Influence of Natural Solutes and Ionic Liquids on the Yield of Enzyme-Catalyzed Reactions: Measurements and Predictions

The maximum yield of enzyme-catalyzed reactions is often limited by thermodynamic equilibrium. The knowledge of influencing factors on limitations of reactions is essential for process optimization to increase yields and to reduce solvent and energy consumption. In this work the effect of solvents/cosolvents [e.g., ionic liquid (IL)] and natural solutes on thermodynamic yield limitations of two enzyme-catalyzed model reactions were investigated, namely, an alcohol dehydrogenase (ADH) reaction (acetophenone + 2-propanol ⇌ 1-phenylethanol + acetone) and an alanine aminotransferase reaction (l-alanine + 2-oxoglutarate ⇌ pyruvate + l-glutamate). Experimental results showed that the equilibrium position and the equilibrium product yield of both reactions in aqueous single-phase systems strongly depend on the type and molality of the present natural solute/IL that were present as additives in the reaction mixture. In addition, the ADH reaction was investigated in pure IL and in an IL/buffer two-phase system. Compared to the aqueous reaction mixtures, the reactant solubility could be increased significantly, but at the cost of a lower product yield. Finally, thermodynamic modeling by means of ePC-SAFT was used to predict the equilibrium product yield of both reactions at different reaction conditions (natural solute/IL type and molality) in the aqueous mixtures as well as in the IL. Experimental and predicted results were in good agreement, showing that ePC-SAFT is a promising tool for predicting yield limitations in different reaction media