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Riboflavin (vitamin B2) serves as a precursor of flavocoenzymes, which have essential roles as redox cofactors in all organisms. The final step in the biosynthesis of the vitamin is catalysed by the enzyme riboflavin synthase. This unusual reaction involves the dismutation of 6,7-dimethyl-8-ribityllumazine. A 4-carbon unit is transferred between two of the identical substrates to form riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione involving the cleavage of two C-N bonds and the formation of two C-C bonds by the transfer of a 4-carbon moiety. The three active sites of the trimer lie between pairs of monomers, although only one active site can be formed and catalytically competent at any one time. The homotrimer is non-existent in humans and is an attractive target for antimicrobial agents.