36] Biosynthesis of riboflavin: Lumazine synthase and riboflavin synthase

Abstract

This chapter discusses the biosynthesis of riboflavin. 6,7-Dimethyl-8-ribityllumazine is developed as the direct biosynthetic precursor of riboflavin. The lumazine derivative is converted to riboflavin by the enzyme riboflavin synthase. The reaction involves the transfer of a four-carbon unit between two identical substrate molecules in an unusual dismutation reaction. The lumazine is biosynthesized from 5-amino-6-ribitylamino-2,4 (1 H ,3 H )-pyrimidinedione and 3,4-dihydroxy-2-butanone 4-phosphate by the enzyme lumazine synthase. The riboflavin synthases of Bacillus subtilis and Escherichia coli are homotrimeric molecules. Members of the Bacillaceae also form a large multimeric enzyme complex with lumazine synthase and riboflavin synthase activity. This complex accounts for 12–40% of the total riboflavin synthase activity in different members of the Bacillaceae. This enzyme complex has been designated “heavy riboflavin synthase.” The protein consists of 60 β subunits (lumazine synthase) that form an icosahedral capsid containing 3 α subunits (riboflavin synthase). The structure of this complex is presented in the chapter.