Active site residues and interaction with SAM. Shown are the active site residues of Ec MAT (light blue; with SAM, PDB entry 1RG9, orange) and the corresponding residues from Tk MAT (blue). Most residues stabilising the adenosine moiety are conserved in both structures, whereas the residues interacting with the methionine carboxyl group and the adenine amino group differ. Asp144 and Leu145 (Tk MAT) are located on the flexible loop and are therefore not in contact with the substrate in the open position. Amino acids marked with * derive from the second monomer.