NADPH and

The second en2ymatic reaction converts NADP" to NADPH and and the appearance of NADPH is measured at 340 nm. 

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

Utilization of Glucose-6-P Depends on the Cell s Need for ATP, NADPH, and Ribose-5-P... 

BOTH NADPH AND ATP ARE NEEDED BY THE CELL, BUT RmOSE-5-P IS NOT Under some conditions, both NADPH and ATP must be provided in the cell. This can be accomplished in a series of reactions similar to case 3, if the fructose-6-P and glyceraldehyde-3-P produced in this way proceed through glycolysis to produce ATP and pyruvate, which itself can yield even more ATP by continuing on to the TCA cycle (Figure 23.40). The net reaction for this alternative is... 

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

A more detailed study of the biological oxidation of sulphoxides to sulphones has been reported165. In this study cytochrome P-450 was obtained in a purified form from rabbit cells and was found to promote the oxidation of a series of sulphoxides to sulphones by NADPH and oxygen (equation 56). Kinetic measurements showed that the process proceeds by a one-electron transfer to the activated enzymatic intermediate [an oxenoid represented by (FeO)3+] according to equation (57). 

The pentose phosphate pathway, present in the cytosol, can account for the complete oxidation of glucose, producing NADPH and COj but not ATP. 

The pathway has an oxidative phase, which is irreversible and generates NADPH and a nonoxidative phase, which is reversible and provides ribose precursors for nucleotide synthesis. The complete pathway is present only in those tissues having a requirement for NADPH for reductive syntheses, eg, lipogenesis or steroidogenesis, whereas the nonoxidative phase is present in all cells requiring ribose. 

Figure28-10. The phenylalanine hydroxylase reaction. Two distinct enzymatic activities are involved. Activity II catalyzes reduction of dihydrobiopterin by NADPH, and activity I the reduction of O2 to HjO and of phenylalanine to tyrosine. This reaction is associated with several defects of phenylalanine metabolism discussed in Chapter 30.

Cytochrome P450s catalyze reactions that introduce one atom of oxygen derived from molecular oxygen into the substrate, yielding a hydroxylated product. NADPH and NADPH-cytochrome P450 reductase are involved in the complex reaction mechanism. 

The hydroxylase that converts 2,4-dichlorophenol into 3,5-dichlorocatechol (Figure 3.14a) before ring fission has been purified from a strain of Acinetobacter sp. (Beadle and Smith 1982), and ixomAlcaligenes eutrophus IMP 134 (Don et al. 1985 Perkins et al. 1990). The reductant is NADPH, the enzyme is a flavoprotein containing FAD, and in the presence of compounds that are not substrates, NADPH and O2 are consumed with the production OfH202. 

Oxidation may take place by a modified tricarboxylic acid cycle in which the production of CO2 is coupled to the synthesis of NADPH and reduced ferredoxin, and the dehydrogenation of succinate to fumarate is coupled to the synthesis of reduced menaquinone. This pathway is used, for example, by Desulfuromonas acetoxidans and in modified form by... 

O2 is consumed by NADPH and 90% of this is released extracellularly as Oi and H2O2 (Huber, 1980). 

To prepare glucuronide conjugates of products formed from initial oxidation (to yield a hydroxyl or carboxylic acid group), the oxidative metabolite should be prepared first and used as a substrate in the incubation. Alternatively, an incubation with parent drug, NADPH and UDPGA may produce the desired glucuronide. 

Human CYPs are multicomponent enzyme systems, requiring at a minimum the CYP enzyme component and a reductase component to be functional. The reductase requires a reduced nicotinamide cofactor, typically NADPH, and this cofactor must be regenerated to provide a steady supply of reducing equivalents for the reductase. Regeneration is accomplished with a separate substrate and enzyme. Glucose-6-phosphate and glucose-6-phosphate dehydrogenase have been widely used for this purpose. The overall complexity of the reaction mixtures and their cost have been barriers to the widespread use of recombinant human CYPs for metabolite synthesis in the past. 

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

At least two enzymes compete for acetyl-CoA - the citrate synthase and 3-ke-tothiolase. The affinities of these enzymes differ for acetyl-CoA (Table l),and at low concentrations of it the citrate synthase reaction tends to dominate, provided that the concentration of 2/H/ is not inhibiting. The fine regulation of the citrate synthases of various poly(3HB) accumulating bacteria has been studied [ 14, 47, 48]. They appear to be controlled by cellular energy status indicators (ATP, NADH, NADPH) and/or intermediates of the TCA cycle. The 3-ketothio-lase has also been investigated [10-14,49, 50]. This enzyme is, above all, inhibited by CoASH [10,14,49]. This important feature will be further considered below. 

The addition of ammonia to the variety of acids derivable from either the breakdown of glucose, glycolysis, or of the pentose shunt reaction products, ribose and NADPH, and from the citrate cycle, gives the amino acids (see Table 4.7 and Figure 4.4) Polymerisation of amino acids in cells gives proteins. In some of the amino acids sulfur and selenium can be incorporated easily. We assume NH3 was present. (Note that Se is in a coded amino acid not in Table 4.7.) Some selective metal-binding properties can be seen in Table 4.7, but amino acid carboxylates can bind all. 

Peroxyl radicals are the species that propagate autoxidation of the unsaturated fatty acid residues of phospholipids (50). In addition, peroxyl radicals are intermediates in the metabolism of certain drugs such as phenylbutazone (51). Epoxidation of BP-7,8-dihydrodiol has been detected during lipid peroxidation induced in rat liver microsomes by ascorbate or NADPH and during the peroxidatic oxidation of phenylbutazone (52,53). These findings suggest that peroxyl radical-mediated epoxidation of BP-7,8-dihydrodiol is general and may serve as the prototype for similar epoxidations of other olefins in a variety of biochemical systems. In addition, peroxyl radical-dependent epoxidation of BP-7,8-dihydrodiol exhibits the same stereochemistry as the arachidonic acid-stimulated epoxidation by ram seminal vesicle microsomes. This not only provides additional... 

Bacteria, fungi, and plants have a thioredoxin reductase system involving Se-containing thioredoxin reductase (EC 1.8.1.9), NADPH, and thioredoxin. Effecting the intercellular redox poise, this system interacts in selenium... 

To supply reducing equivalents for biosynthesis (NADPH) and pentoses for DNA and RNA biosynthesis. 

Reduced flavins (FADH2, FMNH2, and riboflavin) generated by flavin-dependent reductases have been hypothesized to reduce azo dyes in a nonspecific chemical reaction, and flavin reductases have been revealed to be indeed anaerobic azoreductases. Other reduced enzyme cofactors, for example, NADH, NADH, NADPH, and an NADPH-generating system, have also been reported to reduce azo dyes. Except for enzyme cofactors, different artificial redox mediating compounds, especially such as quinines, are important redox mediators of azo dye anaerobic reduction (Table 1). 

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