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Flavoprotein, flavin nucleotides

Like the nicotinamide coenzymes (Fig. 13-15), the flavin nucleotides undergo a shift in a major absorption band on reduction. Flavoproteins that are fully reduced (two electrons accepted) generally have an absorption maximum near 360 nm. When partially reduced (one electron), they acquire another absorption maximum at about 450 nm when fully oxidized, the flavin has maxima at 370 and 440 nm. The intermediate radical form, reduced by one electron, has absorption maxima at 380, 480, 580, and 625 nm. These changes can be used to assay reactions involving a flavoprotein. [Pg.515]

Flavoproteins are often very complex some have, in addition to a flavin nucleotide, tightly bound inorganic ions (iron or molybdenum, for example) capable of participating in electron transfers. [Pg.516]

In higher mammals, riboflavin is absorbed readily from the intestines and distributed to all tis.sues. It is the precursor in the biosynthesis of the cocnzyme.s flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The metabolic functions of this vitamin involve these Iwocoenzymes. which participate in numerous vital oxidation-reduction proces.ses. FMN (riboflavin 5 -phosphate) is produced from the vitamin and ATP by flavokinasc catalysis. This step con be inhibited by phcnothiazincs and the tricyclic antidepressants. FAD originates from an FMN and ATP reaction that involves reversible dinucicotide formation catalyzed by flavin nucleotide pyrophosphorylase. The.se coenzymes function in combination with several enzymes as coenzyme-en-zyme complexes, often characterized as, flavoproteins. [Pg.890]

The hydrogens are accepted by FAD, which is covalently bound to the apoprotein via a histidine residue. In many flavoproteins, the flavin nucleotide is bound to the apoprotein not covalently but rather via ionic linkages with the phosphate group. The reducing equivalents of FADH2 are passed on to coenzyme Q (CoQ or Q) via the iron-sulfur centers. Thus, the overall reaction catalyzed by complex II is... [Pg.254]

Like other biologically active substances we have met, flavin is found combined as a nucleotide, linked to ribose phosphate and sometimes also to adenine nucleotide, forming flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). The flavin nucleotides, unlike NAD, are bound firmly on to proteins and cannot be removed by dialysis. Such proteins are known as flavoproteins (FP). The typical reaction in which the flavoproteins participate is the oxidation of NADH2 ... [Pg.133]

TABLE 13-9 Some Enzymes (Flavoproteins) That Employ Flavin Nucleotide Coenzymes... [Pg.515]

Riboflavin (V.B lactoflavin 6,7-dimethyl-9-(D-r-ribityl)-isoalloxazine) is a water-soluble yellow flavin derivative, occurring chiefly in a bound form in flavin nucleotides or flavoproteins in yeasts, animal products and legume seeds. Milk contains fiee riboflavin. It is required as a precursor of flavin mononudeotide and flavin-adenine-dinucleotide, which are coenzymes of the flavin enzymes. In rats, experimental riboflavin deficiency causes growth ure and dermatitis around the nostrils and eyes. In hinnans, riboflavin deficiency (ariboflavinosis) is characterized by lip... [Pg.717]

At first, the enzymes involved were found to be flavoproteins, capable of transferring an electron from NADH or NADPH to an electron acceptor. NADH and NADPH cytochrome c reductase, as well as NADH diaphorase, were found to have transhydro-genase properties. The flavin nucleotide is not involved in the reaction, because it can be extracted from the protein without interfering with the transhydrogenation. Later, true transhydrogenases were found. A transhydrogenase has been purified from the ammonium sulfate fraction of rat liver mitochondria. Kaplan and his associates purified a transhydrogenase from heart mitochondria. [Pg.34]

In all these reactions, the flavin nucleotide combines with a protein to form a flavoprotein. In the reactions involved in electron transport, the flavoprotein acts as a hydrogen carrier and plays an important role in the transfer of electrons from pyridine nucleotides to cytochromes. Among the flavoproteins of interest in the electron transport chain are (1) coenzyme 1-diaphorase, (2) the Warburg flavoprotein, (3) NADH cytochrome c reductase, (4) NADPH cytochrome c reductase, (5) NADH cytochrome reductase, (6) NADH 2-methyl-l,4maphthoquinone reductase, and (7) succinic cytochrome c reductase. The most important among these are succinic cytochrome c reductase and NADH cytochrome c reductase. The following discussion of these two flavoproteins may serve as an example for the others. [Pg.36]

Both the oxidation-reduction potential and the fluorescence of flavin nucleotides are modified profoundly by attachment of the nucleotide to various proteins. Flavin enzymes have been reported to have oxidation-reduction potentials at pH 7 ranging from —0.4 to 0.187. The combination to proteins also results in shifts of the absorption maxima. The 450 m u band is found at 451 mju in Straub s diaphorase and at 455 m/t in Haas yellow enzyme, while the 375 m/t band appears at 359 m/t and 377 m/t in these preparations. Most flavin enzymes do not fluoresce, and it is assumed that the quenching of fluorescence implies binding of the flavin to the enzyme through N-3. Straub s diaphorase, unlike most other flavoproteins, does fluoresce. This may be evidence that this diaphorase is a partially degraded cytochrome reductase. [Pg.176]

Under physiological conditions the reduced pyridine nucleotides are reoxidized by the flavoproteins. One such flavoprotein, flavin adenine... [Pg.746]

Flavin Nucleotide. Vitamin B2 or riboflavin is a component of the prosthetic group of flavoproteins. The isolation of the vitamin from biologic material was accomplished by Gyorgy and R. Kuhn in 1933. Almost simultaneously, the first yellow enzyme was obtained in pure form and the close relationship between these active substances was recognized. [Pg.98]

In contrast to the nicotinamide nucleotide dehydrogenases, the prosthetic groups FMN and FAD are firmly associated with the proteins, and the flavin groups are usually only separated from the apoen2yme (protein) by acid treatment in water. However, in several covalently bound flavoproteins, the enzyme and flavin coen2ymes are covalently affixed. In these cases, the flavin groups are isolated after the proteolytic digestion of the flavoproteins. [Pg.80]

Flavoprotein dehydrogenases usually accept electrons from reduced pyridine nucleotides and donate them to a suitable electron acceptor. The oxidation-reduction midpoint potential of the FAD of the oxidase has been determined by ESR spectroscopy and shown to be -280 mV. The NADP+/ NADPH redox potential is -320 mV and that of the cytochrome b is -245 mV hence, the flavin is thermodynamically capable of accepting electrons from NADPH and transferring them to cytochrome b. As two electrons are transferred from NADPH, although O2 reduction requires only one electron, the scheme of electron transfer shown in Figure 5.8 has been proposed by Cross and Jones (1991). [Pg.162]

In the transfer of reducing equivalents from the pyridine nucleotide pool, flavoproteins carry out a central role of mediating the conversion of the obligatory 2-electron reductant to 1-electron receptors such as hemes and iron-sulfur redox centers. In such a role, the semiquinone form of the flavin serves as a pivotal intermediate. The reduction of flavins and flavoproteins by reduced pyridine nucleotides has been extensively studied since the initial work of Singer and Kearney which showed that flavin reduction can occur in a non-enzyme catalyzed manner. The reduction proceeds as a 2-electron process since the formation of a nicotinamide semiquinone (a necessary intermediate in a 1-electron process) has been... [Pg.126]

The chemistry of flavins is complex, a fact that is reflected in the uncertainity that has accompanied efforts to understand mechanisms. For flavoproteins at least four mechanistic possibilities must be considered.1533 233 (a) A reasonable hydride-transfer mechanism can be written for flavoprotein dehydrogenases (Eq. 15-23). The hydride ion is donated at N-5 and a proton is accepted at N-l. The oxidation of alcohols, amines, ketones, and reduced pyridine nucleotides can all be visualized in this way. Support for such a mechanism came from study of the nonenzymatic oxidation of NADH by flavins, a reaction that occurs at moderate speed in water at room temperature. A variety of flavins and dihydropyridine derivatives have been studied, and the electronic effects observed for the reaction are compatible with the hydride ion mecha-nism.234 236... [Pg.789]


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See also in sourсe #XX -- [ Pg.54 , Pg.62 , Pg.82 , Pg.83 , Pg.94 , Pg.329 ]




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