Steering the enzymatic activity of proteins by ionic liquids. A case study of the enzyme kinetics of yeast alcohol dehydrogenase

We explore ion-specific effects exerted by ionic liquids (ILs) on the enzyme kinetics of yeast alcohol dehydrogenase. The Michaelis–Menten reaction scheme is used to parameterize the observed kinetics in terms of the apparent dissociation constant of the substrate (Michaelis–Menten constant) KM, the turnover number kcat, which reflects the number of product molecules per enzyme molecule per second, and the enzymatic efficiency kcat/KM of the reaction. Results for fifteen salts are used to deduce Hofmeister anion and cation series. The ion rankings derived from KM, kcat and kcat/KM differ markedly. Only the results for the enzymatic efficiency correspond to expectations from other phenomena, such as the thermal stability of native proteins. Anion variation has a significantly larger effect on the enzymatic efficiency than cation variation. All ILs decrease kcat relative to its value for the IL-free solution, thus driving enzyme deactivation. Enhancements of the enzymatic efficiency by some ions are founded in their effects on the Michaelis–Menten constant. The observed Hofmeister anion and cation series point toward hydrophobic interactions as an important factor controlling ion-specific effects on the enzymatic activity.