Improvement of stability and reaction behavior of enzymes in organic solutions by immobilization onto nano-porous supports

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In this study, we intended to establish theoretical bases, which are required to realize enzymatic syntheses in organic solvents. We studied the synthetic reaction of aspartame precursor, Z-L-Asp-L-PheOMe, from respective amino acid derivatives (Z-Asp and PheOMe) catalyzed by thermolysin. First, we investigated various factors affecting stability of the immobilized thermolysin and analyzed its mechanism. Autolysis was found to be a main cause for inactivation in water-immiscible organic solvents and the immobilized enzyme tended to be inactivated in organic solvents with lower solubility of water. In particular, the immobilized was stable in tert-amyl alcohol probably because that aqueous phase around the enzyme immobilized is thin and as a result autolysis tended to be repressed. The method to predict the overall partition coefficient of the substrates containing acid and amine components was proposed, by taking into consideration the effect of ion-pair complexes formed between the substrates.The synthetic rate with the immobilized enzyme in an organic solvent, containing a small amount of water could be in principle estimated on the basis of the model for the biphasic reaction when the amount of enzyme loaded was not so much. Namely, in the synthetic reactions, the both components of the substrate are partitioned into the water phase inside the support, and then converted to the product by the action of the enzyme, and finally the product is transferred into the bulk organic phase.