Biosynthesis of Riboflavin

In yeasts and fungi, opening of the imidtizole ring is followed by reduction of the ribose side chain to ribitol, deamination, and dephosphorylation to yield amino-ribitylamino-pyiimidinedione in bacteria, deamination occurs before 

The fintil step is a dismutation reaction between two molecules of dimethyl-lumazine, cattdyzed by riboflavin synthtise, yielding riboflavin eind amino-ribityltunino-pyrimidinedione. This latter product can undergo reaction with dihydroxybuttmone 4-phosphate to yield dimethyl-ribityllumazine. 

A number of fungi have a failure of the normal regulation of riboflavin synthesis and are overproducers of the vitamin. Mutants of Ashbya gossypii may accumulate up to 150 /xmol of riboflavin per gram of protein, compared with a normal content of 0.25 /xmol per gram of protein. They can produce and excrete so much that riboflavin crystallizes in the culture medium. Such fungi are used for the commercial production of riboflavin by fermentation, as an alternative to chemical synthesis. 

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Figure 25-20 The biosynthesis of riboflavin and of the dimethylbenzimidazole group of vitamin B12. The fact that two molecules of 6,7-dimethyl-8-ribityllumazine disproportionate to form riboflavin accounts for the need for two molecules of each reactant and product in many steps.

Soon afterward, it was shown that 6,7-dimethyl-8-ribityllumazine (10) was an intermediate in the biosynthesis of riboflavine it was noted that the UV spectrum of this lumazine could not be reconciled with those of... 

Precursors in the biosynthesis of riboflavin include purines, pyrimidines, and ribose. Intermediate in the synthesis is 6,7-diinethyl-8-ribityllumazine. In plants, riboflavin production sites are found in leaves, germinating seeds, and root nodules. Storage sites in animals are heart and liver, with small amounts in Ihe kidneys. Riboflavin in overdose is essentially nontoxic to humans. 

Biosynthesis of 7,8-didemethyl-8-hydroxy-5-deazaflavin starts from guanosine-5 -triphosphate (GTP), which leads through several steps to 5-amino(6-ribitylamino)-2,4( 1 //,3//)-pyrimidinedione 5 -phosphate, whose addition to 4-hydroxybenzoic acid which comes from shikimate gave the target compound <85JA8300>. The biosynthesis of riboflavin and deazaflavins has been studied in Methanobacterium thermoautotrophicum <91JBC9622>. 

The biosynthesis of riboflavin, from the nucleotide guanosine triphosphate (GTP), requires at least six enzymatic activities and is subject to a complex regulation architecture [136, 139]. The genes that encode the enzymes have been identified and cloned [136]. 

Fig. 8.28 Biosynthesis of riboflavin in bacteria. Compounds ArP, 5-amino-6-(ribityl-amino)-2,4-(l /-/,3H)-pyrimidinedione ...

Figure7.3. Biosynthesis of riboflavin in fungi in bacteria, deamination precedes reduction of sugar. GTP cyclohydrolase, EC 3.5.4.25 and riboflavin synthase, EC 2.5. f.9.

BacherA, Eberhardt S, Eisenreich W, Fischer M, Herz S, Illarionov B, Kis K, and Richter G (2001) Biosynthesis of riboflavin. Vitamins and Hormones 61, 1 9. 

Figure 9 Biosynthesis of riboflavine (2) by catalysis ofRiSy Two molecules oflumazine (1) form one molecule of riboflavine and one molecule of 5-amino-6-ribitylamino-2,4-(lH, 3H)-pyrimidinedione (3) by disproportionation...

In terms of coenzyme evolution, it is also noteworthy that the biosynthesis of a given coenzyme frequently requires the cooperation of other coenzymes. For example, the biosynthesis of riboflavin (24) requires tetrahydrofolate (33) for the biosynthesis of GTP serving as precursor (Fig. 3). Pyridoxal 5 -phosphate is required for the biosynthesis of the activated pyrosulfide type protein (2) that serves as the common precursor for iron/sulfur clusters and various sulfur-containing organic coenzymes (Fig. 1). 

Figure 3 Biosynthesis of riboflavin (7). Biosynthetically equivalent atoms are indicated by colored dots. For details, see text.

Walpole CSJ, Wrigglesworth R (1987) Oxido-reductases - Flavoenzymes. In Page MI, Williams A (eds) Enzyme Mechanisms. Royal Society of Chemistry, London, p 506 Bacher A, Eberhardt S, Richter G (1996) Biosynthesis of riboflavin. In Neidhardt FC, Curtis III R, Ingraham JL, Lin ECC, Low KB, Magasanik B, Reznikoff WS, RUey M, Schaechter M, Umbarger HE (eds) Escherichia coli and Salmonella typhimurium-. Cellular and Molecular Biology.Vol 1. ASM, Washington DC,p 657... 

Perkins JB, Pero J (1993) Biosynthesis of riboflavin, biotin, folic acid and cobalamin. In Sonenshein AL, Hoch JA, Losick R (eds) Bacillus subtilis and other gram positive bacteria. ASM, Washington DC, p 319... 

GTP cyclohydrolase II (reaction I) catalyzing the first committed step in the biosynthesis of riboflavin was discovered in the wake of work on GTP cyclohydrolase I (Figure 2), the enzyme catalyzing the first reaction step in the biosynthesis of tetrahydrofolate. ... 

Table 1 Specific activities of enzymes invoived in the biosynthesis of riboflavin of Escherichia coli...

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. 

Figure 4 Biosynthesis of riboflavin in bacteria. In the fungal route, the ribosyl side chain is first reduced and then the imidazole ring is deaminated.

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