A semiempirical study of the reaction of the hemimercaptal of methylglyoxal and glutathione at the active center of glyoxalase I

An AM 1 and MNDO –PM 3 semiempirical study of the reaction mechanism of glyoxalase I is presented. According to some authors, this enzyme serves as a detoxifying agent, transforming ketoaldehydes to hydroxiacids. According to others, however, the enzyme serves as a growth-control agent, destroying 2-oxopropanal (MG), which might be involved in some, mostly unknown, biological pathways. Following a previous study on the conformation of the hemimercaptal of MG and glutathione (HSG) done using molecular mechanics, we study here the possible reaction path of this substrate at a model reaction site in the enzyme. First, the active center of the enzyme was modeled using a Zn²⁺ cation with several ligands. Second, its complex with the hemimercaptal of MG and HSG (SAG) was calculated using semiempirical methods. The reaction proceeds from the substrate to the enediol intermediate in two steps: In the first one, a proton from the activated carbon in SAG is captured by a base–which we model in this work by HO^?, CH3COO^?, and imidazole anion. Our calculations show that the likeliness of the reaction is enhanced if imidazole is used as the base instead of an acid conjugate base or a free hydroxide anion. Moreover, we found a stable intermediate carbanion–described for the first time as an intermediate in this mechanism–whose existence is supported by some of the available experimental evidence. In the second step of the mechanism, a proton is transferred from a Zn-bound water to the intermediate carbanion, giving the enediol species, originally postulated as the intermediate in this mechanism, and leaving a hydroxide anion bound to the Zn²⁺ complex. Comparing this reaction with the same process when no metal complex is present–as if the reaction occurred in aqueous solution–it is shown that the metal ion activates water, facilitating the proton transfer. Our calculations point out the existence of two potential energy barriers to be overcome in the process: the need of the metal center to be coordinated to water as a prerequisite for the second proton transfer to take place, and the fine tuning caused in the first proton transfer by the very nature of the base that initially captures the proton. No significant differences in the qualitative characteristics of the reaction were found in using AM 1 or PM 3 as the method of calculation. © 1992 John Wiley & Sons, Inc.