Enzymes are extraordinary catalysts satisfying the needs of living organisms. Catalytic efficiency of enzymes and their selectivity is also appealing for utilizing them in technological applications [1]. Perhaps surprisingly, a wide range of enzymes do not denature and moreover retain catalytic activity in organic solvents. This opens a path for carrying out new unnatural reactions. Furthermore, the change of reaction medium significantly alters enzyme activity, chemo- and regio- and enantioselectivity. In spite of achirality of the solvent its effects on enantioselectivity are complex; different types of enzymes exhibit qualitatively different behaviour [2,3] and simple rules for rationalization are still missing.
Our aim is to systematically investigate enzyme activity of Candida antarctica lipase B – mainly its intriguing non-monotonic dependence of enantioselectivity on composition of reaction medium. Specifically, we study a transesterification reaction catalysed by this enzyme in acetonitrile, toluene and their mixtures. First, we examine the detailed nature of the rate-limiting step using QM/MM calculations including a test of QM region size. Next, we sample the conformational space of the most important reaction intermediates for both enantiomers by classical molecular dynamics simulations and characterize the productive binding modes. We also inspected the behaviour of the helix α5 located in the vicinity of the active site which could act as a lid and therefore influence the reactivity. The gained molecular level insight will help to understand the puzzling dependence of enantioselectivity on solvent properties.