Galactan（半乳糖体） is a polymer of galactose（半乳糖）, a six-carbon sugar that can be readily fermented¹ by yeast² into ethanol（乙醇） and is a target of interest for researchers in advanced biofuels produced from cellulosic biomass. Now an international collaboration³ led by scientists at the U.S. Department of Energy (DOE)'s Joint⁴ BioEnergy Institute (JBEI) has identified the first enzyme⁵ capable of substantially boosting the amount of galactan in plant cell walls. Unlike ethanol, advanced biofuels synthesized from the sugars in plant cells walls could replace gasoline, diesel⁶ and jet fuels on a gallon-for-gallon basis and be dropped into today's engines and infrastructures⁷ with no modifications⁸ required. Also, adanced biofuels have the potential to be carbon-neutral, meaning they could be burned without adding excess carbon to the atmosphere. Among the key challenges to making advanced biofuels cost competitive is finding ways to maximize the amount of plant cell wall sugars that can be fermented（发酵） into fuels.

 

"We have confirmed the identity of the GT92 enzyme as the first enzyme shown to increase the biosynthesis of galactan," says Henrik Scheller, vice⁹ president for JBEI's Feedstocks Division and director of its Cell Wall Biosynthesis group. "This identification of the first β-1,4-galactan synthase provides an important new tool for the engineering of advanced bioenergy fuel crops."

 

Scheller, who also holds an appointment with DOE's Lawrence Berkeley National Laboratory (Berkeley Lab), is the corresponding author of a paper in the journal Plant Cell that describes this work. The paper is titled "Pectin Biosynthesis: GALS1 in Arabidopsis thaliana is a β-1,4-Galactan β-1,4-galactosyltransferase." Co-authors were JBEI's April Liwanag, Berit Ebert, Yves Verhertbruggen, Emilie Rennie, Carsten Rautengarten, and Ai Oikawa, plus Mathias Andersen and Mads Clausen of the Technical University of Denmark.

 

Galactans are polysaccharide components¹⁰ of pectin, the sticky sugar substance that binds¹² together the individual cells in plant cell walls and is used to make jellies and jams. The β-1,4-galactan component¹¹ of pectin is especially abundant in the "tension wood" that forms in cell walls in response to mechanical stress from wind or snowfall.

 

"Galactans are composed of hexoses, which in contrast to pentoses, are easily utilized¹³ by fermenting¹⁴ microorganisms for the production of biofuels and other compounds," Scheller says. "It would be advantageous¹⁵ to develop plants with increased galactan content instead of hemicelluloses（半纤维素） consisting largely of pentoses."

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