Mechanistic Pathway on Human α-Glucosidase Maltase-Glucoamylase Unveiled by QM/MM Calculations

The excessive consumption of starch in human diets is associated with highly prevalent chronic metabolic diseases such as type 2 diabetes and obesity. α-Glucosidase enzymes contribute to the digestion of starch into glucose and are thus attractive therapeutic targets for diabetes. Given that the active sites of the various families of α-glucosidases have different sizes and structural features, atomistic descriptions of the catalytic mechanisms of these enzymes can support the development of potent and selective new inhibitors. Maltase-glucoamylase (MGAM), in particular, has a N-terminal catalytic domain (NtMGAM) that has shown high inhibitor selectivity. We provide here the first theoretical study of the human NtMGAM catalytic domain, employing a hybrid QM/MM approach with the ONIOM method to disclose the full atomistic details of the reactions promoted by this domain. We observed that the catalytic activity follows the classical Koshland double-displacement mechanistic pathway that uses general acid and base catalysts. A covalent glycosyl-enzyme intermediate was formed and hydrolyzed in the first and second mechanistic steps, respectively, through oxocarbenium ion-like transition state structures. The overall reaction is of dissociative type. Both transition state geometries differ from those known to occur in other glycosidases. The activation free energy for the glycosylation rate-limiting step agrees with the experimental barrier of 15.8 kcal·mol^-1. Both individual mechanistic steps of the reaction are exoergonic. These structural results may serve as the basis for the design of transition state analogue inhibitors that specifically target the intestinal NtMGAM catalytic domain, thus delaying the production of glucose in diabetic and obese patients.