Influence of lid conformation on lipase enantioselectivity

The enantioselectivity of porcine pancreatic lipase (PPL) in the hydrolysis reaction of racemic glycidyl butyrate has been observed to increase substantially upon interfacial activation of the enzyme. The enantioselectivity of Candida antarctica lipase B ( CalB), a lipase that does not display interfacial activation, does not change when the substrate concentration exceeds the solubility limit. A hypothesis, based on a kinetic model, is presented that relates the change of enantioselectivity to the conformational changes that accompany movements of the lid upon interfacial activation. The hypothesis was investigated using various forms of the C. rugosa lipase ( Crl). For several substrates, the enantioselectivities of hydrolysis reactions catalyzed by crude, purified, and crystalline (CLEC®) preparations of Crl in open and closed conformations were measured. As anticipated, the enantioselectivity of open-lid Crl-CLECs in the hydrolysis of racemic ibuprofen methyl ester exceeded that of the closed-lid form. For other esters, however, correlations were less straightforward. It was concluded that apart from affecting the activation barrier leading to the Michaelis–Menten complex, modifications of the lid (open, closed, or modified lid) also induce additional conformational changes in the active site affecting enantioselectivity.