The role of redox balances in the anaerobic fermentation of xylose by yeasts

TLDR: A hypothesis is put forward which explains that ethanol production from xylose by yeast under anaerobic conditions is negligible, and it is suggested that the failure to fermentxylose anaerobically is due to a discrepancy between production and consumption of NADH in the overall conversion of xylOSE to ethanol.

Abstract: The kinetics of glucose and xylose utilization by batch cultures of Candida utilis were studied under aerobic and anaerobic conditions during growth in complex media. Rapid ethanol formation occurred during growth on glucose when aerobic cultures were shifted to anaerobic conditions. However, with xylose as a substrate, transfer to anaerobiosis resulted in an immediate cessation of metabolic activity, as evidenced by the absence of both ethanol formation and xylose utilization. The inability of the yeast to ferment xylose anaerobically was not due to the absence of key enzymes of the fermentation pathway, since the addition of glucose to such cultures resulted in the immediate conversion of glucose to ethanol. Furthermore, when the enzyme xylose isomerase was added to an anaerobic xylose culture, immediate conversion of xylose to ethanol was observed. This indicates that the inability of the yeast to form ethanol from xylose under anaerobic conditions is caused by metabolic events associated with the conversion of xylose to xylulose. A hypothesis is put forward which explains that ethanol production from xylose by yeast under anaerobic conditions is negligible. It is suggested that the failure to ferment xylose anaerobically is due to a discrepancy between production and consumption of NADH in the overall conversion of xylose to ethanol. When a hydrogen acceptor (i.e. acetoin) was added to anaerobic cultures of C. utilis, xylose utilization resumed, and ethanol and acetate were produced with the concomitant stoicheiometric reduction of acetoin to 2,3-butanediol.