Ribitol flavin nucleotides

Aflavoprotein is an enzyme that contains either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) as a coenzyme. FAD and FMN, like NAD and NADP, are coenzymes used in oxidation reactions. As its name indicates, FAD is a dinucleotide in which one of the heterocyclic components is flavin and the other is adenine. FMN contains flavin but not adenine—it is a mononucleotide. (Flavin is a bright yellow compound flavus is Latin for yellow. ) Notice that instead of ribose, the flavin nucleotide has a ribitol group (a reduced ribose). Flavin plus ribitol is called riboflavin. Riboflavin is also known as vitamin B2. A vitamin B2 deficiency causes inflammation of the skin. 

These compounds are derived from the vitamin riboflavin. They are not true nucleotides, because the flavin ring system is attached to ribitol, a sugar alcohol, rather than ribose. 

In riboflavin (7,8-dimethyl-10-(/ )-ribityl isoalloxazine), the polyol ribitol is connected via an amino bond to the chromophore. Riboflavin is not an iV-gly-coside. Accordingly, flavin mononucleotide (FMN, riboflavin S -phosphate) and flavin adenine dinucleotide (FAD riboflavin 5 -adenosine diphosphate) are hydrolyzable only in the nucleotide part (Scheme 7.2.22). 

The nucleotide coenzymes are structurally related to the mononucleotides. Typical nucleotide coenzymes are adenosine triphosphate (ATP), flavin-adenine-dinucleotide (FAD) and numerous other phosphate esters of complex structure, containing adenosine, guanosine, cytidine or uridine. Five coenzymes are known for example, which are derived from cytidine diphosphate (CDP) CDP-choline, CDP-ethanolamine, CDP-diglyceride, CDP-glycerol and CDP-ribitol. 

Flavin adenine dinucleotide (FAD), discovered and characterized by Warburg and Christian 150), has also had its structure confirmed by the elegant synthetic approach of Christie and co-workers 151), It is interesting to note that both FMN and FAD are not strictly nucleotides since the bond is between the isoalloxazine ring and an alditol, ribitol, rather than a sugar. 

In addition to their role as components of nucleoproteins, purines and pyrimidines are vital to the proper functioning of the cell. The bases are constituents of various coenzymes, such as coenzyme A (CoA), adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP), diphosphopyridine nucleotide (DPN), triphosphopyridine nucleotide (TPN), and flavin adenine dinucleotide (FAD). A pyrimidine derivative, cytidine diphosphate choline, is involved in phospholipid synthe another pyrimidine compound, uridine diphosphate glucose, is an important substance in carbohydrate metabolism. Cytidine diphosphate ribitol functions in the biosynthesis of a new group of bacterial cell-wall components, the teichoic acids. While mammals excrete nitrogen derived from protein catabolism in the form of urea, birds eliminate their nitrogen by synthesizing it into the purine compound, uric acid. 

Yellow (hence the name) prosthetic groups,//av/w adenine dinucleotide (FAD) and the somewhat less common flavin mononucleotide (FMN) are linked to protein in flavoproteins (Fig. 29), The nomenclature is not quite accurate since 6,7-dimethyl-isoalloxazine, which is responsible for the color, is not linked to ribose, as in the nucleotides, but to the corresponding sugar alcohol ribitol. In both FAD and FMN the 6,7-dimethyl-... 

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