Effects of ionic liquids on the hydrolysis of casein by lumbrokinase

In this work, the hydrolysis of casein by lumbrokinase in the presence of ionic liquids (ILs) was studied. The experimental results indicated that the effects of ILs on the casein hydrolysis depended on their concentrations; at low IL concentration, the addition of ILs could remarkably increase the lumbrokinase activity due to enhancing the hydrophilicity of casein by forming the IL-casein complex. However, at higher IL concentration, significant decrease in lumbrokinase activity was observed because the presence of ILs disturbed the hydrogen bond network in the active site of lumbrokinase. At the IL concentrations beyond their critical micelle concentrations, the incorporation of substrate into the micellar pseudo-phase also contributed to the decrease in lumbrokinase activity. When a more hydrophilic substrate was adopted, the effects of ILs on the lumbrokinase activity were negative in all cases. Fluorescence analysis indicated that hydrogen bonding was the main driving force ruling the interaction of ILs and lumbrokinase.