R. Carlson, A. Wlaschin, N. Vijayasankaran, and F. Srienc*
Department of Chemical Engineering and Materials Science and BioTechnology Institute, University of Minnesota, Minneapolis/St. Paul, MN 55455/55108, USA
Keywords: E. coli, S. cerevisiae, Pichia pastoris, PHB, network analysis
Polyhydroxyalkanoic acids (PHA) typically function as carbon and energy storage materials in prokaryotic cells and accumulate under imbalanced growth conditions when the carbon source is present in excess. PHAs have traditionally been considered to be aerobic storage compounds since PHA synthesis has been investigated almost exclusively in aerobic organisms such as Ralstonia eutropha. Also commercial production of PHAs has focused primarily on aerobic production processes that require specific design consideration to accommodate the aeration requirements. Elementary mode analysis has revealed that the metabolic pathway structure of the central metabolism of Saccharomyces cerevisiae [1] and of E. coli can accommodate synthesis of PHAs also under anaerobic growth conditions. These theoretical predictions have been confirmed in anaerobic cultivation experiments with E. coli and with yeasts that have been genetically engineered to synthesize PHA. The recombinant E. coli anaerobically accumulated PHA to more than 50% of its cell dry weight during cultivation in either growth or non-growth supporting medium. The by-product secretion profiles differed significantly between the PHB synthesizing strain and the control strain. For instance during cultivation under non-growth conditions, the PHB synthesizing culture produced approximately 50% less acetate but produced about 40% more ethanol on a glucose yield basis as compared to a control culture. A theoretical biochemical network model was used to provide a rational basis to interpret the experimental results and to study network capabilities. The used approach is generally useful for analyzing conditions for redirecting metabolite fluxes in microbes while maintaining a favorable redox balance. Anaerobic PHA synthesis could provide a useful basis for commercial PHA production.
