Flavin mononucleotide reactions involving

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 (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. 

It carries its physiological function in its active forms, flavin mononucleotide (FMN) and flavin adenine dinucleotide. These coenzymes are involved in various biochemical reactions. 

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). 

Flavoenzymes are widespread in nature and are involved in many different chemical reactions. Flavoenzymes contain a flavin mononucleotide (FMN) or more often a flavin adenine dinucleotide (FAD) as redox-active prosthetic group. Both cofactors are synthesized from riboflavin (vitamin B2) by microorganisms and plants. Most flavoenzymes bind the flavin cofactor in a noncovalent mode (1). In about 10% of aU flavoenzymes, the isoalloxazine ring of the flavin is covalently linked to the polypeptide chain (2, 3). Covalent binding increases the redox potential of the flavin and its oxidation power, but it may also be beneficial for protein stability, especially in flavin-deficient environments. 

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. 

Riboflavin, also known as vitamin B2, is an essential component of FAD and flavin mononucleotide (FMN)—coenzymes tliat are involved in many redox reactions. 

Whereas redox reactions on metal centres usually only involve electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer, but hydrogen transfer as well — hence the frequently used denomination dehydrogenase . Note that most of these dehydrogenase reactions are reversible. Redox reactions in biosynthetic pathways usually use NADPH as their source of electrons. In addition to NAD and NADP+, which intervene in redox reactions involving oxygen functions, other cofactors like riboflavin (in the form of flavin mononucleotide, FMN, and flavin adenine dinucleotide, FAD) (Figure 5.3) participate in the conversion of [—CH2—CH2— to —CH=CH—], as well as in electron transfer chains. In addition, a number of other redox factors are found, e.g., lipoate in a-ketoacid dehydrogenases, and ubiquinone and its derivatives, in electron transfer chains. 

Flavoenzymes catalyze a diverse set of oxidation-reduction reactions involving tightly bound FAD or flavin mononucleotide (FMN) (78, 79) [Eq. (7)] ... 

FIGURE 15.4 The structures of riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). Even in organisms that rely on the nicotinamide coenzymes (NADH and NADPH) for many of their oxidation-reduction cycles, the flavin coenzymes fill essential roles. Flavins are stronger oxidizing agents than NAD and NADP. They can be reduced by both one-electron and two-electron pathways and can be reoxidized easily by molecular oxygen. Enzymes that use flavins to carry out their reactions—flavoenzymes—are involved in many kinds of oxidation-reduction reactions. 

Flavins (El) catalyze many different bioreactions of physiological importance [7-9]. Riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) have the 7,8-dimethyl isoalloxazine ring in common but differ in the side chain attached to NIO. With their five redox states, fully oxidized, one-electron reduced semiquinoid (F1H and F1 ), and fully reduced hydroquinone (FIH2 and F1H ), flavins are involved in one-electron and two-electron transfer reactions [10]. 

Figure 31. An electron transport system and redox potentials in mitochondria. FMN refers to Flavin mononucleotide in NADH2 dehydrogenase, FAD refers to Flavin adenine dinucleotide in succinate dehydrogenase, I, II, and III correspond to the reaction processes which may be involved in phosphorylation, Fe—S non-heme iron, Cyt Heme in cytochromes (after ref. 171).

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). 

Riboflavin is an important constituent of the flavoproteins.The prosthetic group of these compound proteins contains riboflavin in the form of the phosphate (flavin mononucleotide, FMN) or in a more complex form as flavin adenine dinucleotide (FAD). There are several flavoproteins that function in the animal body they are all concerned with chemical reactions involving the transport of hydrogen. Further details of the importance of flavoproteins in carbohydrate and amino acid metabolism are discussed in Chapter 9. Flavin adenine dinucleotide plays a role in the oxidative phosphorylation system (see Fig. 9.2 on p. 196) and forms the prosthetic group of the enzyme succinic dehydrogenase, which converts succinic acid to fumaric acid in the citric acid cycle. It is also the coenzyme for acyl-CoA dehydrogenase. 

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. 

STEP 7 OF FIGURE 25.1 OXIDATION The final step in PLP biosynthesis is oxidation of the primary alcohol group in pyridoxine 5 -phosphate to the corresponding aldehyde. Typically, as we ve seen on numerous occasions, alcohol oxidations are carried out hy either NAD" " or NADP+. In this instance, however, flavin mononucleotide (FMN) is involved as the oxidizing coenzyme and reduced flavin mononucleotide (FMNH2) is the hy-product. The details of the reaction are not clear, but evidence suggests that a hydride transfer is involved, just as in NAD" " oxidations. 

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