Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/2/2008
Publication Date: 12/1/2008
Citation: Liu, Z., Moon, J., Andersh, B.J., Slininger, P.J., Weber, S.A. 2008. Multiple Gene Mediated NAD(P)H-Dependent Aldehyde Reduction is a Mechanism of in situ Detoxification of Furfural and HMF by Saccharomyces cerevisiae. Applied Microbiology and Biotechnology. 81:743-753. Interpretive Summary: Furfural and 5-hydroxymethylfurfural (HMF) are major inhibitory compounds generated from dilute acid hydrolysis biomass pretreatment. Remediation of inhibitors adds cost and generates extra waste products. Few yeast strains tolerant to inhibitors are available. Development of stress tolerant ethanologenic yeast is an alternative approach for cost-efficient biomass to ethanol conversion. Understanding mechanisms of the tolerance and detoxification of the inhibitors is necessary for such tolerant strain development. The recent identification of 2-furandimethanol (FDM, 2,5-bis-hydroxymethylfuran), HMF metabolic conversion product, contributed significantly to inhibitor stress tolerance studies. It allowed metabolic profiling analysis for a better understanding of mechanisms in the tolerance and detoxification. Using a tolerant ethanologenic yeast strain, this study provided efficient biotransformation of furfural and HMF as an initial culture without pre-building biomass. It demonstrated aldehyde reduction as a mechanism of in situ detoxification of furfural and HMF. This research benefits the bioethanol community for tolerant yeast strain development by providing better understanding of the mechanisms of the tolerance and detoxification of the inhibitors.
Technical Abstract: Furfural and 5-hydroxymethylfurfural (HMF) are representative inhibitors to ethanologenic yeast generated from biomass pretreatment using dilute acid hydrolysis. Few yeast strains tolerant to inhibitors are available. In this study, we report a tolerant strain 12HF10 of Saccharomyces cerevisiae having enhanced biotransformation ability to convert furfural to furan ethanol (FM), HMF to furan di-methanol (FDM), and is able to produce a normal yield of ethanol as an initial culture. Our recent identification and synthesis of the HMF metabolic conversion product allowed studies on fermentation metabolic kinetics in the presence of HMF and furfural. Individual genes encoding enzymes possessing aldehyde reduction activities demonstrated different cofactor preference for NADH or NADPH. Collectively, protein extract form the yeast whole cell showed equally strong aldehyde reduction activities couples with either cofactor. A single gene deletion of candidate genes, including ADH6 and ADH7, did not affect the yeast to grow in the presence of the inhibitors. Absolute quantification of mRNA expression demonstrated that some candidate genes were significantly enhanced while others repressed, indicating not all candidate genes were able to function contributing the detoxification under immediate challenges of furfural and HMF. Furfural and HMF apparently affect cellular redox balance in a yeast culture. Conversion pathways of furfural and HMF relevant to glycolysis and ethanol production were refined. Our results suggest that it is possible to in situ detoxify furfural and HMF by ethanologenic yeast S. cerevisiae; and a mechanism of the detoxification is due to NAD(P)H-dependent aldehyde reduction, not furan conversion, accomplished through multiple gene-mediated functions rather than a single gene.