Bio-products

SPECIALTY CHEMICALS FROM FERMENTATION

Researchers: Mark A. Eiteman and Elliot Altman, Engineering

Biochemical conversion is one approach to generating products from lignocellulosic biomass. This process follows a series of general process operations which include identifying the biomass, various separation/extraction steps, pretreatments, the conversion itself and subsequent purification steps. Although the particular substrate and chemical product determine the details of each operation, the complex structure of biomass invariably necessitates that its components be broken down by various depolymerization strategies. Biochemical conversion itself relies on microorganisms and/or enzymes to break down the complex structure ultimately into simpler and useable fuels and chemicals. Microorganisms and enzymes have been used for thousands of years for a wide variety of processes and products such as fuel ethanol, amino acids, vitamins, antibiotics, and even contact lens cleaning solutions.

At the University of Georgia, a major research thrust is applying fermentation technology to convert complex mixtures derived from lignocellulosic biomass into fuels and chemicals. Such an approach involves both characterizing and improving microorganisms, as well as developing bioprocess strategies which optimize the formation of a desired product.

We are currently working on several technologies and chemical products.

SUCCINIC ACID


Researcher Sarah Lee works with a fermenter producing succinic acid using metabolically engineered organisms.
Succinic acid is a four carbon dicarboxylic acid which has diverse applications in the food, pharmaceutical and cosmetics industries, and can also serve as a four carbon building block for polymers. Moreover, the production of succinic acid actually requires the use of carbon dioxide, and therefore the process offers one means of "sequestering" this greenhouse gas. Although succinic acid is a ubiquitious biochemical, microorganisms will generally not accumulate this product sufficiently for commercial production. Our group has developed microorganisms which produce significant quantities of succinic acid by using the enzyme pyruvate carboxylase.

Reference
R.R. Gokarn, M.A. Eiteman, E. Altman, "Metabolically engineered E. coli for enhanced production of oxaloacetate-derived biochemicals." U.S. Patent 6,455,284.

PYRUVIC ACID

Pyruvic acid is a three carbon ketoacid and an important raw material for the production of L-tryptophan, L-tyrosine, 3,4-dihydroxyphenyl-L-alanine, and for the synthesis of many drugs and biochemicals. Alanine is the smallest chiral amino acid and can be synthesized in one step from pyruvate via the enzyme alanine dehydrogenase. We are studying the production of pyruvate, alanine and other compounds derived from pyruvate in microorganisms. In order to accumulate compounds which are so central in metabolism, the cells must be unable to metabolize pyruvate. We have therefore developed approaches to terminate metabolism abruptly, and cause cells to accumulate several three-carbon compounds.

References
Y. Zhu, M.A. Eiteman, K. DeWitt, E. Altman, "Homolactate Fermentation by Metabolically Engineered Escherichia coli," Applied and Environmental Microbiology, 73(2), 456-464 (2007).
G.M. Smith, S.A. Lee, K.C. Reilly, M.A. Eiteman, E. Altman, "Fed-batch two-phase production of alanine by a metabolically engineered Escherichia coli," Biotechnology Letters, 28, 1695-1700 (2006).
M.A. Eiteman, E. Altman, "Microbial production of pyruvate and other metabolites," U.S. Patent application 20050255572.

SIMULTANEOUS CONVERSION OF MIXED SUGARS

The hydrolysis of lignocellulosic biomass generates a complex mixture of sugars and inhibitors. Surprisingly, a single microorganism alone is unable to convert multiple sugars simultaneously. Instead, any given microorganism has a complex regulatory network which forces sugars to be metabolized sequentially. This sequential nature invariably reduces the overall rate of a fermentation process to generate the desired product. We have developed a novel approach that successfully overcomes the inability to convert a mixture of sugars and inhibitors, thereby increasing the process yield and rate. The concept centers on the fact that we can readily develop a single organism that will only utilize one substrate. The approach is illustrated by considering xylose and glucose, just two components of lignocellulosic hydrolysate. Using this example, an organism can be developed which is unable to consume xylose. In a bioreactor containing both xylose and glucose, this organism would never consume xylose but would consume glucose normally. Similarly, another organism can be made to consume only consume xylose. The advantage occurs when the two organisms are used together in one bioreactor. By having both organisms simultaneously into a medium containing glucose and xylose, each strain will act on one sugar alone and be unaffected by the presence of the other sugar or the other strain. In this way the mixture of xylose and glucose can be converted much more efficiently than any single microorganism can accomplish.

Reference
M.A. Eiteman, S.A. Lee, E. Altman, "A co-fermentation strategy to consume sugar mixtures effectively," Journal of Biological Engineering, 2, 3 (2008).