The active site of PLE facilitates both substrate binding and hydrolysis. A key serine residue in the active site promotes hydrolysis, but the substrate must present an ester group to this residue after binding to the enzyme active site for hydrolysis to take place. Whether the substrate is able to present an ester group to the catalytic serine residue depends on its bound conformation in the active site, which is dictated by amino acid  side-chains in the active site. Thus, active site models of PLE have been advanced with the goal of predicting from the structure of the substrate which of two enantiotopic ester groups will be hydrolyzed (or whether hydrolysis is likely to occur at all).
A simple model for the binding conformation of an ester in the active site of PLE is shown below. This model accurately predicts the configuration of hydrolyzed glutarates and similar substrates.