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FAD isoalloxazine

There are three tyrosine residues in Fre, Tyr 35, Tyr 72, and Tyr 116, close to the flavin-binding pocket (Fig. 1.2). Fluorescence lifetime measurements of the wild-type and mutant Fre/flavin complexes showed that electron transfer from Tyr 35 to the excited FAD isoalloxazine is responsible for the fluorescence quenching [6]. The average positions of the bound FAD and the three tyrosine residues of the protein are shown in the figure. To study the fluctuations in the... [Pg.6]

Figure 4.6. Arrangement of FMN, FAD, and NADP+ in the rat CPR crystal structure (pdb code I AMO). The Trp677 aromatic ring has to move away from the FAD isoalloxazine ring to allow access of nicotinamide ring to FAD for hydride transfer. Figure 4.6. Arrangement of FMN, FAD, and NADP+ in the rat CPR crystal structure (pdb code I AMO). The Trp677 aromatic ring has to move away from the FAD isoalloxazine ring to allow access of nicotinamide ring to FAD for hydride transfer.
Three residues Ser457, Asp675, and Cys630, lie in close proximity to the FAD isoalloxazine ring in CPR, and have been closely examined... [Pg.123]

MAO inhibitors with a propargylamine moiety inhibit the enzyme in a mechanism-based fashion. They first bind competitively to MAO and then are oxidized by the FAD moiety of the enzyme to active intermediates which bind quantitatively to C-5 of the FAD isoalloxazine ring (Scheme 17) (for review see Gerlach and coworkers ). Since this deactivation reaction is irreversible and quantitative, chlorgyline and selegiline have been used to map and quantitate MAO A and MAO B activities in the tissues of various animal species Most of the quantitation of MAO A and B in human brains was done... [Pg.762]

Riboflavin contains an isoalloxazine ring linked to the reduced sugar ribitol. The sugar unit in riboflavin is the non-cyclic ribitol, so that FAD and FMN differ somewhat from the nucleotides we encounter in nucleic acids. [Pg.456]

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. [Pg.635]

Flavin redox states in a dual flavin enzyme. (Left) Single-electron reduction of the isoalloxazine ring generates the semiquinone radical, while reduction by two electrons generates the fully reduced species. (Right) Five possible oxidation levels of a dual flavin enzyme, where the FMN reduction potential is held at a more positive value relative U) FAD. The flavins can theoretically accept a maximum of four electrons obtained from two NADPH. However, in NADPH-cytochrome P450, reductase, full reduction of the flavins is not normally reached when NADPH serves as the reductant. [Pg.159]

Riboflavin (vitamin B2 6.18) consists of an isoalloxazine ring linked to an alcohol derived from ribose. The ribose side chain of riboflavin can be modified by the formation of a phosphoester (forming flavin mononucleotide, FMN, 6.19). FMN can be joined to adenine monophosphate to form flavin adenine dinucleotide (FAD, 6.20). FMN and FAD act as co-enzymes by accepting or donating two hydrogen atoms and thus are involved in redox reactions. Flavoprotein enzymes are involved in many metabolic pathways. Riboflavin is a yellow-green fluorescent compound and, in addition to its role as a vitamin, it is responsible for the colour of milk serum (Chapter 11). [Pg.196]

Riboflavin (vitamin B2) 6,7-dimethyl-9-(D-l-ribityl)isoalloxazine (63), was discovered as a coloring matter in milk in 1879, but its importance was not then realized. Deficiency causes lesions of the eye and of the angle of the mouth. Riboflavin is phosphorylated by adenosine triphosphate (ATP) to give riboflavin 5 -phosphate (flavinadenine mononucleotide, FMN) and then flavinadenine dinucleotide (FAD) (64 R = riboflavin). These function as prosthetic groups in a number of flavoproteins which are dehydrogenation catalysts by virtue of the oxidation-reduction properties of the isoalloxazine system. [Pg.155]

Flavin adenine dinucleotide (FAD) (fig. 10.8) and flavin mononucleotide (FMN) are the coenzymatically active forms of vitamin B2, riboflavin. Riboflavin is the NI0-ribityl isoalloxazine portion of FAD, which is enzymatically converted into its coenzymatic forms first by phosphorylation of the ribityl C-5 hydroxy group to FMN and then by ade-nylylation to FAD. FMN and FAD are functionally equivalent coenzymes, and the one that is involved with a given enzyme appears to be a matter of enzymatic binding specificity. [Pg.207]

Flavin-containing enzymes are known as flavoproteins and, when purified, normally contain their full complements of FAD or FMN. The bright yellow color of flavoproteins is due to the isoalloxazine chromophore in its oxidized form. In a few flavoproteins, the coenzyme is known to be covalently bonded to the protein by means of a sulfhydryl or imidazole group at the C-8 methyl group and in at least one case at C-6. In most flavoproteins, the coenzymes are tightly but noncovalently bound, and many can be resolved into apoenzymes that can be reconstituted to holoenzymes by readdition of FAD or FMN. [Pg.207]

The functional end of the flavin coenzymes FMN and FAD is the tricyclic isoalloxazine system, with the numbering system shown in structure I, the air-stable, yellow, oxidized form. The other two functionally important redox states are the one-electron-reduced semiquinone, II (pKa = 8.4 for dissociation at N(5)), and the two-electron-reduced, colorless dihydroflavin, III. In the dihydro form N(5), C(4a), C(la), andN(l) form a diaminoethylene system and it was anticipated that nitrogen at the 5 and 1 positions would be key to coenzymatic function. [Pg.124]

However, recent x-ray studies on p-hydroxybenzoate-p-hydroxybenzoate hydroxylase binary complex crystals clearly show the aromatic substrate is bound at the flavin 4a-5 edge and orthogonal to the isoalloxazine plane (29). Unless this binary complex structure is highly misinformative, it can be inferred that in the 02, p-hydroxy-benzoate, enzyme ternary active complex, oxygen transfer is in the 4a,5 region, not the la, 1 region of the bound FAD, which rules out la-OOH derivatives as important oxygenating intermediates for this enzyme. [Pg.135]

Riboflavin is a combination of the isoalloxazine ring and ribitol. Conversion of the vitamin to FMN and FAD occurs via phosphorylation and adenylation, respectively, as indicated in Table 6.1. In the case of FAD or FMN, both the electrons and the hydrogen atoms are bound by the isoalloxazine ring structure of the riboflavin portion. The site of this reduction is pointed out in Table 6.1. FAD is often bound very tightly to the enzyme. For both the NAD+ and FAD-type coenzymes, the adenylate portion of the coenzyme is necessary for binding to the enzyme. Table 6.3 also lists some representative reactions in which FAD and FMN are cofactors. [Pg.133]

The molecule consists of a d-ribitol unit attached to an isoalloxazine ring (Figure 9-15). Anything more than a minor change in the molecule results in a loss of vitamin activity. Aqueous solutions of riboflavin are yellow with a yellowish-green fluorescence. The vitamin is a constituent of two coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). FMN is... [Pg.267]

Figure 21.4 Cartoon picture of the PAMO crystal structure. The FAD co-factor is shown in sticks in the center together with the active site R337. Residues that are <15 A from the isoalloxazin moiety of the FAD co-factor are shown in dark gray balls, while more distant residues are represented as light gray balls. Figure 21.4 Cartoon picture of the PAMO crystal structure. The FAD co-factor is shown in sticks in the center together with the active site R337. Residues that are <15 A from the isoalloxazin moiety of the FAD co-factor are shown in dark gray balls, while more distant residues are represented as light gray balls.
As shown in Figure 7.1, riboflavin consists of a tricyclic dimethyl-isoalloxazine ring conjugated to the sugar alcohol ribitol. The metabolically active coenzymes are riboflavin 5 -phosphate and flavin adenine dinucleotide (FAD). In some enzymes the prosthetic group is riboflavin, bound covalently at the catalytic site. [Pg.172]

The ribityl moiety is not linked to the isoalloxazine ring by a glycosidic linkage, and it is not strictly correct to caU FAD a dinucleotide. Nevertheless, this trivial name is accepted, as indeed is the even less correct term flavin mononucleotide for riboflavin phosphate. [Pg.174]

FIGURE 1. Isoalloxazine ring of FAD and FMN. The R represents either a ribityl phosphate group (FMN) or an adenosine ribityl phosphate group (FAD). [Pg.30]


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See also in sourсe #XX -- [ Pg.5 ]




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