Density functional theory was used to study the catalytic reaction mechanism of human Glyoxalase I. This zinc enzyme converts hemithioacetal to S-D-lactoyl glutathione which in turn is produced from methylglyoxal and glutathion reaction. Glyoxalase I accepts both enantiomers of hemithioacetal and converts them to only S-D-enantiomer of the product. On the basis of earliest reports of catalytic mechanism of Glyoxalase I, we have performed DFT calculation on a model of the enzyme active site, including models of S and Renantiomers of the substrates. Our study show that Glu172 and Glu99 starts the catalytic reaction by abstracting a proton from S and R substrates, respectively. Then, a proton from Glu172 transfers to the endiolate intermediate. As a result only S-D enantiomer of product is produced from both S and R substrates. Geometrical structures of the stationary points along the reaction path were optimized, and the energy profile of the reaction was drown from the stationary points energies. Also, we fixed a number of the active site atoms at their crystallographic positions at two different states for both S and R substrates. The results show that the position of the fixed atoms is important to propose an accurate mechanism for the reaction. The results show that the more rigid cluster model of the active site predict the accurate mechanisms which are in accordance with experimental observations.In conclusion, while reaction proceeds in the enzyme active site, its structure does not change too much during the reaction. on the other hand the substrate and the active site connection is due to Lock and Key hypothesis.
