Effect of NADPH availability on free fatty acid production in Escherichia coli

Microbial conversion of renewable carbon sources to free fatty acids has attracted significant attention in recent years. Accumulation of free fatty acids in Escherichia coli by overexpression of an acyl-ACP thioesterase which can break the fatty acid elongation has been well established. Various efforts have been made to increase fatty acid production in E. coli by enhancing the enzymes involved in the fatty acid synthesis cycle or host strain manipulations. The current study focused on the effect of NADPH availability on free fatty acids (FFAs) productivity. There are two reduction steps in the fatty acid elongation cycle which are catalyzed by beta keto-ACP reductase (FabG) and enoyl-ACP reductase (FabI), respectively. It is reported that FabI can use either NADH or NADPH as cofactor, while FabG only uses NADPH in E. coli. Fatty acid production dropped dramatically in the glucose-6-phosphate dehydrogenase (encoded by the zwf gene) deficient strain. Similarly, the pntB (which encodes one of the subunit of proton-translocating membrane bounded transhydrogenase PntAB) and udhA (which encodes the energy dependent cytoplasmic transhydrogenase UdhA) double mutant strain also showed an 88.8% decrease in free fatty acid production. Overexpression of PntAB and NadK restored the fatty acid production capability of these two mutant strains. These results indicated that the availability of NADPH played a very important role in fatty acid production.