5,6-dimethylbenzimidazole synthase (EC 1.14.99.40, BluB) is an enzyme with systematic name FMNH2 oxidoreductase (5,6-dimethylbenzimidazole forming).
[1][2][3] This enzyme catalyses the following chemical reaction The C-2 of 5,6-dimethylbenzimidazole is derived from C-1' of the ribityl group of FMNH2 and 2-H from the ribityl 1'-pro-S hydrogen.
This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B12) in bacteria.