| 1. |
- Ng, C, et al.
(författare)
-
Phase stability and tensile properties of Co-free Al 0.5CrCuFeNi2 high-entropy alloys
- 2014
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388. ; 584, s. 530-537
-
Tidskriftsartikel (refereegranskat)abstract
- High-entropy alloys (HEAs) are becoming new research frontiers in the metallic materials field. The phase stability of HEAs is of critical significance, but a convincing understanding on it has been somewhat held back by the slow diffusion kinetics, which prevents the completion of diffusion assisted phase transformations toward the equilibrium state. Here a unique methodology, combining both the thermomechanical treatments and thermodynamic calculations, was employed to reveal the phase stability of HEAs, exemplified using the newly developed Al0.5CrCuFeNi2 alloy. The metastable nature of the solid solution phases in this high-entropy alloy was uncovered through thermomechanical treatments induced phase transformations, and supported by the thermodynamic calculations. Meanwhile, the tensile properties for both the as-cast and thermomechanically treated alloys were measured, and correlated to their indentation behavior. © 2013 Published by Elsevier B.V.
|
|
| 2. |
- de Jong, Bouke Wim, 1983, et al.
(författare)
-
Improved production of fatty acid ethyl esters in Saccharomyces cerevisiae through up-regulation of the ethanol degradation pathway and expression of the heterologous phosphoketolase pathway
- 2014
-
Ingår i: Microbial Cell Factories. - : Springer Science and Business Media LLC. - 1475-2859. ; 13:1
-
Tidskriftsartikel (refereegranskat)abstract
- Background: Due to an increasing demand of transportation fuels, a lower availability of cheap crude oil and a lack of sustainability of fossil fuels, a gradual shift from petroleum based fuels towards alternative and renewable fuel resources will be required in the near future. Fatty acid ethyl esters (FAEEs) have properties similar to current crude diesel and could therefore form an important contribution to the development of sustainable transportation fuels in future. It is important to develop novel cell factories for efficient production of FAEEs and their precursors. Results: Here, a Saccharomyces cerevisiae cell factory expressing a heterologous wax ester synthase (ws2) from Marinobacter hydrocarbonoclasticus was used to produce FAEEs from ethanol and acyl-coenzyme A (acyl-CoA). The production of acyl-CoA requires large amounts of NADPH and acetyl-CoA. Therefore, two metabolic engineering strategies for improved provision of NADPH and acetyl-CoA were evaluated. First, the ethanol degradation pathway was employed to re-channel carbon flow towards the synthesis of acetyl-CoA. Therefore, ADH2 and ALD6 encoding, respectively, alcohol dehydrogenase and acetaldehyde dehydrogenase were overexpressed together with the heterologous gene acs(SE)(L641P) encoding acetyl-CoA synthetase. The co-overexpression of ADH2, ALD6 and acs(SE)(L641P) with ws2 resulted in 408 +/- 270 mu g FAEE gCDW-1, a 3-fold improvement. Secondly, for the expression of the PHK pathway two genes, xpkA and ack, both descending from Aspergillus nidulans, were co-expressed together with ws2 to catalyze, respectively, the conversion of xylulose-5-phosphate to acetyl phosphate and glyceraldehyde3-phosphate and acetyl phosphate to acetate. Alternatively, ack was substituted with pta from Bacillus subtilis, encoding phosphotransacetylase for the conversion of acetyl phosphate to acetyl-CoA. Both PHK pathways were additionally expressed in a strain with multiple chromosomally integrated ws2 gene, which resulted in respectively 5100 +/- 509 and 4670 +/- 379 mu g FAEE gCDW(-1), an up to 1.7-fold improvement. Conclusion: Two different strategies for engineering of the central carbon metabolism for efficient provision of acetyl-CoA and NADPH required for fatty acid biosynthesis and hence FAEE production were evaluated and it was found that both the ethanol degradation pathway as well as the phosphoketolase pathway improve the yield of FAEEs.
|
|
| 3. |
- Shi, Shuobo, 1981, et al.
(författare)
-
Engineering of Chromosomal Wax Ester Synthase Integrated Saccharomyces Cerevisiae Mutants for Improved Biosynthesis of Fatty Acid Ethyl Esters
- 2014
-
Ingår i: Biotechnology and Bioengineering. - : Wiley. - 0006-3592 .- 1097-0290. ; 111:9, s. 1740-1747
-
Tidskriftsartikel (refereegranskat)abstract
- In recent years, significant advances have been made to engineer robust microbes for overproducing biochemical products from renewable resources. These accomplishments have to a large extend been based on plasmid based methods. However, plasmid maintenance may cause a metabolic burden on the host cell and plasmid-based overexpression of genes can result in genetically unstable strains, which contributes to loss in productivity. Here, a chromosome engineering method based on delta integration was applied in Saccharomyces cerevisiae for the production of fatty acid ethyl esters (FAEEs), which can be directly used as biodiesel and would be a possible substitute for conventional petroleum-based diesel. An integration construct was designed and integrated into chromosomal delta sequences by repetitive transformation, which resulted in 1-6 copies of the integration construct per genome. The corresponding FAEE production increased up to 34 mg/L, which is an about sixfold increase compared to the equivalent plasmid-based producer. The integrated cassette in the yeast genome was stably maintained in nonselective medium after deletion of RAD52 which is essential for efficient homologous recombination. To obtain a further increase of FAEE production, genes encoding endogenous acyl-CoA binding protein (ACB1) and a bacterial NADP(+)-dependent glyceraldehyde-3-phosphate dehydrogenase (gapN) were overexpressed in the final integration strain, which resulted in another 40% percent increase in FAEE production. Our integration strategy enables easy engineering of strains with adjustable gene copy numbers integrated into the genome and this allows for an easy evaluation of the effect of the gene copy number on pathway flux. It therefore represents a valuable tool for introducing and expressing a heterologous pathway in yeast.
|
|
| 4. |
- Shi, Shuobo, 1981, et al.
(författare)
-
Functional expression and characterization of five wax ester synthases in Saccharomyces cerevisiae and their utility for biodiesel production
- 2012
-
Ingår i: Biotechnology for Biofuels. - : Springer Science and Business Media LLC. - 1754-6834 .- 1754-6834. ; 5, s. Art. no. 7-
-
Tidskriftsartikel (refereegranskat)abstract
- BackgroundWax ester synthases (WSs) can synthesize wax esters from alcohols and fatty acyl coenzyme A thioesters. The knowledge of the preferred substrates for each WS allows the use of yeast cells for the production of wax esters that are high-value materials and can be used in a variety of industrial applications. The products of WSs include fatty acid ethyl esters, which can be directly used as biodiesel.ResultsHere, heterologous WSs derived from five different organisms were successfully expressed and evaluated for their substrate preference in Saccharomyces cerevisiae. We investigated the potential of the different WSs for biodiesel (that is, fatty acid ethyl esters) production in S. cerevisiae. All investigated WSs, from Acinetobacter baylyi ADP1, Marinobacter hydrocarbonoclasticus DSM 8798, Rhodococcus opacus PD630, Mus musculus C57BL/6 and Psychrobacter arcticus 273-4, have different substrate specificities, but they can all lead to the formation of biodiesel. The best biodiesel producing strain was found to be the one expressing WS from M. hydrocarbonoclasticus DSM 8798 that resulted in a biodiesel titer of 6.3 mg/L. To further enhance biodiesel production, acetyl coenzyme A carboxylase was up-regulated, which resulted in a 30% increase in biodiesel production.ConclusionsFive WSs from different species were functionally expressed and their substrate preference characterized in S. cerevisiae, thus constructing cell factories for the production of specific kinds of wax ester. WS from M. hydrocarbonoclasticus showed the highest preference for ethanol compared to the other WSs, and could permit the engineered S. cerevisiae to produce biodiesel.
|
|
| 5. |
- Shi, Shuobo, 1981, et al.
(författare)
-
Prospects for microbial biodiesel production
- 2011
-
Ingår i: Biotechnology journal. - : Wiley. - 1860-6768 .- 1860-7314. ; 6:3, s. 277-285
-
Tidskriftsartikel (refereegranskat)abstract
- As the demand for biofuels for transportation is increasing, it is necessary to develop technologies that will allow for low-cost production of biodiesel. Conventional biodiesel is mainly produced from vegetable oil by chemical transesterification. This production, however, has relatively low land-yield and is competing for agricultural land that can be used for food production. Therefore, there is an increasing interest in developing microbial fermentation processes for production of biodiesel as this will allow for the use of a wide range of raw-materials, including sugar cane, corn, and biomass. Production of biodiesel by microbial fermentation can be divided into two different approaches, (1) indirect biodiesel production from oleaginous microbes by in vitro transesterification, and (2) direct biodiesel production from redesigned cell factories. This work reviews both microbial approaches for renewable biodiesel production and evaluates the existing challenges in these two strategies.
|
|
| 6. |
- Valle Rodriguez, Juan Octavio, 1984, et al.
(författare)
-
Metabolic engineering of Saccharomyces cerevisiae for production of fatty acid ethyl esters, an advanced biofuel, by eliminating non-essential fatty acid utilization pathways
- 2014
-
Ingår i: Applied Energy. - : Elsevier BV. - 1872-9118 .- 0306-2619. ; 115, s. 226-232
-
Tidskriftsartikel (refereegranskat)abstract
- Microbial production of fatty acid derived chemicals and fuels is currently of great interest due to the limited resources and increasing prices of petroleum and petroleum-based products. The development of Saccharomyces cerevisiae as a fatty acid ethyl ester (FAEE) cell factory would represent an opportunity for biodiesel production due to its successful history in the biotechnology area. However, fatty acid (FA) biosynthesis is highly regulated and usually not high enough for developing an efficient production process. In S. cerevisiae, FAs are degraded by beta-oxidation and a large fraction is utilized to synthesize steryl esters (SEs) and triacylglycerols (TAGs), which are not essential for the cell. Here, by eliminating nonessential FA utilization pathways, we developed a metabolic engineering strategy resulting in a S. cerevisiae strain that can overproduce FAs and in turn use these for producing FAEEs (biodiesel). Compared to the wild-type, there is an about 3-fold increase in free FA content in a strain devoid of both TAG and SE formation, a 4-fold increase in free FA content in a strain that is incapable of beta-oxidation, and a 5-fold increase of free FAs in a strain lacking all of these non-essential FA utilization pathways. It is also demonstrated that there are similar positive effects on FAEE production in these deletion strains. The highest production of FAEEs is 17.2 mg/l in the strain in which all these pathways were blocked. The results of this study serve as a basis for further strategies to improve the production of FA derivatives in S. cerevisiae. (C) 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
|
|
