studied the mechanism of sialic acid hydrolysis catalyzed by influenza A N9 neuraminidase.
This enzyme is what is called a retaining glycosidase because if the starting glycoside has the α-configuration (as shown) then the product that is formed will also have the α-configuration.
In common with many glycosidase enzymes, its active site features a pair of carboxyl residues (Asp 151 and Glu 277 in the N9 neuraminidase they studied) which play central roles in the enzyme's catalytic mechanism. The proposed mechanism is shown below.