New class of enzymes discovered that could increase efficiency of cellulosic ethanol production

In a breakthrough that could make the production of cellulosic ethanol less expensive, Cornell University researchers have discovered a class of plant enzymes that potentially could allow plant materials used to make ethanol to be broken down more efficiently than is possible using current technologies.

Biofuels such as corn ethanol or rapeseed biodiesel are unsustainable, because they have a very weak energy balance and do not reduce greenhouse gas emissions much. For this reason, scientists are looking into developing 'second-generation' biofuels, made from cellulose, which is a far more abundant feedstock than the oil and sugar currently obtained from grains, canes and oil seeds. Cellulose is the most abundant biological material on earth.

Even though 'first-generation' biofuels made from tropical feedstocks, such as sugarcane or cassava already do have a strong energy balance and reduce CO2 emissions considerably, utilizing the cellulosic waste biomass obtained during their production as a feedstock for ethanol would likewise increase these balances even further and make tropical biofuels extremely energy efficient.

However, the production of biofuels from cellulose in mass quantities is still quite costly. The development of thermochemical conversion technologies - which involve gasification or pyrolysis of biomass - is making good progress, even though cost and downstream processing into useable fuels remains an obstacle. The same is true for the biochemical conversion path, which is based on the use of enzymes to break down the cell walls of plants and to release the sugars they contain that can then be fermented into alcohol.

With current technologies, this critical step of breaking down cell walls relies on microbial enzymes called "cellulases" to digest the cellulose. The microbial enzymes have a structure that makes them very efficient at binding to and digesting plant cell wall material called lignocellulose (a combination of lignin and cellulose).

But now, a new class of plant enzymes with a similar structure has been discovered, potentially offering researchers new properties for producing ethanol even more efficiently. A team working with Jocelyn Rose, Cornell assistant professor of plant biology, published its paper [*abstract] on the new class of enzymes in the April 20 issue of the Journal of Biological Chemistry. Breeanna Urbanowicz, a graduate student in Rose's laboratory, was the paper's lead author.

"The bottleneck for conversion of lignocellulose into ethanol is efficient cellulose degradation. The discovery of these enzymes suggests there might be sets of new plant enzymes to improve the efficiency of cellulose degradation." - Jocelyn Rose, Cornell assistant professor of plant biology

For an enzyme to break down cellulose, a structure called a cellulose-binding module attaches to the cellulose. Once attached, a catalyst then breaks the cell wall material into small units, which can then be turned into ethanol. While researchers have known that plants have cellulase-like enzymes, it was previously thought that they did not have a cellulose-binding module, and so could not attach to cellulose or digest it very effectively -- until now:
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"This is the first example of a cellulose-binding domain in a plant cell wall enzyme," said Rose. While the new enzyme was found in a tomato plant, Rose and colleagues have evidence of a set of such plant proteins in many species that potentially could be used for biofuel production. Biofuel research may also help uncover exciting new uses for these enzymes, said Rose. Researchers may, for example, breed for plants with high levels of these proteins.

Though the scientists stress that more study is needed to understand how plants use this class of enzymes, Rose speculates that they may be needed when growing tissues rapidly expand and require loosening of tightly bound strands of cellulose, called microfibrils, that make up a cell wall's structure. The binding enzymes may also be part of the process of breaking down tissues, e.g., when fruits -- such as tomatoes -- soften.

Among others, co-authors included Carmen Catalá, a research associate previously working in the Department of Plant Biology, who originally identified the gene for the tomato enzyme, and David Wilson, Cornell professor of molecular biology and genetics.

Image: This schematic diagram shows the newly discovered class of plant enzymes with a cellulose-binding module (shown in blue), sticking to a plant cell wall. The binding module of the enzyme helps the catalytic region of the enzyme (shown in more detail in gray in the pullout part of the picture) break down the crystalline cellulose. Courtesy: Daniel Ripoll and Chris Pelkie/Cornell Theory Center.

More information:
Breeanna R. Urbanowicz, Carmen Catalá, Diana Irwin, David B. Wilson, Daniel R. Ripoll, and Jocelyn K. C. Rose, "A Tomato Endo-beta-1,4-glucanase, SlCel9C1, Represents a Distinct Subclass with a New Family of Carbohydrate Binding Modules (CBM49)" [*abstract], J. Biol. Chem., Vol. 282, Issue 16, 12066-12074, April 20, 2007

Cornell University: Newly discovered plant enzymes could lead to more efficient -- and less costly -- ethanol production from cellulose - April 24, 2007.