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.