Catalytic mechanism of glyoxalase I: a theoretical study

Hybrid density functional theory is used to study the catalytic mechanism of human glyoxalase I (GlxI). This zinc enzyme catalyzes the conversion of the hemithioacetal of toxic methylglyoxal and glutathione to nontoxic (S)-D-lactoylglutathione. GlxI can process both diastereomeric forms of the substrate, yielding the same form of the product. As a starting point for the calculations, we use a recent crystal structure of the enzyme in complex with a transition-state analogue, where it was found that the inhibitor is bound directly to the zinc by its hydroxycarbamoyl functions. It is shown that the Zn ligand Glu172 can abstract the substrate C1 proton from the S enantiomer of the substrate, without being displaced from the Zn ion. The calculated activation barrier is in excellent agreement with experimental rates. Analogously, the Zn ligand Glu99 can abstract the proton from the R form of the substrate. To account for the stereochemical findings, it is argued that the S and R reactions cannot be fully symmetric. A detailed mechanistic scheme is proposed.