction is outlined in equation (38), whereas the pathway whereby deuterium could be incorporated is portrayed in equation (39). These reactions support the proposed mechanism for the hydride reduction of sulphones as outlined in Section III.A.l, namely that attack at sulphur by hydride ions may occur, but will be competitive with proton abstraction in cases when the attack at sulphur is not facilitated. 

As described in Section 4-1. one important class of chemical reactions involves transfers of protons between chemical species. An equally important class of chemical reactions involves transfers of electrons between chemical species. These are oxidation-reduction reactions. Commonplace examples of oxidation-reduction reactions include the msting of iron, the digestion of food, and the burning of gasoline. Paper manufacture, the subject of our Box, employs oxidation-reduction chemishy to bleach wood pulp. All metals used in the chemical industry and manufacturing are extracted and purified through oxidation-reduction chemistry, and many biochemical pathways involve the transfer of electrons from one substance to another. 

Castro CE, DM Riebeth, NO Belser (1992b) Biodehalogenation the metabolism of vinyl chloride by Methylosinus trichosporium OB-3b. A sequential oxidative and reductive pathway through chloroethylene oxide. Environ Toxicol Chem 11 749-755. 

Figure 13.13.2 Alternative reduction pathway of trimethylamine oxide to dimethy-lamine and formaldehyde.

The activity of PK and NRPSs is often precluded and/or followed by actions upon the natural products by modifying enzymes. There exists a first level of diversity in which the monomers for respective synthases must be created. For instance, in the case of many NRPs, noncanonical amino acids must be biosynthesized by a series of enzymes found within the biosynthetic gene cluster in order for the peptides to be available for elongation by the NRPS. A second level of molecular diversity comes into play via post-synthase modification. Examples of these activities include macrocyclization, heterocyclization, aromatization, methylation, oxidation, reduction, halogenation, and glycosylation. Finally, a third level of diversity can occur in which molecules from disparate secondary metabolic pathways may interact, such as the modification of a natural product by an isoprenoid oligomer. Here, we will cover only a small subsection of... 

Nucleic acid pathways (Mg, Zn, Fe/S, B12 or Fe20) P-carbon oxidation/reduction (flavin, Fe)... 

The In vitro metabolic reduction of nitro PAHs was first reported in 1967 (124). Since then a number of similar investigations have been conducted however, most of these studies have been performed under only anaerobic conditions. The oxidative metabolism of nitro PAHs has been examined only recently, but as considered in the following sections, both reductive and oxidative metabolic pathways may be important in the metabolic activation of nitro PAHs. 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

The difference in the self-exchange rates of the two cobalt couples favors the oxidative pathway by a factor of 300. (For a further discussion of the above and other self-exchange rates, see B. S. Brunschwig, C. Creutz, D. H. Macartney, T.-K. Sham, and N. Sutin, Faraday Discuss. Chem. Soc., No 74, in press). Evidently the difference in the intrinsic barriers is large enough to compensate for the less favorable driving force for the oxidative pathway. As a result the latter pathway can compete favorably with the reductive pathway. 

This system illustrates the importance of both the thermodynamic and intrinsic barriers in determining the direction of electron transfer within a given reactant pair. In addition, systems such as the one considered here in which the oxidative and reductive pathways possess comparable rate constants afford an opportunity of controlling or switching the direction of electron transfer by modifying one of the barriers. 

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