Enzymes are remarkably efficient catalysts and their recent use in non-aqueous organic solvents is opening a tremendous range of applications in synthetic chemistry: since, surprisingly, most enzymes do not denature in these non-natural environments, new reactions involving e.g. water-insoluble reagents can be catalyzed, while unwanted degradation side reactions are suppressed.However, a key challenge for these applications is to overcome the greatly reduced catalytic activity compared to aqueous conditions. Empirically, adding activators such as salts or small amounts of water dramatically enhances the activity, but the underlying mechanisms have remained elusive, thus preventing a rational optimization.Through analytic modeling and numerical simulations, our project will provide the first atomic-scale detailed description of enzyme catalysis in organic solvents, including the key role of the environment. We will then use this unprecedented molecular insight to design rigorous new procedures for the rational engineering of systems with dramatically enhanced activities, both through optimized choices of solvents and additives, and through targeted protein mutations.Specifically, we will first rigorously establish the influence of enzyme flexibility on catalytic activity through an original model accounting for the dynamic disorder arising from conformation fluctuations. Second, we will provide the first molecular explanation of the commonly invoked “lubricating” action of added water. Third, the underlying mechanism of the much employed salt-induced activation will be determined, probably calling for a radical change from the currently used picture of a water-mediated action.Far-reaching practical impacts are expected for the numerous industrial syntheses already employing biocatalysis in non-aqueous media.