Riboflavin, FMN, and FAD

Riboflavin, a 7,8-dimethyl-10-(D-T-ribityl)isoalloxazine, also known as Vitamin Bj, is the precursor of flavin mononucleotide (EMN) and PAD, essential redox cofactors, and also involved in numerous physiological processes such as light sensing [281,282], photorepair of DNA [283], circadian time-keeping, and bioluminescence (Pigure 6.67) [284-287]. 

Whereas plants, fiingi, and certain microorganisms obtain riboflavin by biosynthesis, higher animals depend on dietary sources. The biosynthesis of riboflavin has been reviewed repeatedly [288-294]. 

The biosynthetic pathway of riboflavin (and FMN/FAD), for bacteria/archaea/ fungi, requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates (Pigure 6.68) [295, 296]. The first biosynthetic step is catalyzed by the bifunctional GTP cyclohydrolase II/3,4-dihydroxy-2-butanone-4-phosphate synthase, rib A, and involves the hydrolytic release of formate and pyrophosphate from GTP. The product 2,5-diamino-6-ribosylamino-4(3//)-pyrimidinone 5 -phosphate 48 is converted to 5-amino-6-ribitylamino-2,4(l/7,3//)-pyrimidinedione 5 -phosphate 50 by two reaction steps, catalyzed by the bifunctional riboflavin-specific deam-inase/reductase, ribG, involving the hydrolytic cleavage of the 2-amino group of 

The intermediate 5-amino-6-ribitylamino-2,4(I//,3//)-pyrimidinedione 5 -phos phate 50 is dephosphorylated through an unknown process, forming 5-amino-6-ribitylamino-2,4(l//,3//)-pyrimidinedione 52. The latter is converted into 6,7-dimethyl-8-ribityllumazine 53 by a lumazine synthase or riboflavin synthase P-chain, nfeH, through condensation with 3,4-dihydroxy-2-butanone 4-phosphate 54 obtained from ribulose 5-phosphate. The final step involves the unusual dismutation of 6,7-dimethyl-8-ribityllumazine 53, catalyzed by riboflavin synthase or riboflavin synthase a-chain, n feB, affording riboflavin and the dephosphorylated biosynthetic intermediate 52. Riboflavin is then converted to the coenzyme forms FMN and FAD. 

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Anderson RF (1983a) Energetics of the one-electron reduction steps of riboflavin, FMN and FAD to their reduced forms. Biochim Biophys Acta 722 158-162 Anderson RF (1983b) The bimolecular decay rates of the flavosemiquinones of riboflavin, FMN and FAD. Biochim Biophys Acta 723 78-82... 

Direct measurement of riboflavin, FMN, and FAD in plasma or erythrocytes may be made by HPLC, usually with fluorescence detection after protein precipitation or by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF). In a study of riboflavin status and FMN and FAD concentrations in plasma and erythrocytes from elderly subjects at baseline and after low-dose riboflavin supplementation, using both activation coefficient measurements and CE-LIF, it was concluded that concentrations of aU Ba vitamers except plasma FAD are potential... 

Riboflavin, FMN, and FAD metabolism RibU/EcfAA T - riboflavin transporter Firmicutes 20-22 Genetics and uptake assays in Bacillus subtilis, 21,23, 24... 

Table 3 Functional roles included in riboflavin, FMN, and FAD biosynthesis subsystems... 

The completely amidated cobyrinic acid without isopropanol residues is called cobyric acid. Cobyric acid containing isopropanol is named cobinic acid, and its amide, cobinamide. L-threonine is the precursor for l-amino-2-propanol (Ford and Friedman, 1976). The complete corrinoids contain DMB as the lower (Co-a) nucleotide ligand. In anaerobic producers of corrinoids the synthesis of DMB differs from its aerobic synthesis. Aerobic and aerotolerant microorganisms form this base from FMN. Propionibacteria can use different forms of flavins riboflavin, FMN and FAD as precursors of the vitamin B nucleotide ligand (Jaszewski et al., 1995). 

Another milestone in flavin research was the characterization of the flavo-semiquinones, the first example of a stable flavin radical. In addition to riboflavin, FMN, and FAD, a number of flavin analogs with biological activities have been found in micro-organisms and plants. Among these, the coenzyme factor F420 isolated from methanogenic bacteria should be mentioned, which has 5-deazaflavin as its chromophore. 

Figure 1 Structural formulae of riboflavin, FMN, and FAD (1) riboflavin in oxidized (FL j) or reduced form (FL, ) (2) FMN, flavin mononucleotide (3) FAD, flavin adenine dinucleotide. ...

Flavin analogs are of special use in the study of structure/function relationship within different flavin dependent enzymes. Even before 1989, a variety of flavin analogs at the riboflavin, FMN, and FAD level were separated, quantitated, or isolated by FIPLC, including 5-deazaflavin, 1-deazaflavin, and malonylriboflavin (85). A simultaneous separation of 12 different flavin analogs by reversed-phase FIPLC is demonstrated in Figure 11 (85). 

Rg. 15 A Reversible photochemical formation of the FMN-cysteine adduct in naturally-occurring FMN-binding proteins B Chemical structures of riboflavin, FMN and FAD. 

Figure 1 Molecular structure and numbering scheme of 7,8-dimethylisoalloxazine, riboflavin, FMN and FAD in the fully oxidized state, together with the anion and neutral radicals...

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