Process Design for the Biocatalysis of Value-Added Chemicals from Carbon Dioxide

摘要

This report describes results toward developing a process to sequester CO{sub 2} centered on the enzyme pyruvate carboxylase. The process involves the use of bacteria to convert CO{sub 2} and glucose as a co-substrate and generates succinic acid as a commodity chemical product. The phases of research have included strain development and process development. Though we continue to work on one important component of strain development, the research has principally focused on process development. In the previous year we constructed several strains which would serve as templates for the CO{sub 2} sequestration, including the knock-out of genes involved in the formation of undesirable byproducts. This project period the focus has been on the integration of the pyruvate carboxylase gene (pyc) onto the E. coli chromosome. This has proven to be a difficult task because of relatively low expression of the gene and resulting low enzyme activity when only one copy of the gene is present on the chromosome. Several molecular biology techniques have been applied, with some success, to improve the level of protein activity as described herein. Progress in process development has come as a result of conducting numerous fermentation experiments to select optimal conditions for CO{sub 2} sequestration. This process-related research has progressed in four areas. First, we have clarified the range of pH which results in the optimal rate of sequestration. Second, we have determined how the counterion used to control the pH affects the sequestration rate. Third, we have determined how CO{sub 2} gas phase composition impacts sequestration rate. Finally, we have made progress in determining the affect of several potential gaseous impurities on CO{sub 2} sequestration; in particular we have completed a study using NO{sub 2}. Although the results provide significant guidance as to process conditions for CO{sub 2} sequestration and succinate production, in some cases we do not yet understand the underlying mechanism or reason for the observation. Also, process development has used the ''baseline'' organism in the absence of the pyruvate carboxylase gene. In some cases the conclusions regarding the process may change when the ''final'' strain is used which incorporates the key CO{sub 2} sequestration technology.