Flavin coenzymes Adenine Dinucleotide

Complex II, also known as succinate-coenzyme Q reductase, accepts electrons from succinate formed during the TCA cycle (see the previous chapter). Electrons flow from succinate to FAD (the flavin-adenine dinucleotide) coenzyme, through an iron-sulfur protein and a cytochrome bsso protein (the number refers to the wavelength where the protein absorbs), and to coenzyme Q. No protons are translocated by Complex II. Because translocated protons are the source of the energy for ATP synthesis, this means that the oxidation of a molecule of FADH2 inherently leads to less ATP synthesized than does the... 

Flavin adenine dinucleotide in biochemical pathways, 1, 252 Flavin coenzymes... 

Direct hydroxylation of an aromatic ring to yield a hydroxybenzene (a phenol) is difficult and rarely done in the laboratory., but occurs much more frequently in biological pathways. An example is the hydroxylation of p-hydroxyphenyl acetate to give 3,4-dihydroxyphenyl acetate. The reaction is catalyzed by p-hydroxyphenylacctate-3-hydroxylase and requires molecular oxygen plus the coenzyme reduced flavin adenine dinucleotide, abbreviated FADH2. 

This thiol-disulfide interconversion is a key part of numerous biological processes. WeTJ see in Chapter 26, for instance, that disulfide formation is involved in defining the structure and three-dimensional conformations of proteins, where disulfide "bridges" often form cross-links between q steine amino acid units in the protein chains. Disulfide formation is also involved in the process by which cells protect themselves from oxidative degradation. A cellular component called glutathione removes potentially harmful oxidants and is itself oxidized to glutathione disulfide in the process. Reduction back to the thiol requires the coenzyme flavin adenine dinucleotide (reduced), abbreviated FADH2. 

Step 1 of Figure 29.3 Introduction of a Double Bond The /3-oxidation pathway begins when a fait)7 acid forms a thioester with coenzyme A to give a fatty acyl Co A. Two hydrogen atoms are then removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently jn biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

The fused system, pyrimido[4,5-h]pyrazine (219), is more commonly known as pteridine, a ring system which is present in such biologically important molecules as alloxazine, riboflavin, leucopterin, xanthopterin, and the coenzyme, flavin-adenine dinucleotide. 

Flavin Adenine Dinucleotide (FAD) (C27 H33 N9 O15P2) is a coenzyme that acts as a hydrogen acceptor in dehydrogenation reactions in an oxidized or reduced form. FAD is one of the primary cofactors in biological redox reactions. 

Riboflavin is heat-stable in the absence of light, but extremely photosensitive. It has a high degree of natural fluorescence when excited by UV light. This property can be used for detection and determination. Two coenzymes (Fig. 2), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), are derived from riboflavin. 

Riboflavin fulfills its role in metabolism as the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) (Figure 45-10). FMN is formed by ATP-dependent phosphorylation of riboflavin, whereas FAD is synthesized by further reaction of FMN with ATP in which its AMP moiety is transferred to the... 

Figure45-10. Riboflavin and the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).

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). 

The term NOS is used to denote a family of three related but distinct isoenzymes neuronal NOS (nNOS) endothelial NOS (eNOS, endothelium and platelets) and inducible NOS (iNOS, endothelium, vascular smooth muscle and macrophage). In addition to reduced nicotinamide adenine dinucleotide phosphate (NADPH) shown in Figure 5.5, NOS enzymes also require flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and tetrahydrobiopterin (BH4) as coenzymes. 

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. 

The flavin-based coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are associated with a wide variety of enzymes that catalyze reactions in critical biosynthetic and catabolic processes (Fig. 16). Unlike other coenzymes, the reactions catalyzed do not conserve specific mechanistic pathways. In each case the apoenzyme serves to steer the course of the reaction through specific interactions with substrate and coenzyme [55]. Nonetheless, there are common features of the interactions of the apoenzymes with the flavin which can be exploited in the design of functional peptides and proteins. 

So what does riboflavin do As such riboflavin does nothing. Like thiamine, riboflavin must undergo metabolic change to become effective as a coenzyme. It fact, it undergoes two reactions. The first converts riboflavin to riboflavin-5-phosphate (commonly known as flavin adenine mononucleotide, FMN), about which we will say no more, and the second converts it to flavin adenine dinucleotide, FAD. The flavins are a class of redox agents of very general importance in biochemistry. FAD is the oxidized form and FADH2 is the reduced form. ... 

Riboflavin (vitamin B2) is a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), coenzymes that play a major role in oxidation-reduction reactions (see Section 15.1.1). Many key enzymes involved in metabolic pathways are actually covalently bound to riboflavin, and are thus termed flavoproteins. 

Riboflavin (from the Latin flavus, yellow) serves in the metabolism as a component of the redox coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD see p. 104). As prosthetic groups, FMN and FAD are cofactors for various oxidoreductases (see p. 32). No specific disease due to a deficiency of this vitamin is known. 

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-... 

CoASH Coenzyme A FAD Oxidized flavin adenine dinucleotide... 

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. 

Riboflavin (vitamin B2) is found in liver, milk, meat, green vegetables, cereals and mushrooms. It is active in the form of two coenzymes, flavin mononucleotide and flavin adenine dinucleotide. As a coenzyme for proton transfer in the respiratory chain it is indispensable for energy-release from carbohydrates, lipids and proteins. Riboflavin deficiency only occurs in combination with deficiencies of other members of the vitamin B family. The symptoms of such deficiency consist of angular stomatitis, lesions of the cornea, dermatoses and normochromic normocytic anaemia. 

Fig. 1. Energy metabolism in the normal myocardium (ATP adenosine-5 -triphosphate, ADP adenosine-5 -diphosphate, P phosphate, PDH pyruvate dehydrogenase complex, acetyl-CoA acetyl-coenzyme A, NADH and NAD" nicotinamide adenine dinucleotide (reduced and oxidized), FADH2 and FAD flavin adenine dinucleotide (reduced and oxidized).

Table 3.1.1 Disorders of organic acid metabolism (in alphabetical order). This table does not include disorders with primary accumulation of amino acids, disorders of mitochondrial fatty acid oxidation, or primary lactic acidemias. Co A Coenzyme A, FAD flavin adenine dinucleotide...

The combined dehydrogenation and decarboxylation of pyruvate to the acetyl group of acetyl-CoA (Fig. 16-2) requires the sequential action of three different enzymes and five different coenzymes or prosthetic groups—thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), coenzyme A (CoA, sometimes denoted CoA-SH, to emphasize the role of the —SH group), nicotinamide adenine dinucleotide (NAD), and lipoate. Four different vitamins required in human nutrition are vital components of this system thiamine (in TPP), riboflavin (in FAD), niacin (in NAD), and pantothenate (in CoA). We have already described the roles of FAD and NAD as electron carriers (Chapter 13), and we have encountered TPP as the coenzyme of pyruvate decarboxylase (see Fig. 14-13). 

Synthesis of NO Arginine, 02, and NADPH are substrates for cytosolic NO synthase (Figure 13.9). Flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), heme, and tetrahydro- biopterin are coenzymes for the enzyme, and NO and citrulline are products of the reaction. Three NO synthases have been identified. Two are constitutive (synthesized at a constant rate regardless of physiologic demand), Ca2+-calmodulin-dependent enzymes. They are found primarily in endothelium (eNOS), and neural tissue... 

Figure 15-7 The flavin coenzymes flavin adenine dinucleotide (FAD) and riboflavin 5 -phosphate (FMN). Dotted lines enclose the region that is altered upon reduction.

Flavin adenine dinucleotide. See FAD Flavin adenine diphosphate. See FAD Flavin coenzymes 766,780 - 795 modified 788, 789 reduced 794 Flavin radicals 792 color of 794 formation constant 794 Flavocytochrome b2 782, 794, 847 Flavodoxins 793, 799, 800 Flavoprotein(s) 513, 788... 

In the biological oxidation-reduction system, reduced NAD (i.e., NADH) is reoxidized to NAD by the riboflavin-containing coenzyme FAD flavin-adenine dinucleotide). 

This reaction is a good example of the interrelationship of vitamin B coenzymes. Four vitamin coenzymes are necessary for this one reaction (1) thiamine (in TPP) for decarboxylation (2) nicotinic acid in nicotinamide adenine dinucleotide (NAD) (3) riboflavin in flavin adenine dinucleotide (FAD) and (4) pantothenic acid in coenzyme A (CoA) for activation of die acetate fragment. 

Riboflavin has been shown to be a constituent of 2 coenzymes (1) Flavin mononucleotide (FMN) and (2) flavin adenine dinucleotide (FAD). The structures are ... 

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. 

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