Fluorine-19 NMR studies on the mechanism of riboflavin synthase. Synthesis of 6-(trifluoromethyl)-7-oxo-8-(D-ribityl)lumazine and 6-(trifluoromethyl)-7-methyl-8-(D-ribityl)lumazine
作者:Mark Cushman、Hemantkumar H. Patel、Johannes Scheuring、Adelbert Bacher
DOI:10.1021/jo00047a015
日期:1992.10
The reactions of hexafluoropropene oxide (19), methyl trifluoropyruvate (21), and 1,1,1-trifluorobutane-2,3-dione (45) with a series of ortho diamines were investigated as an approach to the synthesis of trifluoromethyl-substituted quinoxalinones and lumazines. 6-(Trifluoromethyl)-7-oxo-8-(D-ribityl)lumazine (11) was synthesized by reaction of methyl trifluoropyruvate (21) with 5-amino-6-(D-ribitylamino)-2,4(1H,3H)-pyrimidinedione (3) hydrochloride and utilized as a F-19 NMR probe of the light riboflavin synthase of Bacillus subtilis. The fluorolumazine 11 was found to be an inhibitor of the enzyme with an inhibition constant K(I) = 55 muM. Equilibrium dialysis experiments indicated the binding of six molecules of 11 per enzyme molecule, corresponding to one molecule bound at each of the three donor and three acceptor sites of the enzyme. The apparent dissociation constants K(D) were approximately 4 and 112 muM. The enzyme-bound ligand gave rise to several broad F-19 NMR signals which were shifted to low field. The bound ligand 11 could be displaced from the enzyme by the enzyme product, riboflavin (2), and the product analog, 5-nitroso-6-(ribitylamino)-2,4(1H,3H)-pyrimidinedione (56). 6-(Trifluoromethyl)-7-methyl-8-(D-ribityl)lumazine (13) was synthesized by reaction of the hydrochloride salt of 3 with 1,1,1-trifluorobutane-2,3-dione (45). Three molecules of 13 can be bound relatively tightly per mole of riboflavin synthase, i.e., one ligand molecule per protein subunit. The inhibition constant K(I) was determined to be 75 muM, while dissociation constants of 17 and 70 muM were determined by equilibrium dialysis and F-19 NMR, respectively. The bound ligand 13 could also be displaced by riboflavin and product analog 56. A scheme for the catalytic cycle of riboflavin synthase is proposed.