Density functional theory were used to study the stereospecific proton exchange of Glutathiohydroxyacetone by Glyoxalase І. We used the quantum mechanical cluster method to model the enzyme active site. The results show that abstraction of Hs by Glu-172 is more feasible than abstraction of Hr by Glu-99 and the product of the Hr-transfer is higher in energy than the product of Hs-transfer. We also tested the effect of flexibility of the glutamate residues on the stereospecific reprotonation of HOC-SG. According to the results, neither releasing Glu-172 nor realizing Glu-99 has no significant effect on energy barriers. However, realizing the glutamates makes the reaction energy of each proton transfer close to 12.0 kcal/mol.
