Hydrophobic Surface Induced Activation of Pseudomonas cepacia Lipase Immobilized into Mesoporous Silica

Abstract

Lipase from Pseudomonas cepacia (PCL) was successfully immobilized into siliceous mesocellular foams (MCFs) with various hydrophobic/hydrophilic surfaces. The catalytic performances of immobilized PCL were investigated using the transesterification reaction and hydrolytic reaction as model reactions. The specific activity of immobilized PCL greatly increased with enhanced surface hydrophobicity of MCFs, mainly because of lipase activation via hydrophobic interaction between alkyl groups in MCFs and the surface loop (so-called "lid") of PCL. Conformational changes of immobilized PCL were further investigated using time-resolved fluorescence spectroscopy with Trp as an intrinsic probe. When the immobilized PCL was suspended in phosphate buffer, short-lived τ(1) shortened and the fractional contribution of τ(1) significantly increased with the increasing level of surface hydrophobicity of MCFs. These results revealed that Trp(s) of the immobilized PCL were surrounded by a hydrophilic microenvironment because of the fact that the opened "lid" permitted the diffusion of water to the active site cleft. However, for the immobilized PCL suspended in n-hexane, long-lived τ(3) increased with the increase of surface hydrophobicity of MCFs. The reduced interaction between Trp(s) and the surrounding protein matrix was due to intercalation of n-hexane into the active site cleft when the lipase was in open conformation. The above results demonstrated that PCL immobilized into MCF with hydrophobic surfaces were in an activated open conformation.