Redox coenzymes 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. 

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). 

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 (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. 

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. 

The flavin coenzyme occurs in each of the oxidases discussed here as flavin adenine dinucleotide (FAD). The R group attached to is adenosyldiphosphoribityl. The flavin nucleus can exist in three redox states, each of which can adopt three ionization states (8, 9). Only two redox states—fully oxidized and fully reduced see Equation 1)—are kinetically important in the simple flavoprotein oxidases under discussion. 

FAD. (flavin adenine dinucleotide). The coenzyme of some redox enzymes. It contains riboflavin. 

Vitamin B2 forms the basis of two coenzymes, flavin mononucleotide (FMN) (Fig. lc). and flavin adenine dinucleotide (FAD) (Fig. Id). Both coenzymes are involved in redox reactions. The general scheme is ... 

MAO catalyzes the oxidative deamination of catecholamines, 5-hydroxytryptamine (serotonin), and other monoamines, both primary such as NE, and secondary such as EP. It is one of several oxidase-type enzymes whose coenzyme is the flavin-adenine-dinucleotide (FAD) covalently bound as a prosthetic group (Fig. 9-3). The isoalloxazine ring system is viewed as the catalytically functional component of the enzyme. In a narrow view N-5 and C-4a is where the redox reaction takes place (i.e., +H+, +le or -H+, -le), although the whole chromophoric N-5-C-4a-C-4-N-3-C-2-N-l region undoubtedly participates. Figure 9-3 is a proposed structure of MAO isolated from pig brain (Salach et al., 1976).4... 

These yellow redox systems all contain the conjugated tricyclic isoalloxazine ring system. Riboflavin, vitamin B2, is an isoalloxazine which is phosphorylated and then adenylated to two active redox coenzyme forms - FMN (flavin adenine mononucleotide, R = phosphorylated ribose) and FAD (flavin adenine dinucleotide, adenylated FMN). 

Two coenzymes which are involved in most of the redox reactions of metabolism are nicotine adenine dinucleotide, NAD", and FAD, flavine adenine dinucleotide. Most metabolic oxidations are, in fact, dehydrogenations, and not reactions with oxygen. Nicotinamide is derived from nicotinic acid, and the isoalloxazine ring of FAD is derived from riboflavin. Thiamin is the principal cofactor in enzymatic decarboxylations. Many of the vitamins serve as coenzymes in a wide variety of cellular reactions. 

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. 

Instead, biochemical redox reactions involving the oxidation of alkane to alkene require the participation of a coenzyme such as flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). The reduced forms FADH2 and FMNHj act as hydride donors in the alkene hydrogenation reactions (Figure 1.41). 

It is, however, better known that flavoenzymes (i.e., enzymes utilizing the flavin adenine dinucleotide [FAD FADH2] redox system) mediate the introduction of a,P carbon-carbon double bonds into carboxylic acids and into acetyl Coenzyme A (acetyl CoA) thioesters of long-, medium-, and short-chain fatty acids. In carboxylic acids, such as those of the tricarboxylic acid (citric acid, TCA, or Krebs) cycle (Chapter 11) the oxidation is affected by the enzyme sucdnate dehydrogenase (fumerate reductase— EC 1.3.99.1), which utilizes the cofactor flavin adenine dinucleotide (FAD) The latter is reduced to FADH2 and an ( )-double bond is introduced. The process shown in Scheme 9.105, for the conversion of succinate (1,4-butanedioic acid) to fumerate [(E)-l,4-butenedioic acid], is a fragment of the tricarboxylic acid (citric acid, TCA, or Krebs) cycle (Chapter 11), which is the pathway commonly utilized for oxidative degradation of acetate to carbon dioxide. 

Riboflavin and other flavinoids are found in dairy produce and meat and to a lesser extent in cereals. The RDA is 1.6-2.0 mg. Flavins are stable to heat and acid but destroyed by exposure to light. UV irradiation of riboflavin in acid or neutral solution gives rise to the fluorescent compound lumichrome, whereas in alkaline solutions irradiation produces lumiflavin. Flavins are required in the body as their coenzymes flavin mononucleotide and flavin adenine dinucleotide, which are involved in redox reactions involving one- and two-electron transfers and linked to many energy-dependent processes in the body. 

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). 

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