Coenzyme NAD

IS the oxidation of lactic acid to pyruvic acid by NAD and the enzyme lactic acid coenzyme NAD ... 

Biomimetic oxidation and asymmetric reduction with coenzyme NAD analogs 99YGK512. 

As another example, studies with deuterium-labeled substrates have shown that the reaction of ethanol with the coenzyme NAD+ catalyzed by yeast alcohol dehydrogenase occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of NAD+. 

As its name implies, the citric acid cycle is a closed loop of reactions in which the product of the hnal step (oxaloacetate) is a reactant in the first step. The intermediates are constantly regenerated and flow continuously through the cycle, which operates as long as the oxidizing coenzymes NAD+ and FAD are available. To meet this condition, the reduced coenzymes NADH and FADH2 must be reoxidized via the electron-transport chain, which in turn relies on oxygen as the ultimate electron acceptor. Thus, the cycle is dependent on the availability of oxygen and on the operation of the electron-transport chain. 

Niacin. Figure 2 Structure of the coenzymes NAD+ (nicotinamide-adenine dinucleotid) and NADP+ (nicotinamide-adenine dinucleotid phosphate). 

Niacin was discovered as a nutrient during studies of pellagra. It is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Two compounds, nicotinic acid and nicotinamide, have the biologic activity of niacin its metabolic function is as the nicotinamide ring of the coenzymes NAD and NADP in oxidation-reduction reactions (Figure 45-11). About 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. The niacin content of foods is expressed as mg niacin equivalents = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable, this is discounted. 

The natural substrates which are most frequently used are the nucleotide coenzymes NAD+ and NADP+, which are reversibly reduced by many enzymes ... 

What can enzyme-catalysed reactions that involve the nucleotide coenzyme NAD be monitored by ... 

D-galactose to D-galactonolactone in the presence of the coenzyme NAD+, is more specific and enzyme preparations are available for which the only other substrates are a-L-arabinose and /3-D-fucose. The generation of NADH is conveniently monitored at 340 nm and permits quantitation of the galactose ... 

Allosteric regulatory molecules are small molecular weight compounds which may be coenzymes (NAD+, ATP, etc.) or intermediate substrates, possibly generated by enzymes found within the same pathway as the regulated enzyme. Alternatively, the allosteric modulator may be generated within another, perhaps complementary, pathway. For example, a regulator may be stimulatory for a catabolic route and at the same time inhibitory for the opposing anabolic pathway. 

NMN is basically half of the NAD+ molecule nicotinamide ribose phosphate. NADP+ is NAD+ bearing a phosphate group at C3 of the ribose group attached to the adenine. The redox chemistry is the same in all three forms of the coenzymes. NAD+ is the form most frequently employed for biochemical oxidation reactions in catabohsm and NADP+ (in its reduced form NADPH) is the form usually employed for biochemical reduction reactions in anabohsm. NMN is employed infrequently. 

Niacin the generic name for nicotinic acid and nicotinamide precursors for the coenzymes NAD+ and NADP+. 

Nicotinic acid and nicotinamide are precursors of the coenzymes NAD+ and NADP+, which play a vital role in oxidation-reduction reactions (see Box 7.6), and are the most important electron carriers in intermediary metabolism (see Section 15.1.1). We shall look further at the chemistry of NAD+ and NADP+ shortly (see Box 11.2), but note that, in these compounds, nicotinamide is bound to the rest of the molecule as an A-pyridinium salt. 

In isocitrate, there is a CHOH group that is available for oxidation via the coenzyme NAD+ and the enzyme isocitrate dehydrogenase. NADH will then be reoxidized via oxidative phosphorylation, and lead to ATP synthesis. The oxidation product from isocitrate is oxalosuccinate, a -ketoacid that easily... 

Phosphoric acid molecules can form acid-anhydride bonds with each other. It is therefore possible for two nucleotides to be linked via the phosphate residues. This gives rise to dinucleotides with a phosphoric acid-anhydride structure. This group includes the coenzymes NAD(P) " and CoA, as well as the flavin derivative FAD (1 see p. 104). 

The active center of an LDH subunit is shown schematically in Fig. 2. The peptide backbone is shown as a light blue tube. Also shown are the substrate lactate (red), the coenzyme NAD (yellow), and three amino acid side chains (Arg-109, Arg-171, and His-195 green), which are directly involved in the catalysis. A peptide loop (pink) formed by amino acid residues 98-111 is also shown. In the absence of substrate and coenzyme, this partial structure is open and allows access to the substrate binding site (not shown). In the enzyme lactate NAD"" complex shown, the peptide loop closes the active center. The catalytic cycle of lactate dehydrogenase is discussed on the next page. 

Niacin is present in foods mainly as coenzyme NAD and NADP, which are hydrolyzed in the intestine, and it is adsorbed as nicotinamide or nicotinic acid. The free forms, nicotinamide and nicotinic acid, only allowed to be added in fortified foods [403], occur naturally in limited amounts. Instead, niacin occurs as nicotynil ester bonded to polysaccharides, peptides, and glycopeptides. In general, niacin is widespread in foodstuffs (cereals, seeds, meat, and fish). High concentrations are present in roasted coffee beans as a primarily product of the roasting process [417]. 

Some enzymes associate with a nonprotein cofactor that is needed for enzymic activity. Commonly encountered cofactors include metal ions such as Zn2+ or Fe2+, and organic molecules, known as coenzymes, that are often derivatives of vitamins. For example, the coenzyme NAD+contains niacin, FAD contains riboflavin, and coenzyme A contains pantothenic acid. (See pp. 371-379 for the role of vitamins as precursors of coenzymes.) Holoenzyme refers to the enzyme with its cofactor. Apoenzyme refers to the protein portion of the holoenzyme. In the absence of the appropriate cofactor, the apoenzyme typically does not show biologic activity. A prosthetic group is a tightly bound coenzyme that does not dissociate from the enzyme (for example, the biotin bound to carboxylases, see p. 379). 

Figure 2-13 (A) Stereoscopic view of the nucleotide binding domain of glyceraldehyde phosphate dehydrogenase. The enzyme is from Bacillus stearothermophilus but is homologous to the enzyme from animal sources. Residues are numbered 0-148. In this wire model all of the main chain C, O, and N atoms are shown but side chains have been omitted. The large central twisted P sheet, with strands roughly perpendicular to the page, is seen clearly hydrogen bonds are indicated by dashed lines. Helices are visible on both sides of the sheet. The coenzyme NAD+ is bound at the end of the P sheet toward the viewer. Note that the two phosphate groups in the center of the NAD+ are H-bonded to the N terminus of the helix beginning with RIO. From Skarzynski et al.llla (B) Structural formula for NAD+.

This toxin subunit is an enzyme, an ADP-ribo-syltransferase which catalyzes transfer of ADP-ribosyl units from the coenzyme NAD+ to specific arginine side chains to form N-ADP-ribosyl derivatives of various proteins. Of the proteins modified by cholera toxin, the most significant is the guanyl nucleotide regulatory protein Gs of the adenylate cyclase system.C/f/h ADP ribosylation of arginine 201 of the a subunit of protein Gs inhibits the GTP hydrolysis that normally allows the protein to relax to an unactivated form.e The ADP-ribosylated Gs keeps adenylate cyclase activated continuously and... 

Why are there four major hydrogen transfer coenzymes, NAD+, NADP+, FAD, and riboflavin phosphate (FMN), instead of just one Part of the answer is that the reduced pyridine nucleotides NADPH and NADH are more powerful reducing agents than are reduced flavins (Table 6-7). Conversely, flavin coenzymes are more powerful oxidizing agents than are... 

The oxidation-reduction potential of a pyridine nucleotide coenzyme system is determined by the standard redox potential for the free coenzyme (Table 6-8) together with the ratio of concentrations of oxidized to reduced coenzyme ([NAD+] / [NADH], Eq. 6-64). If these concentrations are known, a redox... 

Figure 15-1 The hydrogen-carrying coenzymes NAD+ (nicotinanide adenine dinucleotide) and NADP+ (nicotinamide adenine dinucleotide phosphate).

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