Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/28/2004
Publication Date: 7/29/2004
Citation: Liu, Z., Slininger, P.J., Dien, B.S., Berhow, M.A., Kurtzman, C.P., Gorsich, S.W. 2004. Adaptive response of yeasts to furfural and 5-hydroxymethylfurfural and new chemical evidence for hmf conversion to 2,5-bis-hydroxymethylfuran. Journal of Industrial Microbiology and Biotechnology. 31:345-352. Available http://dx.doi.org/10.1007/s10295-004-0148-3. Interpretive Summary: In order to utilize low-cost lignocellulosic feedstocks for bioethanol production, the lignocelluloses need to be degraded to simple sugars for microbial utilization in fermentation. However, commonly used dilute acid hydrolysis generates many inhibitory compounds which suppress yeast growth. Consequently, additional detoxification is needed and this increases the already high cost of the bioethanol. Genetically manipulated strains have shown promise of improved microbial performance in effective production of enzymes and fermentative conversion of biomass. Due to the lack of understanding of the stress-tolerant mechanisms, few genetically improved stress-tolerant strains are available. In this study, we evaluated representative yeast strains and found a dose-dependent inhibition of yeast by inhibitors. A previously unidentified compound converted by metabolic transformation from an inhibitor by yeast was isolated, purified, and identified. Thereafter, we proposed a general model for the reduction of inhibitors to less toxic compounds by yeast. These results provided a basis for further studies on stress-tolerance mechanisms. This is the first chemical identification of the yeast transformed product from the inhibitor, and it contributes to the scientific literature. These findings impact bioethanol research and production communities because they provide a strategy to develop novel inhibitor-tolerant strains for more cost-effective conversion of biomass to ethanol biofuel.
Technical Abstract: Renewable lignocellulosic materials are attractive low-cost feedstocks for bioethanol production. Furfural and 5-hydroxymethylfurfural (HMF) are among the most potent inhibitory compounds generated from acid hydrolysis of lignocelluloses to simple sugars for fermentation. We evaluated Saccharomyces cerevisiae strain ATCC 211239, NRRL Y-12632, and Pichia stipitis strain NRRL-7124 and found furfural and HMF inhibition were dose-dependent at concentrations from 10 to 120 mM. The yeast strains were more sensitive to furfural inhibition than HMF at the same concentration while the combined treatment of furfural and HMF synergistically suppressed cell growth. A metabolite transformed from HMF by strain NRRL Y-12632 was isolated from culture supernatant and conclusively identified as 2, 5-bis-hydroxymethylfuran, a previously postulated HMF alcohol, with a molecular weight of 128. Therefore, we propose that in the presence of HMF, yeast reduce the aldehyde group on the furan ring of HMF to an alcohol, in a similar manner as for furfural. The accumulation of this biotransformed metabolite may be less toxic to yeast cultures than HMF as evidenced by the rapid yeast fermentation and growth rates once the HMF conversion has been completed. The ability of yeast to adapt to and transform furfural and HMF offers the potential for in situ detoxification of these inhibitors and suggests a genetic basis for further development of highly tolerant strains for biofuel production.