Riboflavin structural specificity

Isotope incorporation studies showed that the dimethylbenzimidazole moiety of vitamin Bj2 can be biosynthesized under aerobic conditions from riboflavin (10). Specifically, all carbon atoms of dimethylbenzimidazole are derived from the xylene ring and the 1 -carbon of riboflavin. Recently, this transformation was shown to be catalyzed by the BluB protein. Surprisingly, the reaction requires only FMNH (46) and molecular oxygen. The structure of BluB protein of Sinorhizobium meliloti) s been reported (Figure 26). ... 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

Structures of the vitamin riboflavin (a) and the derived flavin coenzymes (b). Like NAD+ and NADP+, the coenzyme pair FMN and FAD are functionally equivalent coenzymes, and the coenzyme involved with a given enzyme appears to be a matter of enzymatic binding specificity. The catalytically functional portion of the coenzymes is shown in red. 

Specifically, an alkaline aqueous solution of 6,7-dimethyl-8-ribityllumazine contains a mixture of at least five anionic species, namely the exomethylene-type anion with an open chain structure of the position 8 substituent and two diastereomers each of the 5-ring and 6-ring forms arising by intramolecular reaction of the 2 and 3 hydroxyl groups. The hypothesis of an involvement of one of the tricyclic species as intermediates in the biosynthesis of riboflavin is not supported by more recent data. The exomethylene form appears likely to be a late intermediate in the mechanism of lumazine synthase as documented by stopped flow analysis. Moreover, the exomethylene form (31) has been proposed as an early intermediate of the riboflavin synthase reaction. This is supported by the early observation that riboflavin synthase accelerates proton exchange at the position 7 methyl group. 

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