Ethanol is the most commonly used biofuel worldwide and is made by fermenting the sugar components of plant materials, usually sugar and starch crops such as sugar cane, corn and wheat. The difficulty in accessing the sugars contained in woody biomass, coupled with criticism that the use of food crops for biofuel production has a detrimental effect on the food supply has prompted research into biofuels that can be made from cellulosic biomass, such as trees and grasses. By looking at the digestive system of termites, researchers have now discovered a cocktail of enzymes that unlocks access to the sugars stored within the cells of woody biomass that could help make it a more viable source of biofuels, such as ethanol.
Until now, a rigid compound that makes up plant cell walls known as lignin has been one of the most significant barriers blocking the access to sugars contained in biomass and inhibiting their use in fuel production. Looking for a way to break down this barrier, Mike Scharf, the O. Wayne Rollins/Orkin Chair in Molecular Physiology and Urban Entomology at Purdue University, and his research partners turned to that scourge of the homeowner, the termite. Their study was the first to measure the sugar output from enzymes created by the termites themselves and the output from small protozoa called symbionts, which live in the termite guts and aid in digestion of woody material.
"For the most part, people have overlooked the host termite as a source of enzymes that could be used in the production of biofuels. For a long time it was thought that the symbionts were solely responsible for digestion," Scharf said. "Certainly the symbionts do a lot, but what we've shown is that the host produces enzymes that work in synergy with the enzymes produced by those symbionts. When you combine the functions of the host enzymes with the symbionts, it's like one plus one equals four."
The researchers separated the termite guts, testing portions that did and didn't contain symbionts on sawdust to measure the sugars created. Once they identified the enzymes, they worked with a protein production company to create synthetic versions. To do this, they inserted the genes responsible for creating the enzymes into a virus and fed it to caterpillars, which then produced large amounts of the enzymes.
They found that the three synthetic enzymes act on different parts of the biomass. Two are responsible for the release of the sugars glucose and pentose, while the third breaks down lignin, a complex chemical compound in the walls of plant cells that provides mechanical strength to the cell wall, and by extension the plant as a whole. Working in combination, the synthetic versions of the host termite enzymes were shown to be very effective at breaking down the woody biomass and releasing sugar.
"We've found a cocktail of enzymes that create sugars from wood," Scharf said. "We were also able to see for the first time that the host and the symbionts can synergistically produce these sugars."
Next, Scharf said his laboratory and collaborators would work on identifying the symbiont enzymes that could be combined with termite enzymes to release the greatest amount of sugars from woody material. Combining those enzymes would increase the amount of biofuel that should be available from biomass.
The team's findings have been published in the early online version of the journal PLoS One.
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