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-butanone 4-phosphate is catalyzed by the enzyme, lumazine synthase. We show that the condensation reaction can proceed without enzyme catalysis in dilute aqueous solution at room temperature and neutral pH. The reaction rate is proportional to e pH. The activation energy of the uncatalyzed reaction is E a = 46.3 kJ mol-1. The regioselectivity of the uncatalyzed reaction increases with pH and temperature (70% at 65 °C and pH 7.75). The data suggest partitioning of the uncatalyzed reaction via two different reaction pathways. The value of k cat /k uncat may be indicative for an entropy driven process for the enzyme-catalyzed reaction.