Riboflavin vitamin flavoprotein enzymes

Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups. FMN and FAD are formed in the body from the vitamin riboflavin (Chapter 45). FMN and FAD are usually tighdy—but not covalendy—bound to their respecdve apoenzyme proteins. Metalloflavopro-teins contain one or more metals as essential cofactors. 

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 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). 

The answer is b. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121-138. Wilson, pp 287-320.) Nicotinamide adenine dinucleotide (NAD+) is the functional coenzyme derivative of niacin. It is the major electron acceptor in the oxidation of molecules, generating NADH, which is the major electron donor for reduction reactions. Thiamine (also known as vitamin Bi) occurs functionally as thiamine pyrophosphate and is a coenzyme for enzymes such as pyruvate dehydrogenase. Riboflavin (vitamin B2) functions in the coenzyme forms of flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). When concentrated, both have a yellow color due to the riboflavin they contain. Both function as prosthetic groups of oxidation-reduction enzymes or flavoproteins. Flavoproteins are active in selected oxidation reactions and in electron transport, but they do not have the ubiquitous role of NAD+. 

More than 100 years ago a fluorescent compound was isolated first fi om whey, and later from different biological materials. When it Ijecame clear that the isolated yellow pigments, named lactochrome, ovoflavin, or lactoflavin, had a common structure, the new compound was named riboflavin (vitamin B2) (for historical review see 2). In the years between 1933 and 1935 the structure and the main chemical reactions of riboflavin were studied and the chemical synthesis was performed. Soon afterward, the coenzyme forms, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), were isolated in pure form, and the structures were determined. In the last 50 years many flavoproteins were isolated and their physicochemical properties were studied. Succinate dehydrogenase was the first enzyme found with the prosthetic group (FAD) covalently bound to the protein. About 20 flavoproteins are now known to contain covalently bound coenzyme (mainly via carbon atom 8a) (3). In mammalian tissue, the number of covalently bound flavoproteins appears to be limited. 

Flavin binding proteins. Flavin-binding chromoproteins are collectively referred to as flavoproteins. Their chromophores are derived from riboflavin (vitamin B2 Fig. 15), which is phosphorylated by the flavokinase enzyme to yield flavin mononucleotide (FMN) and, in a second ATP-dependent reaction, FAD pyrophosphorylase attaches an AMP... 

In species for which ascorbate is not a vitamin, riboflavin deficiency can also lead to considerably reduced synthesis and low tissue concentrations of ascorbate, since gulonolactone oxidase, the key enzyme in ascorbate synthesis (Section 13.2), is a flavoprotein. 

Pyridoxine phosphate oxidase is a flavoprotein, and activation of the erythrocyte apoenzyme hy riboflavin 5 -phosphate in vitro can he used as an index of riboflavin nutritional status (Section 7.4.3). However, even in riboflavin deficiency, there is sufficient residual activity of pyridoxine phosphate oxidase to permit normal metabolism of vitamin Bg (Lakshmi and Bamji, 1974). Pyridoxine phosphate oxidase is inhibited by its product, pyridoxal phosphate, which binds a specific lysine residue in the enzyme. In the brain, the K, of pyridoxal phosphate is of the order of 2 / mol per L - the same as the brain concentration of free and loosely bound pyridoxal phosphate, suggesting that this inhibition may be a physiologically important mechanism in the control of tissue pyridoxal phosphate (Choi et al., 1987). 

Glutathione is discussed further in the section on selenium and glutathione in Chapter 10. The enzyme assay is conducted using glutathione reductase extracted from red blood cells with and without added FAD. Chmnic consumption of a diet deficient in riboflavin allows the continued synthesis of a variety of flavoproteins, but results in the accumulation of apoenzyme without its conversion to holoen-zyme. Addition of chemically pure FAD to a biological fluid containing apoenzyme results In the stimulation of enzyme activity because of the formation of the holoenzyme. It is this stimulation of enzyme activity that is used to determine vitamin status in humans. 

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 is delivered in form of free vitamin, or as its coenzymes, i.e. flavin mononucleotide (FMN) and adenine dinucleotide (FAD), which occurs mainly as a prosthetic group of flavoproteins. Release of coenzymes from flavoproteins by acidification in stomach and proteolysis, both gastric and intestinal, must precede the absorption. This hydrolysis also releases several percentages of covalently bound FAD from 8a-(peptidyl)riboflavins (Chia et al. 1978). Free riboflavin is physiologically preferred form of absorbed vitamin B2 (Daniel et al. 1983). The upper small intestine enzymes which catalyse reversible reactions of conversion nucleotides into riboflavin are located in the brush-border membrane of enterocytes (Figure 36.1). 

Vitamin (riboflavin section 11.7) is important in a wide variety of oxidation and reduction reactions. A few enzymes contain riboflavin itself, while others contain a riboflavin derivative either riboflavin phosphate (sometimes called flavin mononucleotide) or flavin adenine dinucleotide (FAD Figure 2.14). When an enzyme contains riboflavin, it is usually covalently bound at the active site. Although riboflavin phosphate and FAD are not normally covalently bound to the enzyme, they are very tightly bound, and can be regarded as prosthetic groups. The resultant enzymes with attached riboflavin are collectively known as flavoproteins. 

Flavin Nucleotide. Vitamin B2 or riboflavin is a component of the prosthetic group of flavoproteins. The isolation of the vitamin from biologic material was accomplished by Gyorgy and R. Kuhn in 1933. Almost simultaneously, the first yellow enzyme was obtained in pure form and the close relationship between these active substances was recognized. 

Riboflavin or B2 functions as part of enzymes called flavoproteins (FMN and FAD) critically important in respiration and cell metabolism. It plays a major role with thiamin and niacin in oxidation-reduction reactions. The deficiency of this vitamin is characterized by photophobia, angular lip stomatitis, dermatitis, and swelling of the tongue. 

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