Catalytic mechanism of glyoxalase I: A theoretical study

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