Synergy as design principle for metabolic engineering of 1-propanol production in Escherichia coli

Synthesis of a desired product can often be achieved via more than one metabolic pathway. Whether naturally evolved or synthetically engineered, these pathways often exhibit specific properties that are suitable for production under distinct conditions and host organisms. Synergy between pathways arises when the underlying pathway characteristics, such as reducing equivalent demand, ATP requirement, intermediate utilization, and cofactor preferences, are complementary to each other. Utilization of such pathways in combination leads to an increased metabolite productivity and/or yield compared to using each pathway alone. This work illustrates the principle of synergy between two different pathways for 1-propanol production in Escherichia coli. A model-guided design based on maximum theoretical yield calculations identified synergy of the native threonine pathway and the heterologous citramalate pathway in terms of production yield across all flux ratios between the two pathways. Characterization of the individual pathways by host gene deletions demonstrates their distinct metabolic characteristics: the necessity of TCA cycle for threonine pathway and the independence of TCA cycle for the citramalate pathway. The two pathways are also complementary in driving force demands. Production experiments verified the synergistic effects predicted by the yield model, in which the platform with dual pathway for 2-ketobutyrate synthesis achieved higher yield (0.15 g/g of glucose) and productivity (0.12 g/L/h) of 1-propanol than individual ones alone: the threonine pathway (0.09 g/g; 0.04 g/L/h) or the citramalate pathway (0.11 g/g; 0.04 g/L/h). Thus, incorporation of synergy into the design principle of metabolic engineering may improve the production yield and rate of the desired compound.