Biological Conversion

Fermentation is at the heart of the biorefinery concept. It is the primary way to generate products from the sugars that will be the platform chemicals produced by sugar platform technology. In traditional bioprocesses, such as the current large-scale commercial fermentation of starch into ethanol, relatively pure streams of glucose serve as the feedstock for fermentation. Microorganisms well developed for industrial use, such as brewers yeast, are inexpensive and fully adequate.

Sugar streams derived from lignocellulose, however, pose significant technical barriers. These streams contain five sugars, the hexoses glucose, mannose, and galactose, and the pentoses D-xylose and L-arabinose. With five-carbon structure instead of six, the pentoses, in particular, are not metabolized by common yeast. Cost-effective processes will require the rapid, complete and simultaneous fermentation of all five sugars.

In response to this challenge, Biomass Program researchers metabolically engineered the bacterium Zymomonas mobilis to add ability to ferment xylose and arabinose to its natural ability to ferment glucose. One of only three hexose/pentose co-fermenters developed to date, the patented microorganism received a prestigious R&D 100 award and has been licensed to a number of companies for research and development use.

In addition, hydrolysates of lignocellulose contain compounds that are inhibitory to most microorganisms. The goal of Biomass Program strain development research is to facilitate the development of robust "platform" biocatalysts that can ferment biomass sugars into either ethanol or other desired bioproducts with economically viable rates and yields at industrial scales. Tolerance to harsh environments, including elevated temperatures, high salt, and low pH, will be essential. Currently available strains are severely limited in pentose utilization and exhibit poor hydrolysate tolerance.

Currently, primary Biomass Program efforts in strain development are focused on Cooperative Research and Development Agreements (CRADA's) with two partners, DuPont and the National Corn Growers' Association, as described below. More fundamental studies on sugar uptake, metabolite flow and modeling, pathway regulation, and stress tolerance will allow us to address basic informational gaps in biocatalyst development.

Arabinose Yeast CRADA

Biomass Program researchers are working with the National Corn Growers Association (NCGA) to design unique biocatalysts to ferment L-arabinose, one of the major components of the available sugars in corn fiber. Corn fiber is a residue of the corn-to-ethanol process and is considered a low-value by-product. Previous work, performed under a CRADA with Corn Refiners Association (CRA) and NCGA established that one of the major deficiencies in L-arabinose fermentation by the S. cerevisiae strains engineered to express bacterial araA, araB, and araD genes is poor transport of L-arabinose. Current work focuses on improving L-arabinose transport in the engineered strains. The results of these studies, which employ classical genetics, molecular biology, and recombinant DNA technology, will lead to better understanding of the metabolic mechanisms required to efficiently use the residual components in corn fiber.

DuPont CRADA

In collaboration with scientists and engineers at DuPont, Biomass Program researchers are also working to generate a superior ethanologenic Zymomonas mobilis. The new robust biocatalyst will be used in an Integrated Corn-based Bioproducts Refinery to make bioproducts from corn stover/fiber.

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