Biosynthesis of vitamin B-2 (riboflavin)

The biosynthesis of one riboflavin molecule requires one molecule of GTP an d two molecules of ribulose 5-phosphate as substrates. The imidazole ring o f GTP is hydrolytically opened, yielding a 4,5-diaminopyrimidine which is c onverted to 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione by a sequence of deamination, side chain reduction and dephosphorylation. Condensation o f 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione with 3,4-dihydroxy-2-bu tanone 4-phosphate obtained from ribulose 5-phosphate affords 6,7-dimethyl- 8-ribityllumazine. Dismutation of the lumazine derivative yields riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione, which is recycled i n the biosynthetic pathway. The structure of the biosynthetic enzyme, 6,7-d imethyl-8-ribityllumazine synthase, has been studied in considerable detail .