Some of nature's most extreme little fungi could help pretreat auto fuel feedstocks in the near future

Researchers at the U.S. Department of Energy's Joint Genomics Institute (JGI) have been busy tracking down new enzymes that can pretreat feedstocks for biofuels at higher temperatures.

Nature has evolved a host of proteins that can break down a multitude of commonly occuring natural compounds.  The enzymes' catalysts operate at a speed largely depend on the temperature. Higher temperatures means faster reactions, typically, but it also can cause protein denaturation (when proteins lose their carefully ordered structure, via the disolution of hydrogen bonds, etc.).  Thus organisms who operate at higher temperatures require special denaturation-resistant enzymes, but have the advantage of being able to prcoess materials faster.

With biofuels the biggest challenge is converting cellulose -- the primary sugar found in common biomass feedstocks, like fast growing grasses -- into individual sugars that can be fermented.  To do that scientists have borrowed enzymes from nature
Fig. 1:  The key to making biofuel is to break down cellulose -- the starchy sugar found in plant biomass.  There's slight differences in biomass between woody plants and grasses. [Source: Wikimedia Commons]

Current generation enzymes operate at around 20°C-35°C (68°F-95°F) -- in the range varying from room temperature to nearly the human body temperature.  However, these enzymes process materials too slowly, allowing contamination, which in turn reduces yields and makes the resulting biofuel more expensive.

The DOE JGI researchers proposed finding higher temperature enzymes.  They successfully identified some such potential enzymes by comparing the genomes of Thielavia terrestris and Muceliophthora thermophila, fungi which thrive at high-temperature environments of 45°C or above.

Thermophile fungs
Fig. 2: The thermophilic fungus M. thermophila [Source: Adrian Tsang, Concordia University]

Randy Berka from Novozymes in Davis, Calif., joined the researchers in the work and writes in the report, "Thermostable enzymes and thermophilic cell factories may afford economic advantages in the production of many chemicals and biomass-based fuels.  Thermophilic fungi are well-known composting organisms, and their usefulness as a reservoir for thermotolerant enzymes was established decades ago.  These two thermophiles can be considered all-purpose decomposers with respect to their carbohydrate-active enzymes (CAZymes) and their ability to degrade plant polysaccharides."

The team found that the family of glycoside hydrolase enzymes was expanded in the thermophile ("heat loving") fungi to break single, fermentable sugars off polysaccharides (chains of single sugars) at a variety of temperatures.  Such a range was not present in the more common fungus T. reesei.

Thus researchers believe the 38.7-million base pair (Mbp) genome of M. thermophila and the 36.9 Mbp genome of T. terrestris offers the blueprint to dramatic acceleration of enzymatic breakdown of cellulose, reducing impurities, improving yields, and dropping costs.

Comments DOE JGI Fungal Genomics head Igor Grigoriev, who helped lead the research team, "These thermophilic fungi represent excellent hosts for biorefineries where biomass is converted to biofuels as an alternative to modern oil refineries.  The fact that these organisms not only deliver a broad spectrum of heat-tolerant enzymes but can also host new enzymes and be optimized for industrial processes holds great promise for significant improvements over existing systems."

Igor Grigoriev
Fig. 3: DOE JGI Fungal Genomics head Igor Grigoriev helped lead the study on thermophile fungal enzymes. [Source: DOE JGI]

The bacterial enzymes proved efficient at breaking down biomass of both major groups of flowering plants -- Monocotyledonae (grasses, orchids, palms) and Eudicotyledonae (trees and some garden flowers).

The key to further lowering costs will be to synthetically select the bacteria to produce superior versions of the cellulose-chewing enzymes and to overproduce the enzyme(i.e. overexpress).  Currently enzymatic pretreatment of cellulose biomass remains more expensive than other techniques such as plasma gasification.  However, with this new work, the cost of enzymatic biomass pretreatment creeps closer to the cost of these alternative methods.

The new work was funded by the DOE and was published [abstract] in the October 2 edition of the prestigious peer-reviewed journal Nature Biotechnology.

Source: DOE JGI

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