Tuesday, April 20, 2010
11-15

The switch from glucose to xylose stalled by repression of xylose-utilizing enzymes during exposure of Scheffersomyces (Pichia) stipitis to high ethanol concentrations

Patricia J. Slininger, Jaewoong A. Moon, Stephanie R. Thompson, Scott A. Weber, and Z. Lewis Liu. National Center for Agricultural Utilization Research, USDA-ARS, Bioenergy Research Unit, 1815 N University St, Peoria, IL 61604

During the fermentation of lignocellulosic hydrolyzates to ethanol by Scheffersomyces (Pichia) stipitis NRRL Y-7124 (CBS 5773), the switch from glucose to xylose uptake results in a diauxic lag unless process strategies to prevent this are applied.  When cells were grown on glucose, the length of this lag was observed to be a function of the glucose concentration consumed (and consequently, the ethanol concentration accumulated) prior to the switch from glucose to xylose fermentation.  At glucose concentrations of 95 g/L, the switch to xylose utilization was severely stalled such that efficient xylose fermentation did not occur.  The objective of this research was to investigate the impact of ethanol on cellular xylose transport and the induction and maintenance of xylose reductase and xylitol dehydrogenase activities when large cell populations were pre-grown on glucose or xylose and then presented mixtures of glucose and xylose for fermentation.  Ethanol concentrations circa 50 g/L repressed enzyme induction although xylose transport into the cells was observed to be occurring.  Recycled cell populations grown on xylose resulted in faster fermentation rates, particularly on xylose, and eliminated diauxic lag and stalling during mixed sugar conversion by S. stipitis, despite ethanol accumulations in the 60-70 g/L range.  The process strategy of priming cells on xylose was key to the successful utilization of high mixed sugar concentrations because specific enzymes for xylose utilization could be induced before ethanol concentration accumulated to an inhibitory level.

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