Catabolism of NAD P

The nicotinamide nucleotide coenzymes are catabolized by four enzymes, which act on the oxidized, but not the reduced, coenzymes  

NAD pyrophosphatase, which releases nicotinamide mononucleotide. This can either be hydrolyzed by NAD glycohydrolase to release 

NAD glycohydrolase, which releases nicotinamide and ADP-ribose. As discussed in Section 8.4.4, this enzyme also catalyzes the synthesis of cADP-ribose and nicotinic acid ADR which have roles in intracellular signaling. 

ADP-ribosyltransferase, which catalyzes ADP-ribosylation of proteins, releasing nicotinamide (Section 8.4.2). 

Poly(ADP-ribose) polymerase, which catalyzes poly-ADP-ribosylation of proteins, again releasing nicotineimide (Section 8.4.3). 

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The first step of xylose catabolism is its conversion to xylulose. In bacteria, it takes place by the direct isomerization catalysed by xylose isomerase. In PeniciUium chrysogenum, a sequence of enzymes in the initial steps of pentose metabolism was observed that differs from xylose isomerization in bacteria [91, 92]. These enzymes were common in yeast and filamentous fungi. In this oxido-reductive pathway, xylose is first reduced to the xyhtol in the presence of NAD(P)-linked xylose reductase, which is then reoxidized by NAD(P)-hnked dehydrogenase to give xylulose (Fig. 1). It has been assumed that this oxido-reductive pathway is common among fungi [93]. Both the enzymes involved, xylose reductase and xylitol dehydrogenase, were found to be inducible and relatively specific for the D-xylose and xyhtol in F. oxysporum, whereas D-xylose isomerase was not detected. 

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... 

Intracellular reduced pyridine nucleotides NAD(P)H are the primary suppliers of reducing power to anabolic and catabolic pathways. They can be measured because of their fluorescent properties.<16) The fluorescence is caused by the presence of the reduced forms of the pyridine nucleotides NADH and NADPH (jointly referred to as NAD(P)H). These fluorophores absorb light in a wide band around 340 nm, and reemit, or fluoresce, light in a wide band around 460 nm. The phosphorylated and nonphos-phorylated nucleotides have essentially equivalent fluorescence properties while the oxidized forms of these nucleotides are nonfluorescent. 

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... 

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

The total NADase activity of tissues from these four enzymes is very high, and the total tissue content of nicotinamide nucleotides can be hydrolyzed within a few minutes. Two factors prevent this in vivo. Apart from NAD pyrophosphatase, the enzymes that catalyze the release of nicotinamide from NAD(P) are biosynthetic rather than catabolic, and their activity is highly regulated under normal conditions. Furthermore, the values of K n of the enzymes are of the same order of magnitude as those of many of the NAD(P)-dependent enzymes in the cell, so that there is considerable competition for the nucleotides. Only that relatively small proportion of the nicotinamide nucleotide pool in the cell that is free at any one time will be immediately available for hydrolysis. 

Catabolism of tyrosine and tryptophan begins with oxygen-requiring steps. The tyrosine catabolic pathway, shown at the end of this chapter, results in the formation of fumaric acid and acetoaceticacid, Iryptophan catabolism commences with the reaction catalyzed by tryptophan-2,3-dioxygenase. This enzyme catalyzes conversion of the amino acid to N-formyl-kynurenine The enzyme requires iron and copper and thus is a metalloenzyme. The final products of the pathway are acetoacetyl-CoA, acetyl-Co A, formic add, four molecules of carbon dioxide, and two ammonium ions One of the intermediates of tryptophan catabolism, a-amino-P-carboxyrnuconic-6-semialdchydc, can be diverted from complete oxidation, and used for the synthesis of NAD (see Niacin in Chapter 9). 

Somewhat surprisingly, within the mitochondria the ratio [NAD+]/[NADH] is 100 times lower than in the cytoplasm. Even though mitochondria are the site of oxidation of NADH to NAD+, the intense catabolic activity occurring in the P oxidation pathway and the citric acid cycle ensure extremely rapid production of NADH. Furthermore, the reduction state of NAD is apparently buffered by the low potential of the P-hydroxybutyrate-acetoacetate couple (Chapter 18, Section C,2). Mitochondrial pyridine nucleotides also appear to be at equilibrium with glutamate dehydrogenase. ... 

However, since the adenine dinucleotides are important coenzymes for several anabolic and catabolic reactions in the cell, the NAD(P)H pool changes when the metabolism of the microorganisms is affected. These changes as well as some abiotic factors (eg, bubbles or fluorescent components in the medium) could cause problems during biomass estimation with culture fluorescence. 

Polyhydroxyalkanoates biosynthesis is regulated, on one hand, by the activity of 3-ketothiolase (EC 2.3.1.16), and on the other hand of acetoacetyl-CoA reductase (EC 1.1.1.36) intracellular PHA breakdown is dependent on the activity of 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30). Besides these three enzymes, the following compounds can be pointed out as major factors responsible of the activities of the key enzymes acetyl-CoA, free CoA, NAD(P) + (or NAD(P)H2, respectively) and, to a lower extent, ATP, pyruvate and oxalacetate. In any case, acetyl-CoA can be considered as the central metabolite both for biomass formation and PHB biosynthesis. This compound stems from the catabolic break down of carbon substrates like sugars (mainly catabolized by the 2-Keto-3-desoxy-6-phosphogluconate pathway) or fatty acids (converted by 6-oxidation). 

The pyridine nucleotides NAD"" and NADP" (1) are widely distributed as coenzymes of dehydrogenases. They transport hydride ions (2e and 1 see p. 32) and always act in soluble form. NAD" transfers reducing equivalents from catabolic pathways to the respiratory chain and thus contributes to energy... 

That nongrowing animals require niacin implies that it is lost from the body either as intact niacin or as a modified or breakdown product of the vitamin. An amount of niacin equivalent to nearly 90% of our daily intake is excreted in the forms of N-methyl-2-p)nidone-5urinary metabolites can be used to assess niacin status. Loss of the normal quantity in the urine each day indicates that the supply in the diet is adequate. In humans, the healthy adult excretes 4 to 6 mg of N-methyl-nicoti-namide per day. An abnormally low level indicates that the dietary intake is not adequate. Measurement of urinary niacin metabolites has proven useful in determining the amoimt of niacin available in a variety of foods. The body s ability to use niacin in different foods may vary even if the foods contain identical quantities of the vitamin. One contributing factor to the low availability of niacin is the occurrence of the vitamin in the "bound form," as mentioned earlier. Excretion of normal levels of pyridone, for example, depends not only on normal absorption of the vitamin from the diet, but also on its conversion to NAD or NADP, followed by catabolism to the metabolite. 

Glutamate is synthesized from NH4 and ot-ketoglutarate, a citric add cycle intermediate, by the action of glutamate dehydrogenase. We have already encountered this enzyme in the degradation of amino acids (p. 656). Recall that NAD is the oxidant in catabolism, whereas KADPH is the reductant in biosyntheses. Glutamate dehydrogenase is unusual in that it does not discriminate between NADH and NADPH, at least in some species. 

Another catabolic pathway is transamination (Fig. 24-25, rection h) to 3-mercaptopyravate. The latter compound can be reductively cleaved to pyruvate and sulfide. Cysfeine can also be oxidized by NAD and lacfafe dehydrogenase to 3-mercaptopyruvate. An interesting PLP-dependent p-replacement reaction of cysfeine leads to P-cyanoalanine, the lathyritic factor (Box 8-E) present in some plants. This reaction also detoxifies the HCN produced during the biosynthesis of ethylene from ACC. 

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