A team of researchers at the Joint BioEnergy Institute, University of California, Berkeley, and Technical University of Denmark set out to identify the enzymes that catalyze the production of galactan. Their new study published in the journal The Plant Cell reveals a novel enzyme involved in the production of galactans. The enzyme may be used to engineer plants with more desirable attributes for conversion to biofuel.
Galactan is a polymer of galactose, a six-carbon sugar that can be readily fermented by yeast into ethanol and is a target of interest for researchers in advanced biofuels produced from cellulosic biomass.
Unlike ethanol, advanced biofuels synthesized from the sugars in plant cells walls could replace gasoline, diesel and jet fuels on a gallon-for-gallon basis and be dropped into today’s engines and infrastructures with no modifications required. Also, advanced 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 into fuels.
Getting more galactan is one route with big potential.
Henrik Scheller, vice president for JBEI’s Feedstocks Division and director of its Cell Wall Biosynthesis group holder of the appointment with DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), is the corresponding author of the paper.
Scheller said, “We have confirmed the identity of the GT92 enzyme as the first enzyme shown to increase the biosynthesis of galactan. This identification of the first β-1,4-galactan synthase provides an important new tool for the engineering of advanced bioenergy fuel crops.”
Galactans are polysaccharide components of pectin, the sticky sugar substance that binds together the individual cells in plant cell walls and is used to make jellies and jams. The β-1,4-galactan component of pectin is especially abundant in the “tension wood” that forms in cell walls in response to mechanical stress from wind or snowfall.
Scheller explains, “Galactans are composed of hexoses, which in contrast to pentoses, are easily utilized by fermenting microorganisms for the production of biofuels and other compounds. It would be advantageous to develop plants with increased galactan content instead of hemicelluloses consisting largely of pentoses.”
Pectin imparts strength and elasticity to the plant and forms a protective barrier against the environment. Several different kinds of pectic compounds combine to form pectin. The relative proportion of each of these depends on the plant species, location within the plant, and environment. Pectic compounds decorated with β-1,4-galactan (a chain of six-carbon sugars) are of considerable interest to the biofuels industry, because six-carbon sugars are readily converted into ethanol (biofuel) by fermenting microorganisms.
The major enzymes that catalyze pectin production are hard to pin down. Close to 70 enzymes are predicted to underlie pectin synthesis in plants; only about three of these have been identified definitively. Knowledge of these enzymes could be used to boost the production of pectins with desirable characteristics.
GT92 is a family of glycosyltransferase proteins whose genes are found in all plants that have been genetically sequenced. An increased expression of GT92 genes has been observed in studies of tension wood. This observation combined with the knowledge that tension wood is rich in β-1,4-galactan led Scheller and his colleagues to investigate the function of GT92 proteins in Arabidopsis thaliana, a small flowering relative of mustard that serves as a model organism for plant studies.
Arabidopsis has three members of GT92, which Scheller and his colleagues designated as GALACTAN SYNTHASE 1,2 and 3 (GALS1, GALS2 and GALS3). While loss-of-function mutants in all three genes were found to be galactan deficient, Scheller and his colleague isolated and tested GALS1.
Scheller explains, “Overexpression of GALS1 resulted in plants with 50-percent higher β-1,4-galactan content and no adverse phenotype. We expect that the results for GALS2 and GALS3 overexpressors will be similar though we have yet to test them.”
Given that all three Arabidopsis GALS genes showed overlapping but not identical expression, Scheller and his colleagues are now combining mutations of GALS genes to better understand the role of β-1,4-galactan in plants. They’re also carrying out basic studies on these enzymes, including crystallization and structural analysis. In addition, they’re overexpressing the GALS proteins in different combinations to determine if even higher production of β-1,4-galactan results.
Scheller explores the prospects with, “ . . . galactan is an ancient invention, the function of GT92 as a galactan synthase in Arabidopsis should also be applicable to switchgrass, Miscanthus, poplar and other plants being considered as crops for advanced biofuels,” Scheller says. “We do not anticipate any difficulty in being able to overexpress GT92 genes in these plants.”
Over the past few years we’ve seen a lot of quiet action on the galactan sugar opportunity. From seaweed feedstock to new fermentation yeasts, the galactan share for fuel production is getting set to get a process system research program underway.
As much as folks like to grouse about foodstuff ethanol production, the government bio fuel mandate, the corn farmers and the sugar cane producers have laid the foundation for the most practical biomass to fuel system we’re likely to see unless one of the chemical processes get a foothold with very competitive economics.