China 
Africa 
Explore chapters and articles related to this topic
Metabolic Engineering of Yeast, Zymomonas mobilis, and Clostridium thermocellum to Increase Yield of Bioethanol
Published in Ayerim Y. Hernández Almanza, Nagamani Balagurusamy, Héctor Ruiz Leza, Cristóbal N. Aguilar, Bioethanol, 2023
S. Sánchez-Muñoz, M. J. Castro-Alonso, F. G. Barbosa, E. Mier-Alba, T. R. Balbino, D. Rubio-Ribeaux, I. O. Hernández-De Lira, J. C. Santos, C. N. Aguilar, S. S. Da Silva
Pentose fermentation difficulties have been solved using genetically modified organisms or co-culture of two yeast strains [152, 153]. To metabolize xylose, the microorganism must be able to incorporate pentoses into the cell via membrane transporters. Subsequently, two different pathways for xylose isomerization in xylulose can be followed: the balanced redox oxidoreductase and the isomerase pathway. The first occurs when the enzyme xylose reductase (XR) converts D-xylose to xylitol, then, the enzyme xylitol dehydrogenase (XD) transforms xylitol into D-xylulose. Later, xylose is isomerized to xylulose by the enzyme Xylose Isomerase (XI) without the need for a cofactor [149, 154]. Finally, this D-xylulose is further phosphorylated by a xylulokinase in D-xylulose-5-P, which can enter the PPP [232].
Saccharomyces Cerevisiae for First and Second Generation Ethanol Production
Published in Devarajan Thangadurai, Jeyabalan Sangeetha, Industrial Biotechnology, 2017
Fernanda Bravim, Melina Campagnaro Farias, Oeber De Freitas Quadros, Patricia Machado Bueno Fernandes
Different pathways are available in nature for metabolism of arabinose and xylose; which are converted to xylulose 5-phosphate (intermediate compound) to enter the pentose phosphate pathway as shown in Figure 10.5. In yeasts, xylose is first reduced by xylose reductase to xylitol, which in turn is oxidized to xylulose by xylitol dehydrogenase. In bacteria and some anaerobic fungi, xylose isomerase is responsible for direct conversion of xylose to xylulose. Xylulose is finally phosphorylated to xylulose-5-phosphate by xylulokinase. In fungi, L-arabinose is reduced to L-arabitol (by arabinose reductase), L-xylulose (by arabitol dehydrogenase), xylitol (by L-xylulose reductase). Xylitol is finally converted to xylulose (by xylitol dehydrogenase), whose activity is also part of xylose utilization pathways. In bacteria, L-arabinose is converted to L-ribulose (by L-arabinose isomerase), L-ribulose-5-P (by L-ribulokinase) and finally D-xylulose-5-P (by L-ribulose-5-P 4-epimerase) (Bettiga et al., 2008).
Hydrolysis and Fermentation Technologies for Alcohols
Published in Yatish T. Shah, Water for Energy and Fuel Production, 2014
For certain feedstock such as hardwood and herbaceous biomass, xylose amounts to 30%-60% of fermentable sugars. The efficient fermentation of xylose is therefore very important for the overall economics of ethanol from these feedstock. Co-fermentation of both glucose and xylose is most desirable. Xylose fermentation using pentose yeasts is difficult due to (1) the requirement of ( O2 during ethanol production, (2) the acetate toxicity, and (3) the production of xylitol as byproduct. Xylitol is a naturally occurring low-calorie sugar substitute with anticarcinogenic properties. Other approaches to xylose fermentation include conversion of xylose to xylulose using xylose isomerase prior to fermentation by S. cerevisiae and the development of genetically engineered strains [50].
Progress in microbiology for fermentative hydrogen production from organic wastes
Published in Critical Reviews in Environmental Science and Technology, 2019
Khanna et al. (2011) studied the redirection of biochemical pathways for the enhancement of H2 production by Enterobacter cloacae. Since NADH is usually generated by catabolism of glucose to pyruvate through glycolysis, and then hydrogen is produced through the oxidation of NADH. However, the conversion of pyruvate to ethanol and acids like lactic acid and butyric acid consumes NADH. Thus, they attempted to redirect the biochemical pathways to block alcohol and some of the organic acids formation in E. cloacae IIT-BT 08, increase the concentration of available NADH for hydrogen production, hydrogen yield and hydrogen production rate obtained were 2.26 mol H2/mol hexose and 1.25 L H2/L/h, which were 1.2 and 1.6 times higher than the wild type strain. Xiong et al. (2018) introduced xylA (encoding for xylose isomerase) and xylB from Thermoanaerobacter ethanolicus to cellulolytic bacteria Clostridium thermocellum DSM 1313, achieved simultaneous fermentation of xylose, glucose, cellobiose and cellulose. The results showed that both hydrogen and ethanol production were enhanced by twice when xylose and cellulose was consumed simultaneously, and hydrogen yield of 1.2 mol/mol hexose was obtained. Sekar et al. (2017) enhanced the co-production of hydrogen and ethanol from glucose in Escherichia coli by activating pentose-phosphate pathway through deletion of phosphoglucose isomerase and overexpression of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, hydrogen and ethanol yield obtained by the mutant strain was enhanced by 1.2 and 2.05 times over the wild strain, which were 1.74 mol H2/mol hexose and 1.62 mol ethanol/mol hexose, respectively.