 The extra pyrroline ring is incorporated into the active site of several methyltransferases, where it is believed to rotate relatively freely.It is believed that the ring is involved in positioning and displaying the methyl group of methylamine for attack by a corrinoid cofactor.The proposed model is that a nearby carboxylic acid bearing residue, glutamate, becomes protonated, and the proton can then be transferred to the imine ring nitrogen, exposing the adjacent ring carbon to nucleophilic addition by methylamine.