When lignocellulosic biomass is pretreated, a number of undesired degradation products are generated which may inhibit microbial metabolism. The furaldehydes hydroxymethylfurfural (HMF) and furfural are two major inhibitors in lignocellulosic hydrolyzate that have shown to impede growth and limit ethanol productivity of the yeast Saccharomyces cerevisiae.
In the present study, the xylose-utilizing strain VTT C-10883 was grown under anaerobic conditions in continuous and batch cultivations with both glucose and xylose as carbon sources. Intracellular metabolites connected to energy and redox metabolism, such as NAD(P)H, NAD(P)+, ATP, ADP and AMP, were quantified and transcriptome analysis was performed in the presence and absence of inhibitors. The continuous cultivations were performed so that cell growth during stress was allowed whereas growth was completely inhibited during furfural conversion in the batch cultures. In the chemostat cultivations, it was found that both catabolic and anabolic reduction charges were significantly decreased in the presence of HMF and furfural, indicating that HMF and furfural are draining the cells of reductive power during growth. Furthermore, the [ATP]/[ADP] ratio of stressed cells was found to be lower than for non-stressed cells, suggesting that the energy metabolism was affected. Transcriptome analysis revealed that genes involved in xenobiotic transporter activity were significantly enriched among the up-regulated genes. The results from the present study provide valuable insights of how S. cerevisiae deals with stress imposed by HMF and furfural, which potentially can result in development of strategies to improve stress tolerance.