Production of polyhydroxyalkanoate from acetate by metabolically engineered Aeromonas hydrophilia.

Aeromonas hydrophila 4AK4 normally produces the copolymer of 3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx) using lauric acid as the carbon source. In this study we reported the metabolic engineering of A. hydrophila 4AK4 for the production of polyhydroxyalkanoate (PHA) using acetate as a main carbon source. Recombinant A. hydrophila overexpressing ß-ketothiolase and acetoacetyl-CoA reductase could accumulate poly-3-hydroxybutyrate (PHB) from acetate with a polymer content of 1.39 wt%. Further overexpression of acetate kinase/phosphotransacetylase and acetyl-CoA synthetase improved PHB content to 8.75 wt% and 19.82 wt%, respectively. When acetate and propionate were simultaneously supplied as carbon sources, the engineered A. hydrophila overexpressing ß-ketothiolase, acetoacetyl-CoA reductase, and acetyl-CoA synthetase was found able to produce the copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV). The recombinant grew to 3.79 g/L cell dry weight (CDW) containing 15.02 wt% PHBV. Our proposed metabolic engineering strategies illustrate the feasibility for producing PHA from acetate by A. hydrophila.

Metabolic engineering of Escherichia coli for the production of L-malate from xylose.

Malate is regarded as one of the key building block chemicals which can potentially be produced from biomass at a large scale. Although glucose has been extensively studied as the substrate for malate production, its high price and potential competition with food production are serious limiting factors. In this study, Escherichia coli was metabolically engineered to effectively produce malate from xylose, the second most abundant sugar component of lignocellulosic biomass. First, the biosynthetic route of malate was constructed by overexpressing D-tagatose 3-epimerase, L-fuculokinase, L-fuculose-phosphate aldolase, and aldehyde dehydrogenase A. Second, genes encoding malic enzyme, malate dehydrogenase, and fumarate hydratase were knocked out to eliminate malate consumption, resulting in a titer of 1.99 g/l malate and a yield of 0.47 g malate/g xylose. Third, glycolate oxidase and malate synthase were overexpressed to strengthen the conversion of glycolate to malate, which led to a titer of 4.33 g/l malate and a yield of 0.83 g malate/g xylose, reaching 93% of the theoretical yield. Finally, catalase HPII was overexpressed to decompose H2O2 and alleviate its toxicity, which improved cell growth and further boosted malate titer to 5.90 g/l with a yield of 0.80 g malate/g xylose. To the best of our knowledge, this is the first study to report efficient malate production from xylose as the carbon source.