Systematic improvement of isobutanol production from D-xylose in engineered Saccharomyces cerevisiae

As the importance of reducing carbon emissions as a means to limit the serious effects of global climate change becomes apparent, synthetic biologists and metabolic engineers are looking to develop renewable sources for transportation fuels and petroleum-derived chemicals. In recent years, microbial production of high-energy fuels has emerged as an attractive alternative to the traditional production of transportation fuels. In particular, the Baker’s yeast Saccharomyces cerevisiae, a highly versatile microbial chassis, has been engineered to produce a wide array of biofuels. Nevertheless, a key limitation of S. cerevisiae is its inability to utilize xylose, the second most abundant sugar in lignocellulosic biomass, for both growth and chemical production. Therefore, the development of a robust S. cerevisiae strain that is able to use xylose is of great importance. Here, we engineered S. cerevisiae to efficiently utilize xylose as a carbon source and produce the advanced biofuel isobutanol. Specifically, we screened xylose reductase (XR) and xylose dehydrogenase (XDH) variants from different xylose-metabolizing yeast strains to identify the XR–XDH combination with the highest activity. Overexpression of the selected XR–XDH variants, a xylose-specific sugar transporter, xylulokinase, and isobutanol pathway enzymes in conjunction with the deletions of PHO13 and GRE3 resulted in an engineered strain that is capable of producing isobutanol at a titer of 48.4 ± 2.0 mg/L (yield of 7.0 mg/g d-xylose). This is a 36-fold increase from the previous report by Brat and Boles and, to our knowledge, is the highest isobutanol yield from d-xylose in a microbial system. We hope that our work will set the stage for an economic route for the production of advanced biofuel isobutanol and enable efficient utilization of lignocellulosic biomass.

Co-production of bio-oil and propylene through the hydrothermal liquefaction of polyhydroxybutyrate producing cyanobacteria

A polyhydroxybutyrate (PHB) producing cyanobacteria was converted through hydrothermal liquefaction (HTL) into propylene and a bio-oil suitable for advanced biofuel production. HTL of model compounds demonstrated that in contrast to proteins and carbohydrates, no synergistic effects were detected when converting PHB in the presence of algae. Subsequently, Synechocystis cf. salina, which had accumulated 7.5wt% PHB was converted via HTL (15% dry weight loading, 340°C). The reaction gave an overall propylene yield of 2.6%, higher than that obtained from the model compounds, in addition to a bio-oil with a low nitrogen content of 4.6%. No propylene was recovered from the alternative non-PHB producing cyanobacterial strains screened, suggesting that PHB is the source of propylene. PHB producing microorganisms could therefore be used as a feedstock for a biorefinery to produce polypropylene and advanced biofuels, with the level of propylene being proportional to the accumulated amount of PHB.

From first generation biofuels to advanced solar biofuels

Roadmaps towards sustainable bioeconomy, including the production of biofuels, in many EU countries mostly rely on biomass use. However, although biomass is renewable, the efficiency of biomass production is too low to be able to fully replace the fossil fuels. The use of land for fuel production also introduces ethical problems in increasing the food price. Harvesting solar energy by the photosynthetic machinery of plants and autotrophic microorganisms is the basis for all biomass production. This paper describes current challenges and possibilities to sustainably increase the biomass production and highlights future technologies to further enhance biofuel production directly from sunlight. The biggest scientific breakthroughs are expected to rely on a new technology called "synthetic biology", which makes engineering of biological systems possible. It will enable direct conversion of solar energy to a fuel from inexhaustible raw materials: sun light, water and CO2. In the future, such solar biofuels are expected to be produced in engineered photosynthetic microorganisms or in completely synthetic living factories.

Metabolic engineering of Zymomonas mobilis for 2,3-butanediol production from lignocellulosic biomass sugars

BACKGROUND

To develop pathways for advanced biofuel production, and to understand the impact of host metabolism and environmental conditions on heterologous pathway engineering for economic advanced biofuels production from biomass, we seek to redirect the carbon flow of the model ethanologen Zymomonas mobilis to produce desirable hydrocarbon intermediate 2,3-butanediol (2,3-BDO). 2,3-BDO is a bulk chemical building block, and can be upgraded in high yields to gasoline, diesel, and jet fuel.

RESULTS

2,3-BDO biosynthesis pathways from various bacterial species were examined, which include three genes encoding acetolactate synthase, acetolactate decarboxylase, and butanediol dehydrogenase. Bioinformatics analysis was carried out to pinpoint potential bottlenecks for high 2,3-BDO production. Different combinations of 2,3-BDO biosynthesis metabolic pathways using genes from different bacterial species have been constructed. Our results demonstrated that carbon flux can be deviated from ethanol production into 2,3-BDO biosynthesis, and all three heterologous genes are essential to efficiently redirect pyruvate from ethanol production for high 2,3-BDO production in Z. mobilis. The down-selection of best gene combinations up to now enabled Z. mobilis to reach the 2,3-BDO production of more than 10 g/L from glucose and xylose, as well as mixed C6/C5 sugar streams derived from the deacetylation and mechanical refining process.

CONCLUSIONS

This study confirms the value of integrating bioinformatics analysis and systems biology data during metabolic engineering endeavors, provides guidance for value-added chemical production in Z. mobilis, and reveals the interactions between host metabolism, oxygen levels, and a heterologous 2,3-BDO biosynthesis pathway. Taken together, this work provides guidance for future metabolic engineering efforts aimed at boosting 2,3-BDO titer anaerobically.