Flavin, generally adenine dinucleotide

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. ... 

Enzymatic cofactors, such as nicotinamide adenine dinucleotide (NADH), nicotinamide adenine dinucleotide phosphate (NADPH), flavin adenine dinucleotide (EAD), flavin mononucleotide (EMN), and pyridoxal phosphate, are fluorescent and commonly found associated with various proteins where they are responsible for electron transport (see Fig. lb and Table 1). NADH and NADPH in the oxidized form are nonfluorescent, whereas conversely the flavins, FAD and EMN, are fluorescent only in the oxidized form. Both NADH and FAD fluorescence is quenched by the adenine found within their cofactor structures, whereas NADH-based cofactors generally remain fluorescent when interacting with protein structures. The fluorescence of these cofactors is often used to study the cofactors interaction with proteins as well as with related enzymatic kinetics (1, 9-12). However, their complex fluorescent characteristics have not led to widespread applications beyond their own intrinsic function. 

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

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 ... 

Ann O Rexia has been malnourished for some time, and has developed subclinical deficiencies of many vitamins, including riboflavin. The coenzymes FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide) are synthesized from the vitamin riboflavin. Riboflavin is actively transported into cells, where the enzyme flavokinase adds a phosphate to form FMN. FAD synthetase then adds AMP to form FAD. FAD is the major coenzyme in tissues and is generally found tightly bound to proteins, with about 10% being covalently bound. Its turnover in the body is very slow, and people can live for long periods on low intakes without displaying any signs of a riboflavin deficiency. 

Riboflavin 7,8-dimethyl-10-(r-D-ribityl)isoalloxazine (RF), a yellow-green, light-sensitive compound, is widely distributed in animal and plant cells. In biological systems, the two coenzyme forms flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are predominant (Fig. 1). These terms are formally incorrect, because FMN is no nucleotide and FAD is no dinucleotide. However, these names are still accepted. Some authors use also the name riboflavin to designate the coenzyme form of the vitamin and take vitamin B2 as a general term. Some basic physicochemical properties of these flavins are outlined... 

Biochemical status estimates are generally based upon urinary excretion or measurements of erythrocyte glutathione reductase (NADPH oxidized glutathione oxidoreductase EC 1.6.4.2) and its reactivation with flavin adenine dinucleotide (FAD) in red cell lysates. Other biochemical indices, such as plasma or red cell flavin concentrations, have been less widely used, but their potential is increasing with the advent of new assay techniques such as capillary electrophoresis with highly sensitive laser-induced fluorescence detection. Fvmctional indices... 

---