Flavin mononucleotide riboflavin transport

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+. 

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. 

Description. Riboflavin is a water-soluble B-complex vitamin, also known as vitamin B2. In the body, riboflavin functions in the mitochondrial electron transport systan as the coenzymes, flavin adenine dinucleotide and flavin mononucleotide. ... 

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. 

Studies with these model systems have shown that the transport of riboflavin at low (e.g., micromolar) concentrations is temperature- and energy-dependent (it is inhibited by inhibitors of ATP production from energy substrates), it becomes saturated as the concentration of riboflavin increases, and it is sodium ion dependent. These characteristics are shared with many other types of small molecules that are actively transported across the gut wall. More specifically for riboflavin, the active transport mechanism involves phosphorylation (to riboflavin phosphate, also known as flavin mononucleotide, or FMN) followed by dephosphorylation, both occurring within the intestinal cells (Figure 1). This latter process is not shared by several other B vitamins, but it is one of a number of common strategies which the gut may use to entrap essential nutrients, and then relocate them, in a controlled manner and direction. A similar strategy is employed at other... 

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