Highly efficient production of L-homoserine in Escherichia coli by engineering a redox balance route.

L-Homoserine is a nonessential chiral amino acid and the precursor of L-threonine and L-methionine. It has great potential to be used in the pharmaceutical, agricultural, cosmetic, and fragrance industries. However, the current low efficiency in the fermentation process of L-homoserine drives up the cost and therefore limits applications. Here, we systematically analyzed the L-homoserine production network in Escherichia coli to design a redox balance route for L-homoserine fermentation from glucose. Production of L-homoserine from L-aspartate via reduction of the tricarboxylic acid cycle intermediate oxaloacetate lacks reducing power. This deficiency could be corrected by activating the glyoxylate shunt and driving the flux from fumarate to L-aspartate with excess reducing power. This redox balance route decreases cell growth pressure and the theoretical yield of L-homoserine is 1.5 mol/mol of glucose without carbon loss. We fine-tuned the flux from fumarate to L-aspartate, deleted competitive and degradative pathways, enhanced L-homoserine efflux, and generated 84.1 g/L L-homoserine with 1.96 g/L/h productivity and 0.50 g/g glucose yield in a fed-batch fermentation. This study proposes a novel balanced redox metabolic network strategy for highly efficient production of L-homoserine and its derivative amino acids.