Reductants NADH

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

FIGURE3.7 The potential window for the redox chemistry of life. Redox chemistry in living cells is approximately limited by the standard potentials for reduction and oxidation of the solvent water at neutral pH. Approximate standard reduction potentials are also indicated for the commonly used oxidant ferricyanide and reductants NADH and dithionite. 

Hereditary methemoglobinemia arises from a deficiency of the enzyme that catalyzes this reduction, NADH-cytochrome b reductase. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

Similar mechanisms operate in the action of nitrate reductase and nitrite reductase. Both of these substances are produced from ammonia by oxidation. Plants and soil bacteria can reduce these compounds to provide ammonia for metabolism. The common agricultural fertilizer ammonium nitrate, NH4NO3, provides reduced nitrogen for plant growth directly, and by providing a substrate for nitrate reduction. NADH or NADPH is the electron donor for nitrate reductase, depending on the organism. 

Table I also shows the great diversity of organisms in which iron—sulfur proteins have been detected. Thus far there is no organism which when appropriately examined has not contained an iron-sulfur protein, either in the soluble or membrane-bound form. Iron-sulfur proteins catalyze reactions of physiological importance in obligate anaerobic bacteria, such as hydrogen uptake and evolution, ATP formation, pyruvate metabolism, nitrogen fixation, and photosynthetic electron transport. These properties and reactions can be considered primitive and thus make iron-sulfur proteins a good place to start the study of evolution. These key reactions are also important in higher organisms. Other reactions catalyzed by iron-sulfur proteins can be added such as hydroxylation, nitrate and nitrite reduction, sulfite reduction, NADH oxidation, xanthine oxidation, and many other reactions (Table II).

C-tertninal FAD-binding domains of the two proteins are different in size and have different secondary structure elements (Figures 8 and 9). The highly asymmetric charge distribution of AdR was absent from PdR, which has a much lower dipole moment. The reductant NADH approaches and binds on the re side of the FAD in PdR, indicating that Pdx should bind on the si side. Unlike for AdR, the si side surface of PdR is... 

The extraction of energy from organic compounds, carried out by several catabolic pathways (e.g., the citric-acid cycle), involves the oxidation of these compounds to CO2 and H2O with the concomitant production of water-soluble reductants (NADH and succinate). These reductants donate electrons to components of the mitochondrial electron-transfer chain, resulting in the reduction of oxygen to water ... 

Figure 8.17 Enzyme coupled assay. Triose Phosphate Isomerase (TIM) assay system with glyceraldehyde 3-phosphate dehydrogenase (GAPDH) colourimetric coupled assay for detection purposes. In this version of the TIM assay, dihydroxyacetone phosphate (DHAP) is converted enzymically to glyceraldehyde 3-phosphate (GAP) that is onward converted to glycerol phosphate (GP) by means of the coupled enzyme GAPDH enzyme that uses the reverse-colourimetric reductant NADH.

Figure 8.18 Colourimetric assay. Malate dehydrogenase (MDH) assay system. Oxalate is onward converted to L-malate by means of enzyme MDH that uses the reverse-colourimetric reductant NADH to effect catalytic reduction.

Pierre, J.-L. Gagnaire, G. Chautemps, P. An artificial allosteric system Regulation of a biomimetic reduction (NADH model) by potassium ions. Tetrahedron Lett. 1992, 33 (2), 217-220. 

Figure 1 Enzymes as signal transdncers. As drawn, an enzyme specific for only one substrate is immobilized in close proximity to the electrode surface. The substrate (e.g., glutamate) is oxidized by the enzyme to the corresponding enzyme product (e.g., a-ketoglu-tarate) with concurrent rednction of a cofactor in a 1 1 ratio. (NAD" is rednced to NADH for dehydrogenase enzymes, and molecular oxygen is reduced to peroxide in the case of oxidase enzymes.) Of the fonr species present in solution, only the product of the cofactor reduction (NADH or H2O2) is electrochemically active and produces an analytical signal, so the enzyme substrate (glutamate in this example) is transduced to an electroactive species by the enzyme. Barring introduction of NADH (or peroxide) to the solntion, any increase in faradaic cnrrent may then be attributed to the presence of the enzyme snbstrate alone.

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