To study the effect of acetic acid on glucose/xylose co-fermentation by S. cerevisiae 424A(LNH-ST), a genetically engineered yeast strain that can effectively co-ferment both glucose and xylose to ethanol, we first determined the impact of the acetic acid on various yeast performance characteristics. Results showed acetic acid to be inhibitory to cell growth, substrate consumption (especially xylose), and ethanol productivity, and stimulatory to the metabolic ethanol yield.
To further explore and understand these effects of acetic acid, we took a systems biology approach by analyzing intracellular metabolite levels and gene expression levels. Reverse-phase liquid chromatography-mass spectrometry and in vitro 13C labeling was used for the identification and quantification of key intracellular glycolytic and pentose phosphate pathway metabolites. Initial results show significant differences in the concentration of the selected intracellular metabolites between fermentations with and without acetic acid. Microarray technology was used to determine the expression levels of the full yeast genome (with the exception of the genes inserted to allow for xylose fermentation). Preliminary analysis shows minimal differences in the expression of central carbon metabolism genes during glucose fermentation; however, significant differences were seen during xylose fermentation. Relationships between metabolomic, transcriptomic, and fermentation performance will be presented.