A Water-Assisted Catalytic Mechanism in Glycoside Hydrolases Demonstrated on the Staphylococcus aureus Autolysin E

Autolysin E (AtlE), from Staphylococcus aureus, is a cell-wall-degrading enzyme that is a potential drug target. It is a member of the glycoside hydrolase (GH) class, enzymes that commonly have either two catalytic residues and hydrolyze their substrates by inverting or retaining mechanisms or one catalytic residue and undergo retaining, substrate-assisted catalysis. Here, we address the catalytic mechanism of AtlE. Site-directed mutagenesis studies identified Glu138 as the only catalytic residue. Quantum mechanics/molecular mechanics (QM/MM) simulations of the possible reaction pathways suggest that hydrolysis proceeds via a retaining, water-assisted mechanism and an oxocarbenium ion like transition state. These results, on the basis of data from a member of the hydrolase GH73 family, support the hypothesis of the presence of an alternative catalytic mechanism in glycoside hydrolases, which can be considered in the design of future AtlE inhibitors.