Flavin adenine nucleotide

Figure 11.39 summarizes the reactions taking place in this amperometric sensor. FAD is the oxidized form of flavin adenine nucleotide (the active site of the enzyme glucose oxidase), and FAD1T2 is the active site s reduced form. Note that O2 serves as a mediator, carrying electrons to the electrode. Other mediators, such as Fe(CN)6 , can be used in place of O2. 

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). 

Vitamin B2 (riboflavin) acts mainly as the coenzyme FAD (flavin adenine nucleotide) rmd FMN (flavin mononucleotide), which are used in many oxidation-reduction reactions in which hydrogen atoms are received or donated. Particularly noteworthy examples are their uses in... 

Conversion of dihydroorotate to orotate is catalyzed by dihydroorotate dehydrogenase, a metalloflavoprotein that contains nonheme-iron atoms and flavin adenine nucleotides (FMN and FAD). In this reaction, the electrons are probably transported via iron atoms and flavin nucleotides that are reoxidized by NAD+. 

Surprisingly, however, recent evidence showed that dihydrofolate reductase is not a universal enzyme. About one-third of the sequenced eubacteria use a flavin-dependent thymidylate synthetase (FDTS) (ThyX protein, EC 2.1.1.148) that does not require the supply of reduction equivalents by the THF-type cofactor, they are NADPH oxidases that use flavin adenine nucleotide (FAD) to mediate hydride transfer the genomes of these respective organisms do not specify orthologues of dihydrofolate reductase (see Section 7.17.2.3.2(viii)). ... 

Low et al. (2004) have proposed a model to explain thioacetamide-induced hepatotox-icity and cirrhosis in rat livers. The pathways of thioacetamide-induced liver fibrosis were found to be initiated by thioacetamide S-oxide derived from the biotransformation of thioacetamide by the microsomal flavin-adenine nucleotide containing monooxygenase and cytochrome P450 systems and involve oxidative stress and depletion of succinyl-CoA, thus affecting heme and iron metabolism. Karabay et al. (2005) observed such hepatic damage in rats with elevation of total nitrite level in livers and decrease in arginase activity. The authors have reported that nitrosative stress was essentially the critical factor in thioacetamide-induced hepatic failure in rats. 

Both NADPH cytochrome P450 reductase (P450 reductase) and NAD(P)H-quinone oxidoreductase (NQO) are flavin adenine nucleotide-containing enzymes that catalyze the reduction of quinones and quinone-like structures. However, P450 reductase is a microsomal enzyme that catalyzes a one-electron reduction to yield semiquinone radieals that can redox cycle to produce superoxide anion radicals, whereas NQO is a cytosolic enzyme that catalyzes a two-electron reduction to yield hydroquinones (Matsunaga et al., 2006). P450 reductase also can catalyze the one-electron reduction of nitroaromatics to the nitro anion radical, which can redox cycle ... 

The flavin adenine nucleotides act as hydrogen acceptors in a large number of reactions, several of which are listed below ... 

An NADH diaphorase has been purified from human erythrocytes. The enzyme activity can be determined in a system containing 2.6 dichlorophenol-indo-phenol, Tris HCl buffer (pH 7.55), EDTA, and the enzyme. The reduction of 2.6 dichlorophenol-indo-phenol is followed spectrophotometrically. The purified enzyme contains one mole of a flavin adenine nucleotide (probably FAD) per 195,000 g of protein. The enzyme functions with either NADPH or NADH as hydrogen donor, but the affinity for NADH is almost 10 times greater than that for NADPH. 

Coemymes effecting transfer of hydrogen. These include the pyridine nucleotides, nicotinamide-adenine dinucleolide and nicotinamide-adenine dinucleolide phosphate the flavin nucleotides such as flavin-adenine dinucleotide and lipoic acid. 

Flavin adenine dinucleotide (FAD) has been electropolymerized using cyclic voltammetry. Cyclic voltammograms of poly (FAD) modified electrode were demonstrated dramatic anodic current increasing when the electrolyte solution contained NADH compare with the absence of pyridine nucleotide. 

Several classes of vitamins are related to, or are precursors of, coenzymes that contain adenine nucleotides as part of their structure. These coenzymes include the flavin dinucleotides, the pyridine dinucleotides, and coenzyme A. The adenine nucleotide portion of these coenzymes does not participate actively in the reactions of these coenzymes rather, it enables the proper enzymes to recognize the coenzyme. Specifically, the adenine nucleotide greatly increases both the affinity and the speeifieity of the coenzyme for its site on the enzyme, owing to its numerous sites for hydrogen bonding, and also the hydrophobic and ionic bonding possibilities it brings to the coenzyme structure. 

Pantothenic acid, sometimes called vitamin B3, is a vitamin that makes up one part of a complex coenzyme called coenzyme A (CoA) (Figure 18.23). Pantothenic acid is also a constituent of acyl carrier proteins. Coenzyme A consists of 3, 5 -adenosine bisphosphate joined to 4-phosphopantetheine in a phosphoric anhydride linkage. Phosphopantetheine in turn consists of three parts /3-mercaptoethylamine linked to /3-alanine, which makes an amide bond with a branched-chain dihydroxy acid. As was the case for the nicotinamide and flavin coenzymes, the adenine nucleotide moiety of CoA acts as a recognition site, increasing the affinity and specificity of CoA binding to its enzymes. 

Autofluorescence of cells often complicates the studies with fluorescence microscopy (especially the application of green fluorescent substances). There are different reasons for the occurrence of this phenomenon (157) (i) the fluorescent pigment lipofuscin, which settles with rising age in the cytoplasm of cells (ii) cell culture medium, which often contains phenol red that increases autofluorescence (iii) endogen substances such as flavin coenzymes [flavin-adenine dinucleotide (FDA), flavin mononucleotide (FMN) absorp-tion/emission 450/515nm], pyridine nucleotides [reduced nicotinamide adenine dinucleotide (NADH) absorption/emission 340/460nm] or porphyrine (iv) substances taken up by cells (as mentioned above filipin) and (v) preparation of the cells fixation with glutaraldehyde increases autofluorescence. 

Selected entries from Methods in Enzymology [vol, page(s)] Determination of FMN and FAD by fluorescence titration with apoflavodoxin, 66, 217 purification of flavin-adenine dinucleotide and coenzyme A on p-acetoxymercurianiline-agarose, 66, 221 a convenient biosynthetic method for the preparation of radioactive flavin nucleotides using Clostridium kluyveri, 66, 227 isolation, chemical synthesis, and properties of roseoflavin, 66, 235 isolation, synthesis, and properties of 8-hydroxyflavins, 66, 241 structure, properties and determination of covalently bound flavins, 66, 253 a two-step chemical synthesis of lumiflavin, 66, 265 syntheses of 5-deazaflavins, 66, 267 preparation, characterization, and coenzymic properties of 5-carba-5-deaza and 1-... 

Babior, who has studied this enzyme at several stages of its purification, found in lysates of PMNs which were activated with zymosan that of eight potential biological reductants only reduced pyridine nucleotides supported the formation of O ". The K , for NADPH was less than the K , for NADH and the activity was decreased in preparations from three patients with chronic granulomatous disease. In accord with predictions based on reaction 7, 0.55 molecule of O7 was measured per molecule of NADPH oxidized under conditions of saturating concentrations of cytochrome c The enzyme which was extracted with Triton X-100 from a granule-rich fraction from activated PMNs, required an external source of FAD for the formation of O from NADPH . Riboflavin and FMN would not substitute. Flavin adenine dinucleotide was proposed as a necessary cofactor, which was probably lost when the enzyme was treated with the detergent. 

Owing to the great biological importance of this class of compounds, the 13C NMR spectra of a considerable number of nucleosides and nucleotides and their aglycones have been reported in the literature [147,428, 460, 676, 735, 748-760]. As demonstrated by the spectra of flavin adenine dinucleotide (Fig. 5.9), the 13C NMR spectra of nucleosides and nucleotides have two groups of signals ... 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

In addition to their role in genetics, nucleotides play other important roles in biochemistry. Key enzymes and coenzymes such as nicotinamide adenine dinucleotide (NAD), flavin adenine dinucleotide (FAD), and vitamin B12 also include nucleotides as part of their structures. Also, the major component of viruses is DNA. 

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