Chemical modifications by ionic liquid-inspired cations improve the activity and the stability of formate dehydrogenase in [MMIm][Me2PO4]

The formate dehydrogenase (FDH, EC: 1.2. 1.2) from Candida boidinii was found to be inactivated and unstable in the presence of high concentration (>50%) of the water soluble dimethylimidazolium dimethyl phosphate ([MMIm][Me2PO4]) ionic liquid. In order to circumvent this problem, the enzyme was chemically modified by cations usually present in ionic liquids: cholinium (1), hydroxyethyl-methylimidazolium (2) and hydroxypropyl-methylimidazolium (3) cations were activated with carbonyldiimidazole before being reacted with the FDH leading to a heterogeneous population of 6–7 biocatalysts. FDH modified by (1) or (3) led to 3–9 modifications while FDH modified by (2) led to 6 proteins presenting 7–12 grafted cations. Specific activity of the modified enzymes was decreased by a 2.5–3-fold factor (0.10–0.15 μmol min−1 mg−1) compared to the non-modified FDH (0.33 μmol min−1 mg−1) when assayed in carbonate buffer (pH 9.7, 25 mM). After modification, the FDH still present 0.06 μmol min−1 mg−1 in 70% [MMIm][Me2PO4] (v:v) (30–45% of their activity in aqueous buffer) while the native enzyme is inactive at this ionic liquid concentration, proving the efficiency of this strategy. The half-life of the modified enzyme is also increased by a 5-fold factor after modification by (1) (t1/2 of 9 days) and by a 3-fold factor after modification by (2) or (3) (t1/2 of 6 and 5 days respectively) in aqueous solution. When stored in 37.5% [MMIm][Me2PO4] (v:v), both modified and unmodified FDH have an increased half-life (t1/2 of 6–9 days). This grafting strategy is found to be good methods to mimic and study the stabilizing effect of ionic liquids on enzymes.