Pyridine nucleotides reactions catalyzed

Coenzyme Specificity of Dehydrogenases. A number of dehydrogenases have been reported to require DPN, others are specific for TPN, and several react with both, although not necessarily at the same rate. Examples have already been cited of enzymes that catalyze identical reactions but possess different coenzyme requirements. An additional means for studying pyridine nucleotide reactions became available with desamino DPN. Certain dehydrogenases (e.g., liver alcohol dehydrogenase) react at equal rates with DPN and desamino DPN. Others (as... 

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... 

Xanthobacter sp. strain Py2 may be grown with propene or propene oxide. On the basis of amino acid sequences, the monooxygenase that produces the epoxide was related to those that catalyzes the monooxygenation of benzene and toluene (Zhou et al. 1999). The metabolism of the epoxide is initiated by nucleophilic reaction with coenzyme M followed by dehydrogenation (Eigure 7.13a). There are alternative reactions, both of which are dependent on a pyridine nucleotide-disulfide oxidoreductase (Swaving et al. 1996 Nocek et al. 2002) ... 

Alcohol dehydrogenases catalyze oxidation of alcohols in a reaction dependent on the pyridine nucleotide NAD+ [Eq. (5)]. Since the reaction is reversible, alcohol dehydrogenases also catalyze the reduction of aldehydes by... 

Indicine IV-oxide (169) (Scheme 36) is a clinically important pyrrolizidine alkaloid being used in the treatment of neoplasms. The compound is an attractive drug candidate because it does not have the acute toxicity observed in other pyrrolizidine alkaloids. Indicine IV-oxide apparently demonstrates increased biological activity and toxicity after reduction to the tertiary amine. Duffel and Gillespie (90) demonstrated that horseradish peroxidase catalyzes the reduction of indicine IV-oxide to indicine in an anaerobic reaction requiring a reduced pyridine nucleotide (either NADH or NADPH) and a flavin coenzyme (FMN or FAD). Rat liver microsomes and the 100,000 x g supernatant fraction also catalyze the reduction of the IV-oxide, and cofactor requirements and inhibition characteristics with these enzyme systems are similar to those exhibited by horseradish peroxidase. Sodium azide inhibited the TV-oxide reduction reaction, while aminotriazole did not. With rat liver microsomes, IV-octylamine decreased... 

Fig. 1. General scheme of the hydrogen transfer as catalyzed by dehydrogenase reactions. R represents the residue of the pyridine nucleotide molecule.

Both the enzymes were prepared by a special technique from the insoluble portion of guinea pig liver mitochondria, and they are quite specific with respect to the requirement of pyridine nucleotide (H9, Hll). However, dehydrogenases catalyzing reaction (25) with NAD as coenzyme have been reported (Mil, S13, T3), thus confirming the importance of the source of the enzyme and the purification procedure employed. 

V3. Vennesland, B., and Westheimer, F. H., Hydrogen transport and steric specificity in reactions catalyzed by pyridine nucleotide dehydrogenases. In The Mechanism of Enzyme Action (W. D. McElroy, and B. Glass, eds.), pp. 357-379. Johns Hopkins Press, Baltimore, Maryland, 1954. 

This is another example of the generalization that enzyme reactions of the same type have the same stereospecificity for the pyridine nucleotide, no matter what their cellular origin. There is another mevaldic reductase in the cytosol of liver, which catalyzes the reduction of mevaldic acid to mevalonic acid by NAD or NADP, and this enzyme also has A (or pro R) stereospecificity for the pyridine nucleotide but the hydrogen transfer occurs to the pro R position on C—5. This latter enzyme can use either 3R or 3S mevaldic acid that is, it is indifferent to... 

It should be noted, in this connection, that there are pyridine nucleotide dehydrogenases which catalyze redox reactions which must occur in two steps. Hydroxymethylglutaryl CoA reductase (discussed on p. 51) is one example. Another is uridine diphosphate-glucose dehydrogenase, which catalyzes the oxidation of the C—6 of the glucose (i.e., a primary alcohol) to a carboxyl group. In both cases, there are two molecules of pyridine nucleotide required, and the overall reactions are essentially irreversible. The former enzyme, with A stereospecificity for the pyridine nucleotide, catalyzes the reduction of an acyl-CoA group... 

The transfer of a hydride ion (H ) in a reaction. Such transfers occur in many enzyme-catalyzed reactions perhaps the most common being in pyridine nucleotide-linked dehydrogenases in which hydride transfer is stereospecific (i.e., to the re- or xi-face of the coenzyme) ... 

This enzyme [EC 1.6.1.1] (also known as NAD(P)+ trans-hydrogenase (B-specific), pyridine nucleotide transhy-drogenase, and nicotinamide nucleotide transhydro-genase) catalyzes the reversible reaction of NADPH with NAD+ to produce NADP+ and NADH. This FAD-dependent enzyme is B-specific with respect to both pyridine coenzymes. In addition, deamino coenzymes will also serve as substrates. 

The biological function of Factor-420 is to catalyze the electron transport between hydrogen and pyridine nucleotide in Methanobacteria (anaerob). These Arckaebacteria are obligate anaerobes. In this context some proposals have been put forward with respect to the evolution of biological reactions catalyzed by flavoproteins... 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

Another base-catalyzed reaction is the addition of enolate anions derived from ketones to the 4 position of the pyridine nucleotides (Eq. 15-19). The adducts undergo ring closure and in the presence of oxygen are converted slowly to fluorescent materials. While forming the basis for a useful analytical method for determination of NAD+ (using 2-butanone), these reactions also have created a troublesome enzyme inhibitor from traces of acetone present in commercial NADH.132... 

An alternative pathway for synthesis of quinoli-nate from aspartate and a triose phosphate exists in bacteria and in plants and provides the major route of nicotinic acid synthesis in nature. In E. coli the reaction is catalyzed by two enzymes, one an FAD-containing L-aspartate oxidase which oxidizes aspartate to a-iminoaspartate.228 The latter condenses with dihydroxyacetone-P to form quinolinate (Eq. 25-13).229 There are at least two other pathways for synthesis of quinolinic acid as well as five or more salvage pathways for resynthesis of degraded pyridine nucleotide coenzymes.224/230/231... 

Buchanan, Bachofen, and Arnon (29) showed that the conversion of pyruvate to the above amino acids required DPN and NHs- They found a ferredoxin-dependent reduction of DPN with hydrogen gas was catalyzed by Ckromalium extracts, and concluded that DPNH2, formed from reduced ferredoxin, and NH3 were used in reductive amination reactions leading to the synthesis of amino acids. A ferredoxin-dependent reduction of pyridine nucleotides was shown first for spinach chloroplasts (Tagawa and Arnon (99) it was shown later in other photosynthetic bacteria, when ferredoxin was reduced either with dithionite (Yamanaka and Kamen (115)) or with hydrogen gas and hydrogenase (Weaver, Tinker, and Valentine (112)). 

Another variable is the relative amounts of nucleotide component and condensing agent in joining-reactions. The occurrence of side reactions catalyzed by DCC is minor, so a large (5-10-fold) excess of the reagent is routinely used to optimize yields and minimize reaction times. However, after prolonged reaction in pyridine, byproducts... 

The pyridine nucleotide coenzymes, NAD and NADP, provide the reducing equivalents necessary for energy transduction and biosynthesis in living cells. The reduced coenzymes are generated by the transfer of a hydride anion from a wide variety of substrates and catalyzed by an equally wide variety of dehydrogenases, the half reaction being... 

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