Pentose metabolism pathway, ethanol

Yomano LP, York SW, Ingram LO (1998) Isolation and characterization of ethanol-tolerant mutants of Escherichia coli koll for fuel ethanol production. J Ind Microbiol Biot hnol 20 132-138 Zhang M, Eddy C, Deanda K, Finkestein M, Picataggio S (1995) Metabolic engineering of a pentose metabolism pathway in Ethanologenic Zymomonas-mobilis. Science 267 240-243... 

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

Ethanol fermentation from xylose by yeasts can be divided into four distinctive steps. The first step is the reduction of xylose to xylitol mediated by NADPH/ NADH-linked xylose reductase (XR). This is followed by the oxidation of xylitol to xylulose, mediated by NAD-linked xylitol dehydrogenase (XDH). Xylulose-5-phosphate, the key intermediate, is generated from the phosphorylation of xylulose by xylulose kinase. Xylulose-5-phosphate is then channeled into the pentose phosphate pathway for further metabolism (Fig. 9). 

XK activity decreased with the increase in oxygen limitation (Fig. 4a). XK is a key enzyme in xylose metabolism and fulfills the initial steps of xylose metabolism together with XR and XDH, to convert xylose to xylitol, and then to xylulose in series. After that, xylulose is phosphorylated and channeled into the pentose phosphate pathway. The deerease in XK activity could be another reason for the decrease in xylose uptake rate under oxygen-limited conditions. It was reported that overexpression of the XKSl gene encoding xylulose kinase significantly increased xylose utilization and ethanol production in recombinant S. cerevisiae [32]. 

Oxygen limitation had little effect on the activities of TAL and TKL. When OTR varied from 0 to 12.6 mmol/L-h, TAL and TKL aetivities always maintained at low levels (Fig. 4c). TAL and TKL activities always maintained at low levels, to show big control effects on xylose metabolism. Walfi idsson et al. [34] overexpressed TAL in XR- and XDH-expressing S. cerevisiae, whieh increased the cell growth but not ethanol production. Karhumaa et al. [30] found that increased XR and XDH activities redirected the production from xylitol to ethanol, whereas the rate of xylose consumption was governed by the overexpressed nonoxidative pentose phosphate pathway. 

Bacteria using the heterofermentative pathway transform hexoses principally but not exclusively into lactate. The other molecules produced by this metabolism are essentially CO2, acetate and ethanol this is the pentose phosphate pathway. After being transported into the cell, a glucoki-nase phosphorylates the glucose into glucose 6-P (glucose 6-phosphate). Its destination is completely... 

Similarly, another study (Ohno et al. 2013) confirmed that deletion of the ethanol (adhE) and acetate (pta) productiOTi pathways is essential for high-titre production of -butanol in engineered E. coli cells. In this study, constraint-based metabolic flux simulation was used to predict the effect of different triple knockouts on the productivity of butanol and identify knockout candidates. Also, the authors proposed leading the flux through the pentose phosphate pathway to increase the pool of NADPH that could make further NADH available for -butanol productirai because of transhydrogenase activity (Ohno et al. 2013). 

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