Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 8/13/2009
Publication Date: 1/15/2010
Citation: Liu, Z., Blaschek, H.P. 2010. Biomass conversion inhibitors and in situ detoxification. In: Vertes, A., Qureshi, N., Yukawa, H., Blaschek, H., editors. Biomass to Biofuels. West Sussex, UK: John Wiley and Sons Ltd. p.233-258. Interpretive Summary:
Technical Abstract: Inhibitory compounds derived from lignocellulosic biomass pretreatment are classified into aldehydes, ketones, organic acids, and phenols based on their chemical functional group that are toxic to fermentative microorganisms. Inhibitors and effects of inhibition to fermentative microbes vary depending upon divergent biomass sources and strains applied. The removal of inhibitors by physical or chemical means is most likely impractical due to economic concerns. Instead, the use of tolerant strains to in situ detoxify harmful aldehydes, organic acids, and phenols appear to be promising. To overcome inhibitor complexes in biomass hydrolysate remains a significant challenge. Tolerant strain development by directed evolutionary adaptation under laboratory settings is anticipated to play a significant role combining the necessary enhancement of genetic background through recombinant engineering. Current understanding of inhibitor conversion pathways and mechanisms of in situ detoxification will aid tolerant strain development. Furfural is reduced to furan methanol and can be further broken down to furoic acid and formic acid. HMF is reduced into furandimethanol and further to formic acid and levulinic acid. Many genes are identified to code enzymes possessing aldehyde reduction activities, including ADH6, ADH7, ALD4, GRE3, and several uncharacterized genes. A mechanism of in situ detoxification of furfural, HMF, cinnamaldehyde, and other aldehyde inhibitors is recognized as NAD(P)H dependent aldehyde reduction catalyzed by multiple reductases. Members of PDR gene family are significantly involved in response to coping with inhibitor stress conditions. Among eight candidate transcriptional factors identified, Pdr3 and Yap1 are significantly involved positively in regulating gene responses and interactions for HMF stress. Organic acid ferulic acid is metabolized to vinyl guaiacol and dihydroferulic acid. Similarly, 4-hydroxycinnamic acid is decarboxylated to styrene and dihydrocinnamic acid. These reactions are catalyzed by phenylacrylic acid decarboxylase and likely involve other enzymes. Studies of genomic mechanisms of inhibitor stress tolerance allow a better understanding of cell response and in situ detoxification by fermentative microorganisms.