| Abstract: | Escherichia coli and many other microorganisms synthesize aromatic amino acids through the condensation reaction between phosphoenolpyruvate (PEP) and erythrose 4-phosphate to form 3-deoxy-D-arabinoheptulosonate 7-phosphate (DAHP). It has been shown that overexpression of transketolase increases the production of DAHP in an aroB mutant strain (unable to further metabolize DAHP) with elevated DAHP synthase. However, the yield (percent conversion) of DAHP from glucose is still low. Stoichiometric analysis shows that many enzymes compete for intracellular PEP. In particular, the phosphotransferase system, responsible for glucose transport in E. coli, uses PEP as a phosphate donor and converts it to pyruvate, which is less likely to recycle back to PEP. This stoichiometric limitation greatly reduces the yield of aromatic metabolites. To relieve this limitation, we overexpressed PEP synthase in the presence of glucose and showed that it increased the final concentration and the yield of DAHP by almost twofold, to a near theoretical maximum. The PEP synthase effect is not observed without overproduced transketolase, suggesting that erythrose 4-phosphate is the first limiting metabolite. This result demonstrates the utility of pathway analysis and the limitation of central metabolites in the high-level overproduction of desired metabolites. |
