News Unraveling the Mechanism of Lignocellulosic Biomass Acetylation May Lead to Cheaper Bioethanol
Lignocellulosic biomass -- such as energy grasses and wood residues -- contains bound acetate in addition to the coveted sugars in cellulose and its rigid partners, hemicellulose and lignin. The acetate is released upon processing and is a major inhibitor of microbial fermentation of sugars into bioethanol, a popular type of biofuel largely derived from sugary food crops such as corn and sugarcane
A study at the Energy Biosciences Institute (EBI) in Berkeley has identified a plant gene responsible for adding acetate to a sugar-laden hemicellulose in plant cells, thus providing a new avenue for reducing the level of acetylation in plant feedstocks and thereby potentially lowering the cost of biofuel production.
According to the study, published this week in The Plant Cell, mutation of the gene, accomplished in the model plant Arabidopsis, a member of the mustard and cabbage family, eliminates acetylation of a hemicellulose.
A team of researchers at the EBI, headed by Markus Pauly, a plant biologist at the University of California, Berkeley, set out to identify the enzymes that acetylate the polysaccharides, which contain multiple sugars, that are present in lignocellulosic feedstocks. Their initial work focused on xyloglucan, a type of hemicellulose that is abundant in plant cell walls.
Using a mass spectrometric technique, the scientists isolated a mutant from among a mutagenized population of Arabidopsis plants that exhibited a 20 to 45 percent reduction in acetate bound to xyloglucan. The researchers mapped the mutation to a physical location in the Arabidopsis genome and named the gene locus “Altered Hemicellulose Xyloglucan 4,” or AXY4. Blocking the expression of AXY4 in Arabidopsis leads to an elimination of the bound acetate.
“The identification of the first gene to encode a polysaccharide O-acetyltransferase opens the door for identifying similar genes in bioenergy crop feedstocks, such as Miscanthus or other energy grasses,” said Pauly. “These genes can be used as genetic markers to facilitate breeding programs that aim to generate biofuel feedstocks with reduced lignocellulosic acetate content.”
Technologies are being developed to generate bioethanol from non-food sources, such as the lignocellulosics present in switchgrass and trees. The sugars locked in the polymers of cell walls composed of cellulose and hemicellulose packed in lignin can be extracted and fermented by yeast into bioethanol.
A major obstacle to this strategy is that most wall polysaccharides contain acetate groups, and the acetate released from these molecules during processing inhibits the activity of the microbes that ferment sugars into alcohol. Based on techno-economical models, a 20 percent reduction in biomass acetylation is predicted to translate into a 10 percent reduction in bioethanol price. Thus, a major goal in the field of plant biofuel research is to diminish the O-acetate content in the cell walls of plants, possibly by blocking the enzymes that acetylate the cell wall polymers. However, little has been known about the acetylation enzymes in plants.
A natural variety of Arabidopsis growing in northern Scotland also has low levels of xyloglucan O-acetylation. Intriguingly, this variety was found to have a natural mutation in AXY4. This finding demonstrates that lack of xyloglucan O-acetylation does not represent a selective disadvantage for the plant, and supports the feasibility of genetically blocking the expression of the protein that controls O-acetylation in plants destined for biofuel production.
In addition to Pauly, other EBI authors on the Plant Cell paper include Sascha Gille, Amancio de Souza, Guangyan Xiong, Monique Benz, Kun Cheng, and Alex Schultink. Also participating was Ida-Barbara Reca of the DOE Great Lakes Bioenergy Center at Michigan State University.
The Energy Biosciences Institute is a public-private collaboration in which bioscience and biological techniques are being applied to help solve the global energy challenge. The partnership, funded with $500 million for 10 years from the energy company BP, includes researchers from the University of California, Berkeley; the University of Illinois at Urbana-Champaign; and the Lawrence Berkeley National Laboratory. Details about the EBI can be found on the website: www.energybiosciencesinstitute.org
The paper, “O-acetylation of the Hemicellulose Xyloglucan Requires AXY4/AXY4L Proteins with a TBL and DUF231 Domain,” can be accessed at http://www.plantcell.org/content/early/2011/11/14/tpc.111.091728.abstract?papetoc. Plant Cell is the journal of the American Society of Plant Biologists.