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Breaking biomass

Dec. 3, 2012 - The production of biomass is an often time-consuming process that involves a lot of energy to break the strong plant bonds holding that energy. In addition, the high cost of pre-treatment and the low efficiency of enzyme's ability to break down those plant cell walls into sugars are major industry challenges.

However, Dr. Shi-You Ding, a senior scientist at the US Department of Energy BioEnergy Science Center National Renewable Energy Laboratory in Colorado has been investigating a more efficient way to break down these barriers.

Dr. Ding and his team investigated two different natural existing enzyme systems: bacterial cellulosomes (derived from Clostridium thermocellum) and a blended enzyme mixture from the fungus Trichoderma reesei (Hypocrea jecorina) to represent fungal cellulases. The researchers exposed dried corn stover sections to the different cellulase treatments and analyzed the resultant products.

"Our research revealed that fungal cellulases are more efficient in digesting cell wall when lignins are effectively removed and the cell wall architectures remain intact," said Ding.

In fact, they discovered that fungal cellulases acted approximately five-times faster than cellulosomes against both untreated and delignified cell walls.

"This research has elucidated, for the first time, the different mechanisms that fungal cellulases and bacterial cellulosomes exhibit in digestion of biomass materials, and the correlation of nanoscale architecture of the plant cell walls with the enzyme digestibility," he added. "This research may suggest rational strategies for further engineering enzyme systems based on designed pretreatment approaches."

The results of the experiment showed the most important factor in the digestibility of biomass would appear to be the ability of the enzyme to access the cell wall. This can be solved by altering the pre-treatment method: by focusing on getting rid of the lignin while leaving the structural polysaccharides within the cell walls intact, the enzymes will be allowed easier access and help contribute to rapid digestion.

Current pretreatments remove some of the spongier carbohydrate polymers and allow the remainder to collapse into tighter and less-accessible structures for the enzymes, known as thermo-chemical pretreatment, adds Ding. "In this processes, released sugars may be degraded into forms that are unfermentable by microorganisms (and therefore unusable) and may even be inhibitory to the organisms."

In contrast, by only removing the lignin through the use under milder conditions could greatly benefit the industry. "Removal of the lignin is like unlocking all of the doors in the building so that the workers can get in to pull out re-useable materials, but without collapsing the overall structure so that access is blocked," he said.

As well, the fungal enzymes are smaller and are not only able to act on the surface of the cell walls, but also navigate through the pores within the planet's cell wall and access the polysaccharide. This is in contrast to the cellulosomes, which are too big to access the pores and can only act on the surface of the cell walls.

The researchers believe that using fungal cellases could increase the effectiveness on enzymatic digestion of plants, and therefore increase the amount of harvestable fermentable sugars for use in the biomass industry.

"Improvement of enzyme systems to be able to convert the carbohydrate polymers in biomass that has been less severely or more selectively pretreated could reduce this loss, and allow yields to approach those for recovery and use of all the sugar building-blocks of the cell-wall polymers," says Ding.


December 3, 2012
By David Manly


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