Water activity dependence of performance of surface-displayed lipase in yeast cells: A unique water requirement for enzymatic synthetic reaction in organic media

Water activity (aw) is a crucial parameter affecting enzymatic synthetic reactions in organic media. In this paper, we report on the aw dependence of surface-displayed lipases, genetically immobilized on yeast cells via fusion with cell wall proteins. When Saccharomyces cerevisiae displaying Rhizopus oryzae lipase was used for esterification in n-hexane, equilibrating the dried cells with water prior to the reaction markedly increased the reaction rate. An equilibration of the cells with various saturated salt solutions showed that the reaction rate increased with increasing aw of the salt solution, to give the best performance at aw of 1.0. Interestingly, this trend was extremely different from those of lipases in powder or resin-immobilized form. To determine whether the cell surface is responsible for the unique aw profiles, an investigation was carried out similarly using other lipase sources and yeast strains, which indicated that, in all the cells examined, a higher aw resulted in a higher reaction rate. Moreover, increasing aw was found to increase the cell surface hydrophobicity determined by an aqueous-hydrocarbon biphasic partitioning assay. These results indicate that lipases displayed on yeast cells show a unique aw dependence probably because of the variation in cell surface characteristics.