(71a) Development and Optimization of An Escherichia Coli Biocatalyst for the Efficient Production of 1,2-Propanediol From Glycerol

Glycerol has become an inexpensive and abundant carbon source due to its generation as an inevitable by-product of biofuels production. Given the high degree of reduction of carbon in glycerol, fuels and reduced chemicals could be produced from glycerol at yields higher than those obtained from common sugars. Fully realizing this potential requires the metabolism of glycerol in the absence of external electron acceptors (i.e. fermentative metabolism). Unfortunately, only a small number of microorganisms, most of which are not amenable to industrial applications, were known to be capable of fermentative utilization of glycerol prior to our work. In these organisms, the ability to synthesize 1,3-propanediol (1,3-PDO) has long been considered the metabolic property that enables the fermentative utilization of glycerol. Escherichia coli and Saccharomyces cerevisiae, workhorses of modern biotechnology, do not have the capability to synthesize 1,3-PDO and therefore have been deemed unable to conduct glycerol fermentation.

Our recent discovery that this previous view was incorrect and that E. coli is indeed able to ferment glycerol in the absence of external electron acceptors, despite its inability to synthesize 1,3-PDO, has enabled its use as a platform for the conversion of glycerol into fuels and chemicals. Key to the development of efficient biocatalysts for this purpose is both a fundamental understanding of fermentative glycerol metabolism in E. coli as well as the implementation of metabolic engineering strategies based on this fundamental knowledge. The understanding of the pathways and mechanisms mediating fermentative glycerol utilization is used as a knowledge base to engineer E. coli for the efficient conversion of glycerol into fuels and chemicals. Several biocatalysts have been developed for the production of ethanol, hydrogen, formate, succinate, lactate, and 3- and 4- carbon diols from glycerol-rich streams generated during biofuels production. Most recently, we have focused on implementing specific metabolic engineering strategies for the development of a biocatalyst for the conversion of glycerol into 1,2-propanediol (1,2-PDO), a major commodity chemical in the food, drug, and cosmetic industries. High yields of 1,2-PDO were achieved through the overexpression of key enzymes involved the metabolic conversion of glycerol into 1,2-PDO, in combination with blocking the synthesis of by-products. Subsequent optimization of fermentation conditions allowed for further increases in both overall yield and productivity of the microbial conversion of glycerol into 1,2-PDO.