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) K-M, the turnover number k(cat), which reflects the number of product molecules per enzyme molecule per second, and the enzymatic efficiency k(cat)/K-M of the reaction. Results for fifteen salts are used to deduce Hofmeister anion and cation series. The ion rankings derived from K-M, k(cat) and k(cat)/K-M 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 k(cat) 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.