Redistribution of metabolic fluxes in the central aerobic metabolic pathway of E. coli mutant strains with deletion of the ackA-pta and poxB pathways for the synthesis of isoamyl acetate.
Although the bacterium E. coli is chosen as the host in many bioprocesses, products derived from the central aerobic metabolic pathway often compete with the acetate-producing pathways poxB and ackA-pta for glucose as the substrate. As such, a significant portion of the glucose may be excreted as acetate, wasting substrate that could have otherwise been used for the desired product. The production of the ester isoamyl acetate from acetyl-CoA by ATF2, a yeast alcohol acetyl transferase, was used as a model system to demonstrate the beneficial effects of reducing acetate production. All strains tested for ester production also overexpressed panK, a native E. coli gene that previous studies have shown to increase free intracellular CoA levels when fed with pantothenic acid. A recombinant E. coli strain with a deletion in ackA-pta produces less acetate and more isoamyl acetate than the wild-type E. coli strain. When both acetate-producing pathways were deleted, the acetate production was greatly reduced. However, pyruvate began to accumulate, so that the overall ester production remained largely unchanged. To produce more ester, a previously established strategy of increasing the flux from pyruvate to acetyl-CoA was adopted by overexpressing pyruvate dehydrogenase. The ester production was then 80% higher in the poxB, ackA-pta strain (0.18 mM) than that found in the single ackA-pta mutant (0.10 mM), which also overexpressed PDH.