Cellulosic breakthrough; enzyme discovery could reduce production costs
Researchers have found that an enzyme discovered more than a decade ago can digest cellulose in cellulosicethanol feedstocks such as switch grass, corn stover and wheat straw twice as fast as any enzyme currently on the commercial market, a scientist at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) said.
While researchers have continued to make discoveries that have led to improved cellulosic-ethanol production volumes at demonstration and pilot-scale plants in recent years, the enzyme CelA has the potential to be a game changer, the lead researcher on the NREL discovery told DTN.
Roman Brunecky, NREL scientist and lead author of the paper, “Revealing Nature’s Cellulase Diversity: The Digestion Mechanism of Caldicellulosiruptor bescii CelA,” said the enzyme is like no other.
NREL discovered an enzyme produced by a microorganism found in the Valley of Geysers on the Kamchatka Peninsula in Russia in 1990, according to the paper published in the journal Science.
The enzyme has the potential to make cellulosic biofuels more cost-competitive with corn-based ethanol.
It’s a potential breakthrough that comes at a time when a budding U.S. cellulosic-ethanol industry faces a wide array of technological and policy challenges as it prepares to launch at least some commercial production in 2014 and 2015.
The NREL discovery also comes at a time when the U.S. Environmental Protection Agency is considering an overall cut to the 2014 biofuel volumes in the Renewable Fuel Standard (RFS), including the cellulosic-ethanol mandate. Companies developing the technology have indicated that an RFS cut may lead them to reconsider their plans.
If used for commercial cellulosic ethanol production, the enzyme has the potential to help the industry scale up production closer to the volumes current corn-ethanol plants produce—to between 50-100 million gallons per year—researchers say. Companies that have plans to launch cellulosic-ethanol production in 2014 announced plans to produce between 20 million and 30 million gallons per year with current enzymes.
For many years, cellulosicethanol researchers have been looking for ways to more efficiently break down lignocellulose materials in biomass into usable sugars.
If an enzyme can produce sugars more efficiently, it can lead to lower cost for an enzyme cocktail, Brunecky said, which typically is a major cost of converting biomass into usable sugars for biofuels.
“Because enzyme costs are one of the major costs in a biorefinery,” he said, “reducing the total number of enzymes required in a formulation and the total amount of formulation required is very important from a cost-reduction standpoint.”
CelA is unique in that while it acts to remove materials from the surface of biomass, “it can also bore down into the cellulose micro fibril, something never seen before in fungal enzyme systems,” Brunecky said.
So far, the enzyme discovered by NREL has performed at the laboratory scale. The enzyme may perform differently at a larger commercial scale. However, Brunecky said he believes existing enzyme companies have the necessary technology to commercialize such an enzyme.
“Commercialization primarily requires large amounts of the enzyme being efficiently produced,” he said. “Most of the large enzyme companies in the world have the necessary expertise to carry this out, so I would say the chances for commercialization of CelA-based formulations are good.”
Brunecky said biomass consists of three types of polymers that together form plant cell walls. They include cellulose, xylan and lignin. Each of these cellulose and xylan polymers requires several types of enzymes to “deconstruct” them into soluble sugars for ethanol fermentation.
Current enzyme formulations on the market are derived from fungal sources such as Trichoderma reesi, he said. These systems use a model of one enzyme catalyzing one type of reaction.
“Since biomass is a complex polymer of cellulose, xylan and lignin linkages, many different types of enzymes acting together synergistically are required to deconstruct biomass,” Brunecky said.
“Therefore, most commercial formulations contain tens or even hundreds of enzymes. Also, since these enzymes are produced by the fungus, they are optimized to work in a specific temperature range.”
These enzymes essentially remove material from the surface of biomass, he said. The CelA enzyme is seven times more efficient in converting cellulose to fermentable sugars, Brunecky said, and has the ability to deconstruct both cellulose and xylan.
According to an NREL news release, most commercial operations use enzyme cocktails that combine 15-20 different enzymes to convert plant materials into sugars that are valuable to the biofuels industry.
The CelA enzyme was discovered 15 years ago. The organism was initially grown on biomass by scientists from the University of Georgia. — Todd Neeley, DTN