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In order to rationally manipulate the cellular metabolism of Escherichia coli for D: -lactate production, single-gene and multiple-gene deletions with mutations in acetate kinase (ackA), phosphotransacetylase (pta), phosphoenolpyruvate synthase (pps), pyruvate formate lyase (pflB), FAD-binding D-lactate dehydrogenase (dld), pyruvate oxidase (poxB), alcohol dehydrogenase (adhE), and fumarate reductase (frdA) were tested for their effects in two-phase fermentations (aerobic growth and oxygen-limited production). Lactate yield and productivity could be improved by single-gene deletions of ackA, pta, pflB, dld, poxB, and frdA in the wild type E. coli strain but were unfavorably affected by deletions of pps and adhE. However, fermentation experiments with multiple-gene mutant strains showed that deletion of pps in addition to ackA-pta deletions had no effect on lactate production, whereas the additional deletion of adhE in E. coli B0013-050 (ackA-pta pps pflB dld poxB) increased lactate yield. Deletion of all eight genes in E. coli B0013 to produce B0013-070 (ackA-pta pps pflB dld poxB adhE frdA) increased lactate yield and productivity by twofold and reduced yields of acetate, succinate, formate, and ethanol by 95, 89, 100, and 93%, respectively. When tested in a bioreactor, E. coli B0013-070 produced 125 g/l D-lactate with an increased oxygen-limited lactate productivity of 0.61 g/g h (2.1-fold greater than E. coli B0013). These kinetic properties of D-lactate production are among the highest reported and the results have revealed which genetic manipulations improved D-lactate production by E. coli.

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Biodiesel has attracted considerable attention as one of the best choices among alternative and renewable fuels. Large quantities of crude glycerol are produced as a main co-product with increasing biodiesel production. Currently, the problem of waste glycerol utilization needs to be crucially addressed, not only for environmental protection but also for the economy of the biodiesel industry. In this paper, the use of crude glycerol for the production of D-lactate by engineered Escherichia coli was investigated. Engineered E. coli B0013-070 with a homolactic pathway for D-lactate synthesis by elimination of byproduct pathways (ethanol, succinate, formate and acetate) could convert 20 g L −1 of crude glycerol to 13.6 g L −1 of D-lactate with a yield of 0.67 g g −1 glycerol. Overexpression of D-lactate dehydrogenase by a low-copy vector in E. coli B0013-070 resulted in the increased production and yield of D-lactate, in which 14.5 g L −1 of D-lactate was produced with a yield of 0.72 g g −1 glycerol from crude glycerol. The effect of temperature on D-lactate fermentation by the engineered strain E. coli B0013-070-pTHldhA was also investigated, and 34 °C and 40 °C were found to be the optimal temperatures for cell growth and lactate production, respectively. The engineered strain B0013-070-pTHldhA produced 100.3 g L −1 of D-lactate with 99.97% optical purity from 531.5 g of crude glycerol with an overall productivity of 2.78 g L −1 h −1 and a yield of 75.4 g per 100 g glycerol (0.77 mol mol −1 ) using two phase fermentation combined with a temperature shifting strategy in a 7 L bioreactor. In summary, this paper shows that crude glycerol could be directly converted to D-lactate without any prior purification. † Electronic supplementary information (ESI) available. See

L-Lactic acid, one of the most important chiral molecules and organic acids, is produced via pyruvate from carbohydrates in diverse microorganisms catalyzed by an NAD+-dependent L-lactate dehydrogenase. Naturally, Escherichia coli does not produce L-lactate in noticeable amounts, but can catabolize it via a dehydrogenation reaction mediated by an FMN-dependent L-lactate dehydrogenase. In aims to make the E. coli strain to produce L-lactate, three L-lactate dehydrogenase genes from different bacteria were cloned and expressed. The L-lactate producing strains, 090B1 (B0013-070, ΔldhA::diflldD::Pldh-ldhLca), 090B2 (B0013-070, ΔldhA::diflldD::Pldh-ldhStrb) and 090B3 (B0013-070, ΔldhA::diflldD::Pldh-ldhBcoa) were developed from a previously developed D-lactate over-producing strain, E. coli strain B0013-070 (ack-ptappspflBdldpoxBadhEfrdA) by: (1) deleting ldhA to block D-lactate formation, (2) deleting lldD to block the conversion of L-lactate to pyruvate, and (3) expressing an L-lactate dehydrogenase (L-LDH) to convert pyruvate to L-lactate under the control of the ldhA promoter. Fermentation tests were carried out in a shaking flask and in a 25-l bioreactor. Strains 090B1, 090B2 or 090B3 were shown to metabolize glucose to L-lactate instead of D-lactate. However, L-lactate yield and cell growth rates were significantly different among the metabolically engineered strains which can be attributed to a variation between temperature optimum for cell growth and temperature optimum for enzymatic activity of individual L-LDH. In a temperature-shifting fermentation process (cells grown at 37°C and L-lactate formed at 42°C), E. coli 090B3 was able to produce 142.2 g/l of L-lactate with no more than 1.2 g/l of by-products (mainly acetate, pyruvate and succinate) accumulated. In conclusion, the production of lactate by E. coli is limited by the competition relationship between cell growth and lactate synthesis. Enzymatic properties, especially the thermodynamics of an L-LDH can be effectively used as a factor to regulate a metabolic pathway and its metabolic flux for efficient L-lactate production.HighlightsThe enzymatic thermodynamics was used as a tool for metabolic regulation. ► minimizing the activity of L-lactate dehydrogenase in growth phase improved biomass accumulation. ► maximizing the activity of L-lactate dehydrogenase improved lactate productivity in production phase.

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To find a lipase for synthesis of flavor esters in food processing, a total of 35 putative lipases from Aspergillus niger F0215 were heterologously expressed and their esterification properties in crude preparations were examined. One of them, named An-lipase with the highest esterification rate (23.1%) was selected for further study. The purified An-lipase had the maximal activity at 20 °C and pH 6.5 and the specific activity of 1293 U/mg. Sixty percent of the activity was maintained in a range of temperatures of 0-30 °C and pHs of 3.0-8.5. The highest hydrolysis activity of An-lipase was towards pNPC (C8), followed by pNPB (C4) and pNPA (C2), then pNPL (C12). K m , V max , k cat, and k cat /K m towards pNPC were 26.7 mmol/L, 129.9 mmol/ (L h), 23.2 s −1 , and 0.8/mM/s, respectively. The ethyl lactate, butyl butyrate, and ethyl caprylate flavor esters were produced by esterification of the corresponding acids with conversion efficiencies of 15.8, 37.5, and 24.7%, respectively, in a soybeanoil-based solvent system. In conclusion, An lipase identified in this study significantly mediated synthesis of predominant flavor esters (ethyl lactate, butyl butyrate, and ethyl caprylate) in a soybean-oil-lacking other toxic organic solvents, which has potential application in food industries.