dimethylbenzimidazole (dmb-on) according to the pH. When bound to the cobalamin-binding domain, the dimethylbenzimidazole ligand is replaced by the active histidine (His-on) of the DXHXXG motif . The replacement of dimethylbenzimidazole by histidine allows switching between the catalytic and activation cycles. [ 6 ] In methionine synthase the cobalamin cofactor is