Metabolic engineering of Saccharomyces cerevisiae for increased bioconversion of lignocellulose to ethanol

Engineering Zymomonas mobilis for the Production of Xylonic Acid from Sugarcane Bagasse Hydrolysate

Sugarcane bagasse is an agricultural residue rich in xylose, which may be used as a feedstock for the production of high-value-added chemicals, such as xylonic acid, an organic acid listed as one of the top 30 value-added chemicals on a NREL report. Here, Zymomonas mobilis was engineered for the first time to produce xylonic acid from sugarcane bagasse hydrolysate. Seven coding genes for xylose dehydrogenase (XDH) were tested. The expression of XDH gene from Paraburkholderia xenovorans allowed the highest production of xylonic acid (26.17 ± 0.58 g L-1) from 50 g L-1 xylose in shake flasks, with a productivity of 1.85 ± 0.06 g L-1 h-1 and a yield of 1.04 ± 0.04 gAX/gX. Deletion of the xylose reductase gene further increased the production of xylonic acid to 56.44 ± 1.93 g L-1 from 54.27 ± 0.26 g L-1 xylose in a bioreactor. Strain performance was also evaluated in sugarcane bagasse hydrolysate as a cheap feedstock, which resulted in the production of 11.13 g L-1 xylonic acid from 10 g L-1 xylose. The results show that Z. mobilis may be regarded as a potential platform for the production of organic acids from cheap lignocellulosic biomass in the context of biorefineries.

Improved robustness of an ethanologenic yeast strain through adaptive evolution in acetic acid is associated with its enzymatic antioxidant ability

AIMS

To investigate multiple tolerance of Saccharomyces cerevisiae obtained through a laboratory strategy of adaptive evolution in acetic acid, its relation with enzymatic ROS detoxification and bioethanol 2G production.

METHODS AND RESULTS

After adaptive evolution in acetic acid, a clone (Y8A) was selected for its tolerance to high acetic acid concentrations (13 g l^-1 ) in batch cultures. Y8A was resistant to multiple stresses: osmotic, thermic, oxidative, saline, ethanol, organic acid, phenolic compounds and slow freeze-thawing cycles. Also, Y8A was able to maintain redox homeostasis under oxidative stress, whereas the isogenic parental strain (Y8) could not, indicating higher basal activity levels of antioxidative enzyme Catalase (CAT) and Gluthatione S-transferase (GST) in Y8A. Y8A reached higher bioethanol levels in a fermentation medium containing up to 8 g l^-1 of acetic acid when compared to parental strain Y8.

CONCLUSIONS

A multiple-stress-tolerant clone was obtained using adaptive evolution in acetic acid. Stress cross-tolerance could be explained by its enzymatic antioxidative capacity, namely CAT and GST.

SIGNIFICANCE AND IMPACT OF THE STUDY

We demonstrate that adaptive evolution used in S. cerevisiae was a useful strategy to obtain a yeast clone tolerant to multiple stresses. At the same time, our findings support the idea that tolerance to oxidative stress is the common basis for stress cotolerance, which is related to an increase in the specific enzymes CAT and GST but not in Superoxide dismutase, emphasizing the fact that detoxification of H₂ O₂ and not O₂ ˙ is a key condition for multiple stress tolerance in S. cerevisiae.

Strategies to Improve Saccharomyces cerevisiae: Technological Advancements and Evolutionary Engineering

Bakery industries are thriving to augment the diverse properties of Saccharomyces cerevisiae to increase its flavor, texture and nutritional parameters to attract the more consumers. The improved technologies adopted for quality improvement of baker's yeast are attracting the attention of industry and it is playing a pivotal role in redesigning the quality parameters. Modern yeast strain improvement tactics revolve around the use of several advanced technologies such as evolutionary engineering, systems biology, metabolic engineering, genome editing. The review mainly deals with the technologies for improving S. cerevisiae, with the objective of broadening the range of its industrial applications.

Insights into enzyme secretion by filamentous fungi: comparative proteome analysis of Trichoderma reesei grown on different carbon sources

Trichoderma reesei is the main industrial producer of lignocellulolytic enzymes, and the secretory behavior of this fungus strongly depends on the carbon sources. To gain insights into how the T. reesei adapts to various carbons and regulates enzyme production, the extra- and intracellular proteomes of T. reesei grown in defined medium with lactose or xylose as the carbon source were investigated. Results indicated that the composition of extracellular proteome differed considerably depending on the carbons. The main cellobiohydrolases, i.e. Cel7a/Cel6 were the most abundant cellulolytic enzymes identified in both media, and found to be more abundant in lactose-grown culture. As compared to lactose, xylose can serve as a potent inducer of xylanolytic enzymes. Interestingly, most identified intracellular proteins are involved in carbon metabolism. Enzymes involved in utilization of xylose, such as d-xylose reductase (Xyl1p) and d-xylose dehydrogenase (Xyl2p), were present at elevated levels in the culture growing on xylose but only in minor amounts in the lactose culture. However, lactose induction significantly activated the expression of key enzymes involved in glycolysis pathway and citrate cycle. Importantly, the protein Xyl1p which participates both in the lactose and the xylose catabolism was verified as a potential regulator for cellulase formation in T. reesei.

BIOLOGICAL SIGNIFICANCE

This study not only gives an overview of the ubiquitous cellular changes induced by the two conventional carbon substrates, but offers the framework for understanding the mechanisms behind the carbon-dependent induction of extracellular enzymes in T. reesei. Moreover, this study provides a potential target (Xyl1p) that could be tentatively used for metabolic engineering of T. reesei for cost-effective cellulase production.

Intensification of Fructosyltransferases and Fructo-Oligosaccharides Production in Solid State Fermentation by Aspergillus awamori GHRTS

The present work was aimed to investigate the impact of the solid substrates mixture on Fructosyltransferases (FTase) and Fructo-oligosaccharides (FOS) production. An augmented simplex lattice design was used to optimize a three component mixture for FTase production. Among selected substrates corn cobs has highest impact on FTase production followed by wheat bran and rice bran. All two substrates and three substrate combinations showed the highest enzyme production than their individual levels. Among the tested various models quadratic model was found to be the best suitable model to explain mixture design. Corncobs, wheat bran and rice bran in a ratio of approximately 45:29:26 is best suitable for the FTase production by isolated Aspergillus awamori GHRTS. This study signifies mixture design could be effective utilize for selection of best combination of multi substrate for improved production of high value products under solid state fermentation.