Hydroxynitrile lyase from Arabidopsis thaliana: Identification of reaction parameters for enantiopure cyanohydrin synthesis by pure and immobilized catalyst

The (R)-selective hydroxynitrile lyase from Arabidopsis thaliana (AtHNL) is a promising biocatalyst for the synthesis of a broad range of chiral cyanohydrins. However, the enantiomeric excess of the reaction is strongly compromised by a non-catalyzed side reaction resulting in racemic cyanohydrins besides the chiral product obtained by enzymatic catalysis. This reaction is influenced by the pH value, the temperature and the water content of the reaction medium. In aqueous media this side reaction can be suppressed at low pH (4-5) and by lowering the temperature. However both approaches are not possible with AtHNL, since the enzyme is rapidly inactivated below pH 5.4, which prevents its use in aqueous media or two-phasic aqueous-organic reaction systems. Alternatively the side reaction can be suppressed by lowering the water concentration in the reaction system as far as possible. This approach was successfully tested for AtHNL using buffer-saturated methyl tert-butyl ether (MTBE) as a reaction medium for the hydrocyanation of aromatic, heteroaromatic and aliphatic aldehydes. Here we compare the activity and stability of AtHNL immobilized on celite (celite-AtHNL) and in solgel (solgel-AtHNL) relative to the precipitated enzyme, which was directly used in the organic solvent. Surprisingly, AtHNL was activated (up to 10-fold) upon solgel immobilization, an effect that was up to now only described for solgel-immobilized lipases. In contrast to lipases, AtHNL is not stabilized by the solgel. Best results were obtained with AtHNL adsorbed on celite, which is an easy and efficient way of immobilization and shows good recyclability (> 5 cycles), storability (tau(1/2) = 71 days, 4 degrees C) and excellent catalytic properties upon adjustment of a relative water content of 26% water per g celite-AtHNL as determined by thermogravimetry