Molecular Mechanism behind Solvent Concentration-Dependent Optimal Activity of Thermomyces lanuginosus Lipase in a Biocompatible Ionic Liquid: Interfacial Activation through Arginine Switch

Thermomyces lanuginosus lipase (TLL) is an industrially significant catalyst for the production of biodiesel due to its operability over a wide range of pH's and temperatures. Molecular dynamics simulations of TLL in aqueous solutions of a biocompatible ionic liquid (IL), cholinium glycinate (ChGly), have been carried out to investigate the microscopic reasons for the experimentally observed enhancement in the activity of TLL upon addition of a room temperature IL (RTIL), especially at an optimal concentration. Eight different TLL systems, in both their open and closed forms, at various concentrations of the RTIL in water have been studied. A special orientation of the lid residue, W89, in the closed form, which enables an optimal substrate-binding rate, has been identified, which can be probed via fluorescence spectroscopy. The flipping and consequent exposure of W89 in the open form of TLL induce a change in the lid helicity and orientation in such a way that residue R84 from the front lid hinge gets trapped around a particular region in all systems except at a 0.5 M IL concentration. At that concentration, R84 exhibits considerable fluxionality and moves back and forth via a water channel that is formed because of the chaotropic nature of the cholinium cation. Arginine switch is well established to be the primary signature of interfacial activation of TLL, which is observed here at an optimal IL concentration (0.5 M) without the use of a substrate or surfactant. The present work can pave the way for development of a broader platform for understanding lipases and their application in environment-friendly catalysis.