Combined SCC-DFTB/CHARMM calculations were carried out to analyze the origin for the functional specificities of triosephosphate isomerase (TIM) and methylglyoxal synthase (MGS). The two enzymes bind to the same substrate, dihydroxyacetone phosphate (DHAP), and have rather similar active sites. However, they catalyze different reactions; TIM catalyzes the isomerization of DHAP to glyceraldehyde 3-phosphate (GAP), while MGS catalyzes the elimination of phosphate from DHAP. Similar to previous suggestions, the calculations confirmed that GAP formation is prohibited in MGS due primarily to the reduced flexibility of the catalytic base (Asp 71) compared to that in TIM (Glu 165). For the suppression of phosphate elimination in TIM, the calculations show that the widely accepted stereoelectronic argument that invokes the different phosphoryl torsion angles observed in the X-ray structures of inhibitor complexes of the two enzymes is not as important as electrostatic contributions from the protein and water molecules surrounding the phosphoryl.