Water activity (a w) is a crucial parameter affecting enzymatic synthetic reactions in organic media. In this paper, we report on the a w 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 a w of the salt solution, to give the best performance at a w 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 a w profiles, an investigation was carried out similarly using other lipase sources and yeast strains, which indicated that, in all the cells examined, a higher a w resulted in a higher reaction rate. Moreover, increasing a w 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 a w dependence probably because of the variation in cell surface characteristics.
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