crystal structures for the "e. coli" scs provide evidence that the coenzyme a binds within each α-subunit (within a rossmann fold) in close proximity to a histidine residue (his246α). this histidine residue becomes phosphorylated during the succinate forming step in the reaction mechanism. the exact binding location of succinate is not well-defined. the formation of the nucleoside triphosphate occurs in an atp grasp domain, which is located near the n-terminus of the each β subunit. however, this grasp domain is located about 35 å away from the phosphorylated histidine residue. this leads researchers to believe that the enzyme must undergo a major change in conformation to bring the histidine to the grasp domain and facilitate the formation of the nucleoside triphosphate. mutagenesis experiments have determined that two glutamate residues (one near the catalytic histidine, glu208α and one near the atp grasp domain, glu197β) play a role in the phosphorylation and dephosphorylation of the histidine, but the exact mechanism by which the enzyme changes conformation is not fully understood.