text: Enzymes, which are composed of chiral amino acids, catalyze chemical reactions with high stereoselectivity. Specifically, esterase enzymes catalyze the hydrolysis of esters to carboxylic acids. This transformation may be rendered asymmetric if two enantiotopic ester groups exist in the substrate or if a racemic mixture of chiral esters is used. In the former case (desymmetrization), the chiral environment of the enzyme active site leads to selective hydrolysis of the ester that is closer to the catalytically active serine residue when the substrate is bound to the enzyme. In the latter case (kinetic resolution), one of the enantiomers is hydrolyzed faster than the other, leading to an excess of hydrolyzed product from one enantiomer. Both strategies rely on the fact that the transition states for hydrolysis of enantiotopic or enantiomorphic ester groups by the chiral enzyme are diastereomeric.
  Pig liver esterase (PLE) is a widely used enzyme for asymmetric ester hydrolysis. Although it was originally used for the desymmetrizing hydrolysis of glutarate esters, PLE also hydrolyzes malonates, cyclic diesters, monoesters, and other substrates. Active site models have been advanced to explain the selectivity of PLE.