Redox coenzymes nicotinamide adenine dinucleotide

Niacin and niacinamide (nicotinamide and nicotinic acid amide) are converted to the ubiquitous redox coenzymes nicotinamide-adenine dinucleotide (NAD)" and nicotinamide-adenine dinucleotide phosphate (NADP). ... 

Enzyme-catalyzed redox processes involve the transfer of two electrons in either one or two steps, each process requiring different coenzymes. Nicotinamide adenine dinucleotide (NAD) and its 2 -phosphate (NADP) coenzymes are involved in many two-electron... 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

Although the structures for molecules having niacin activity are simple, the forms in which they act in human biochemistry are not so simple. Nicotinic acid and nicotinamide are precursors for three complex coenzymes in multiple oxida-tion/reduction (redox) reactions nicotinamide mononucleotide, NMN nicotinamide adenine dinucleotide, NAD+ and nicotinamide adenine dinucleotide phosphate, NADP. I shall use NAD+ as representative of the class. NADH is the corresponding reduced form. ... 

Two derivatives of nicotinamide (pyridine-3-carboxylic amide), one of the B2 vitamins, nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ), serve as redox coenzymes. Of the three heterocyclic ring systems found in these coenzymes, i.e. those of purine, ribose and pyridine, it is the pyridine portion that is reactive in redox reactions. Biologically, two oxidation states are important the oxidized form, NAD(P)+, and the 1,4-dihydro isomer of the two-electron reduced form, NAD(P)H (Scheme 1). Nicotinamide coenzymes interconvert between these two oxidation states in... 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

Several reactions in metabolism are oxidation-reduction (or redox) reactions. Two of the principal redox carriers are nicotinamide adenine dinucleotide (NAD+) and coenzyme Q. Remember that we live in an oxidizing world, so species that are in the reduced form are frequently high-energy compounds that react exothermically with oxygen. Also recall that organic molecules are reduced by adding bonds to hydrogen. 

Niacin is a water-soluble vitamin. The RDA of niacin for the adult man is 19 mg. Niacin is converted in the bi>dy to the cofactor nicotinamide adenine dinucleotide (NAD). NAD also exists in a phosphorylated form, NADP The phosphate group occurs on the 2-hydrr>xyl group of the AMP half of the coenzyme, NAD and NADP are used in the catalysis of oxidation and reduction reactions. These reactions are called redox reactions. NAD cycles between the oxidized form, NAD, and the reduced form, NADH + H. The coenzyme functions to accept and donate electrons. NADP behaves in a similar fashion. It occurs as NADP and NADPH + HT The utilization of NAD is illustrated in the sections on glycolysis, the malatc-aspartate shuttle, ketone body metabolism, and fatty acid oxidation. The utilization of NADP is illustrated in the sectirrns concerning fatty acid synthesis and the pentose phosphate pathway. 

NADI Oxidized form of nicotinamide adenine dinucleotide phosphate. Note that despite the plus sign in the symbol, the coenzyme is anionic under normal physiological conditions. An enzyme commonly associated with biosynthetic reactions. NADP is a hydrogen carrier in a wide range of redox reactions. 

Figure 7.70 Structures of resazurin and resorufin. The transformation from blue resazurin to red fluorescing resorufin by reduction is catalysed by the redox reaction of nicotinamide adenine dinucleotide (NAD+, coenzyme 1) to NAD dehydrogenase.

Oxidoreductase dehydrogenase and flavoenzymes. Nicotinamide adenine dinucleotide (NAD+)/nicotinamide adenine dinucleotide phosphate (NADF) and flavin mononucleotide (FMN)/flavin adenine dinucleotide (FAD) are the two major types of redox coenzymes. 

A major coenzyme of redox enzymes is nicotinamide adenine dinucleotide (NAD) (Table 2). The cofactor is composed of an adenosine monophosphate residue linked by a phosphodiester to a 5-phosphoribosyl-l-nicotinamide. 

Alcohol dehydrogenase and aldehyde dehydrogenase were used as the enzymes for anodic ethanol oxidation. Together with the coenzyme NAD" " (nicotinamide adenine dinucleotide), they formed the NAD+/NADH redox system. Aided by these enzymes, this reaction occurs in two steps. First, the former enzyme oxidizes ethanol to acetaldehyde, and then the latter enzyme oxidizes the aldehyde to acetic acid. 

Nicotinamide-adenine-dinucleotide, NAD, di-phosphopyrUUne nudeotide, DPN, codehydrogenase I, coenzyme I, cozymase a pyridine nucleotide coenzyme involved in many biochemical redox processes. It is the coenzyme of a large number of oxidoreducta-ses, which are classified as pyridine nucleotide-dependent dehydrogenases. Mechanistically, it serves as the electron acceptor in the enzymatic removal of hydrogen atoms from specific substrates. 

Nicotinamide adenine dinucleotide and its phosphate derivative are widely distributed as coenzymes for biological redox reactions. Structures of their oxidized and reduced forms are depicted above. Within the coenzyme molecule, the nicotinamide moiety which is linked... 

In biological systems, the redox coenzymes NADH (reduced nicotinamide adenine dinucleotide phosphate) and NADPH, the 2 -phosphoric acid derivative of NADH, belong to the most prominent and important examples of... 

Specific redox characteristics of a catalyst derived from CV scans are also used to confirm an enzyme s ability for bioelectrocatalysis by either direct electron transfer (DET) or mediated electron transfer (MET) to the electrode. DET and MET are two distinct mechanisms of bioelectrocatalysis. MET has the advantage of being compatible with almost all naturally occurring oxidoreductase enzymes and coenzymes, but it requires additional components (either smaU-molecule redox mediators or redox polymers) because the enzymes cannot efficiently transfer electrons to the electrode. These additional components make the system more complex and less stable [8]. The vast majority of oxidoreductase enzymes that require MET to an electrode are nicotinamide adenine dinucleotide (NAD" ) dependent. Two of the most commonly encountered NAD -dependent enzymes in BFC anodes are glucose dehydrogenase (GDH) and alcohol dehydrogenase (ADH). These enzymes have been thoroughly characterized in respect to half-cell electrochemistry and have been demonstrated for operation in BFC. More information about MET can be found in Chapter 9. 

The two pyridine nucleotide coenzymes, formerly known as coenzymes I and II, then for a period as DPN and TPN, and known nowadays as NAD and NADP (nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate), are involved in hundreds of enzyme-catalyzed redox reactions in vivo. Although a minority of these diverse reactions can use either of the two... 

The nicotinamide nucleotide coenzymes function as electron carriers in a wide variety of redox reactions. In addition, NAD is the precursor of adenine dinucleotide phosphate (ADP)-ribose for ADP-ribosylation and poly(ADP-ribosylation) of proteins and cADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). They act as second messengers and stimulate increases in intracellular calcium concentrations. 

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