Xylose

Xylose

Electrochemistry for the Generation of Renewable Chemicals: One-Pot Electrochemical Deoxygenation of Xylose to delta-Valerolactone.[Pubmed:28332296]

ChemSusChem. 2017 May 9;10(9):2015-2022.

In this study, the electrochemical conversion of Xylose to delta-valerolactone via carbonyl intermediates is demonstrated. The conversion was achieved in aqueous media and at ambient conditions. This study also demonstrates that the feedstock for production of renewable chemicals and biofuels through electrochemistry can be extended to primary carbohydrate molecules. This is the first report on a one-pot electrochemical deoxygenation of Xylose to delta-valerolactone.

Establishing a novel biosynthetic pathway for the production of 3,4-dihydroxybutyric acid from xylose in Escherichia coli.[Pubmed:28342964]

Metab Eng. 2017 May;41:39-45.

3-Hydroxy-gamma-butyrolactone (3HBL) is an attractive building block owing to its broad applications in pharmaceutical industry. Currently, 3HBL is commercially produced by chemical routes using petro-derived carbohydrates, which involves hazardous materials and harsh processing conditions. Only one biosynthetic pathway has been reported for synthesis of 3HBL and its hydrolyzed form 3,4-dihydroxybutyric acid (3,4-DHBA) using glucose and glycolic acid as the substrates and coenzyme A as the activator, which involves multiple steps (>10 steps) and suffers from low productivity and yield. Here we established a novel five-step biosynthetic pathway for 3,4-DHBA generation from D-Xylose based on the non-phosphorylative D-Xylose metabolism, which led to efficient production of 3,4-DHBA in Escherichia coli. Pathway optimization by incorporation of efficient enzymes for each step and host strain engineering by knocking out competing pathways enabled 1.27g/L 3,4-DHBA produced in shake flasks, which is the highest titer reported so far. The novel pathway established in engineered E. coli strain demonstrates a new route for 3,4-DHBA biosynthesis from Xylose, and this engineered pathway has great potential for industrial biomanufacturing of 3,4-DHBA and 3HBL.

Improved Xylose Metabolism by a CYC8 Mutant of Saccharomyces cerevisiae.[Pubmed:28363963]

Appl Environ Microbiol. 2017 May 17;83(11). pii: AEM.00095-17.

Engineering Saccharomyces cerevisiae for the utilization of pentose sugars is an important goal for the production of second-generation bioethanol and biochemicals. However, S. cerevisiae lacks specific pentose transporters, and in the presence of glucose, pentoses enter the cell inefficiently via endogenous hexose transporters (HXTs). By means of in vivo engineering, we have developed a quadruple hexokinase deletion mutant of S. cerevisiae that evolved into a strain that efficiently utilizes d-Xylose in the presence of high d-glucose concentrations. A genome sequence analysis revealed a mutation (Y353C) in the general corepressor CYC8, or SSN6, which was found to be responsible for the phenotype when introduced individually in the nonevolved strain. A transcriptome analysis revealed altered expression of 95 genes in total, including genes involved in (i) hexose transport, (ii) maltose metabolism, (iii) cell wall function (mannoprotein family), and (iv) unknown functions (seripauperin multigene family). Of the 18 known HXTs, genes for 9 were upregulated, especially the low or nonexpressed HXT10, HXT13, HXT15, and HXT16 Mutant cells showed increased uptake rates of d-Xylose in the presence of d-glucose, as well as elevated maximum rates of metabolism (Vmax) for both d-glucose and d-Xylose transport. The data suggest that the increased expression of multiple hexose transporters renders d-Xylose metabolism less sensitive to d-glucose inhibition due to an elevated transport rate of d-Xylose into the cell.IMPORTANCE The yeast Saccharomyces cerevisiae is used for second-generation bioethanol formation. However, growth on Xylose is limited by pentose transport through the endogenous hexose transporters (HXTs), as uptake is outcompeted by the preferred substrate, glucose. Mutant strains were obtained with improved growth characteristics on Xylose in the presence of glucose, and the mutations mapped to the regulator Cyc8. The inactivation of Cyc8 caused increased expression of HXTs, thereby providing more capacity for the transport of Xylose, presenting a further step toward a more robust process of industrial fermentation of lignocellulosic biomass using yeast.

Butyric acid production from lignocellulosic biomass hydrolysates by engineered Clostridium tyrobutyricum overexpressing xylose catabolism genes for glucose and xylose co-utilization.[Pubmed:28343058]

Bioresour Technol. 2017 Jun;234:389-396.

Clostridium tyrobutyricum can utilize glucose and Xylose as carbon source for butyric acid production. However, Xylose catabolism is inhibited by glucose, hampering butyric acid production from lignocellulosic biomass hydrolysates containing both glucose and Xylose. In this study, an engineered strain of C. tyrobutyricum Ct-pTBA overexpressing heterologous Xylose catabolism genes (xylT, xylA, and xylB) was investigated for co-utilizing glucose and Xylose present in hydrolysates of plant biomass, including soybean hull, corn fiber, wheat straw, rice straw, and sugarcane bagasse. Compared to the wild-type strain, Ct-pTBA showed higher Xylose utilization without significant glucose catabolite repression, achieving near 100% utilization of glucose and Xylose present in lignocellulosic biomass hydrolysates in bioreactor at pH 6. About 42.6g/L butyrate at a productivity of 0.56g/L.h and yield of 0.36g/g was obtained in batch fermentation, demonstrating the potential of C. tyrobutyricum Ct-pTBA for butyric acid production from lignocellulosic biomass hydrolysates.