Biosynthesis of riboflavin. The reaction catalyzed by 6,7-dimethyl-8-ribityllumazine synthase can proceed without enzymatic catalysis under physiological conditions

6,7-Dimethyl-8-ribityllumazine is the biosynthetic precursor of the vitamin , riboflavin. The biosynthetic formation of the lumazine by condensation of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione and 3,4-dihydroxy-2-buta none 4-phosphate is catalyzed by the enzyme, lumazine synthase. We show tha t the condensation reaction can proceed without enzyme catalysis in dilute aqueous solution at room temperature and neutral pH. The reaction rate is p roportional to e PH The activation energy of the uncatalyzed reaction is E- a = 46.3 kJ mol(-1). The regioselectivity of the uncatalyzed reaction incre ases with pH and temperature (70% at 65 degreesC and pH 7.75). The data sug gest partitioning of the uncatalyzed reaction via two different reaction pa thways. The value of k(cat)/k(uncat) may be indicative for an entropy drive n process for the enzyme-catalyzed reaction.