NREL enzyme enables conversion of biomass to sugar up to 14x faster than current alternatives; changing the economics of conversion
Scientists at the US Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) have developed an enzyme that can enable the conversion of biomass to sugars up to 14 times faster and more cheaply than competing catalysts in enzyme cocktails today. The enzyme called CelA, a cellulase from the bacterium Caldicellulosiruptor bescii, could thus could change the economics of biofuel conversion.
In one scenario, the best commercially used enzyme converted sugars at a 30% extent in seven days. CelA converted to double that extent. And while it took the alternative enzyme seven days to achieve that conversion, CelA, with a small boost from an extra beta glucosidase, achieved double in just about two days. Among CelA’s many attributes:
Biomass comprises three types of polymers that are intermeshed to form plant cell walls: cellulose, xylan, and lignin. Each of the three polymers typically requires several types of enzymes to deconstruct them to soluble species that can then be upgraded to ethanol, drop-in fuels, or chemicals. But unlike most catalysts which digest one major component in biomass, CelA can digest two: both cellulose and xylan.
CelA works in two mechanical realms, not just one. It is an ablater, scraping the valuable material off the cell walls of the plants. But it is also a borer, digging deep into the wall to grab more of the digestible biomass. It is the only enzyme known to dig pits into biomass; others only ablate.
It can operate at much higher temperatures than other enzymes; higher temperatures mean faster action.
CelA also works faster on raw biomass than on biomass pre-treated with chemicals.
Because it can operate above the boiling point of alcohol, the alcohol is separated naturally, saving a costly step in the conversion process—and the high temperatures kill many of the microorganisms that would otherwise interfere with the process.
If you can achieve in one day what typically takes seven, you are saving the better part of a week of processing. And that can have a huge economic impact.—Senior Scientist Roman Brunecky
CelA alone is four to five times faster at breaking down sugars than the enzymes in today’s cocktails. A more typical usage would be CelA combined with a beta glucosidase—an improvement that makes it 14 times faster.
If the enzyme continues to perform well in larger tests, it could help drive down the price of converting cellulose and, with it, the price of everything from jet fuel to ethanol, butanol, drop-in fuels, and numerous chemicals. NREL has filed for patent protection on the enzyme formulation and the improvements made to the unusual enzyme.
The discovery was the unexpected result of very thorough imaging and analysis by Bryon Donohoe of NREL’s Biomass Surface Characterization Laboratory research team.
C. bescii, producer of the CelA enzyme, was first discovered in warm-water pools in the Valley of Geysers on Russia’s Kamchatka Peninsula in the 1990s. Russian scientists found the enzyme somewhat promising, but it wasn’t until the NREL researchers conducted a thorough analysis and added improvements that its potential was realized.
NREL found some surprising properties. For example, even though CelA contains naturally occurring endoglucanase, the researchers found that adding more of the substance greatly accelerated the rate at which it broke down sugars.
CelA has its own beta-glucosidase activity, but NREL researchers checked to see how it would perform if more of the beta-glucosidase was added. That’s when the rate really accelerated.
You’d think that nature would already have evolved an optimal mix in a single enzyme. You expect to see maybe a 20% to 30% improvement when you add a beta glucosidase to a cocktail—not the doubling or tripling of the rate of conversion and the increase of rates by a factor of 10 that we got with CelA. Nobody expected the improvement to be this high.—Roman Brunecky, NREL and lead author of a paper on CelA in Science
In a Perspective on a 2013 paper in Science describing CelA’s digestion mechanism, Alex Berlin of biotechnology company Novozymes noted that CelA’s ability to operate at high temperatures “would be seen by many in the biomass biorefinery industry as an advantage” because it “dramatically reduces the chances of bacterial contamination” while lowering the viscosity of the mixtures.
Acting alone or in combination with a β-glucosidase, CelA breaks down highly recalcitrant cellulose faster than does an enzyme mixture commonly used in commercial biomass enzyme products. Furthermore, CelA shows higher specific activity on raw biomass than on physicochemically pre-treated biomass. These results may be a turning point in the development of future bioconversion technologies that do not require biomass pretreatment or the participation of a large number of enzyme components.—Berlin (2013)
The NREL researchers are examining the other enzymes secreted by C. bescii. They’re also using what they’ve learned from CelA to help improve cellulase enzymes that are more compatible with the enzyme formulations used today.
The next step toward using CelA at commercial scale is to express large amounts of the enzyme in existing production systems, and to boost enzyme yields from the native organism, C. bescii.
Roman Brunecky, Markus Alahuhta, Qi Xu, Bryon S. Donohoe, Michael F. Crowley, Irina A. Kataeva, Sung-Jae Yang, Michael G. Resch, Michael W. W. Adams, Vladimir V. Lunin, Michael E. Himmel, and Yannick J. Bomble (2013) “Revealing Nature’s Cellulase Diversity: The Digestion Mechanism of Caldicellulosiruptor bescii CelA” Science 342 (6165), 1513-1516 doi: 10.1126/science.1244273
Alex Berlin (2013) “No Barriers to Cellulose Breakdown” Science 342 (6165), 1454-1456. doi: 10.1126/science.1247697