Synthesis and biochemical evaluation of bis(6,7-dimethyl-8-D-ribityllumazines) as potential bisubstrate analogue inhibitors of riboflavin synthase

The reaction catalyzed by riboflavin synthase utilizes two identical 6,7-dimethyl-8-n-ribityllumazine substrate molecules. Three bis(6,7-dimethyl-8-D-ribityllumazines) were, therefore, synthesized in which the two lumazine moieties were connected through their N-3 nitrogen atoms by polymethylene linker chains containing three, four, and five carbon atoms. The compounds with three and five carbon linkers were found to be very weak inhibitors of riboflavin synthase, having inhibition constants of 320 and >1000 mu M, respectively. In contrast, the bis(lumazine) with a four-carbon linker was much more potent, with an inhibition constant of 37 mu M. These results have potential implications for understanding the distance between the donor and acceptor sites of riboflavin synthase and the orientations of the two 6,7-dimethyl-8-D-ribityllumazine substrate molecules which occupy these two sites.