Fuel matters: Insights into enzymatic conversion

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  • Published: Jul 1, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Fuel matters: Insights into enzymatic conversion

Cellulose goes non-native

An enzyme (shown in blue) pulls out individual cellulose chains (pink) from the pretreated nanofiber surface (green) and then breaks them apart into simple sugars. Image credit - Shishir Chundawat, Great Lakes Bioenergy Research Center

Altering the crystalline structure of cellulose from its native form to another can lower its binding partition coefficient for fungal cellulose enzymes by 40-50% but surprisingly boost hydrolytic activity. This new finding could thus help open the road to more efficient enzymatic production of biofuels from biomass rather than petroleum.

Researchers at the Los Alamos National Laboratory (LANL) and the Great Lakes Bioenergy Research Center (GLBRC) recognise that improving the enzymatic processing of cellulose nanofibres so that they can be broken down to sugars more effectively and at a lower cost is important to the development of a viable biofuel production industry. This would faciliate a gradual societal transition from reliance on dwindling petroleum resources to a more sustainable source of energy and chemical feedstocks. Now, the team has used earlier data from X-ray techniques to investigate the unique properties of crystalline cellulose nanofibres that should allow the development of novel chemical pre-treatments and designer enzymes for converting cellulose derived from non-food crop sources, or biomass, for biofuel production.

Splitting nanofibres

GLBRC's Shishir Chundawat explains how cellulose - long polymers of simple glucos molecules - is present within plant cell walls as nanoscopic, crystalline fibres, that act like reinforcement struts to help maintain the structural integrity of the cells. As such, the key to making cheaper biofuel rests on unravelling these tightly packed nanofibres efficiently to release, using fewer enzymes, soluble glucose molecules that can then be converted into advanced biofuels.

Writing in the journal Proceedings of the National Academy of Sciences, the team has demonstrated something apparently paradoxical about the enzymatic treatment of cellulose polymers. One might imagine that increased binding of enzymes to the substrate would accelerate the desirable break down to sugars, given that substrate binding is usually a critical rate-limiting step. However, Chundawat's team has shown that a novel biomass pre-treatment that generates crystalline cellulose III from cellulose reduced native enzyme binding but increased sugar yields by as much as five fold.

"The ability of this unconventional pre-treatment strategy, currently under development at GLBRC, to selectively alter the cellulose crystal structure may lead to an order of magnitude reduction in enzyme usage" Bruce Dale of Michigan State University suggests. "This will be critical for cost-effective cellulosic biofuel production," Dale adds.

Cellulose alterations

In earlier work, the team had found that altering the crystal structure of native cellulose to cellulose III would speed up enzymatic break down, but the latest work now reveals that sugar yield rises counter-intuitively given that the levels of bound enzyme observed are much reduced. In order to resolve this seeming paradox, Chundawat and colleagues at LANL including Gnana Gnanakaran and Anurag Sethi have developed a mechanistic kinetic model that uncovers a much more complex relationship between enzyme affinity for cellulose and catalytic efficiency than had been anticipated. Their experiments have shown that cellulose III has a less "sticky" surface, which makes it more difficult for native enzymes to bind to break it down. The model also hints that the enhanced enzyme activity without strong binding arises because of the relative ease with which the enzymes can extract individual cellulose III chains from the surface of the pre-treated nanofibres and thence cleave them to release simple sugar molecules, without getting stuck on the cellulose surface non-productively. The team is looking to carry out more experiments in the near future to provide further supporting evidence for their model's predictions.

"These findings are exciting because they may catalyse future development of novel engineered enzymes that are further tailored for conversion of cellulose III rich pre-treated biomass to cheaper fuels and other useful compounds that are currently derived from non-renewable fossil fuels," Gnanakaran enthuses. Chundawat revealed to SpectroscopyNOW that, "Overall our goal is to develop cost-effective and sustainable processes for production of next-generation biofuels and biochemicals from cellulosic biomass."

Related Links

Proc Natl Acad Sci 2013, online: "Increased enzyme binding to substrate is not necessary for more efficient cellulose hydrolysis"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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