Pathway for xylose utilization

Although many facultatively fermentative yeasts utilize xylose as the carbon source for growth, the ability of these yeasts to produce ethanol from xylose is limited. Yeast strains that utilize xylose often produce xylitol from xylose extra-cellularly as a normal metabolic activity. However, only a few can produce significant quantities of ethanol. The prominent strains that produce ethanol from xylose include Pachysolen tannophilus, Candida shihatae and Pichia stipitis. However, the efficient production of ethanol from xylose is limited by the regulation of dissolved oxygen as well as by the imbalance of cofactors in the metabolic pathway during xylose utilization. In recent years, much effort has been put into improving yeast strains in order to produce ethanol from xylose more efficiently. 

Besides S. cerevisiae, Zymomonas mobilis and C. acetobutylicum have been also engineered with metabolic pathway for xylose and/or arabinose utilization. Clostridium... 

Figure 18.2 Metabolic pathways for the utilization of the pentose sugar xylose. XR xylose reductase and XDH xylitol dehydrogenase.

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]. 

Computational methods can also be focused on one selected pathway with finer detail, as opposed to the broad-sweeping computational methods described in Section 18.2.1.3. A kinetic model of xylose utilization by S. cerevisiae for ethanol production aimed to identify which portion of the poorly functioning pathway should be improved [85]. This analysis concluded that higher xylulokinase activity was needed. The authors experimentally verified that increasing xylulokinase activity via the expression of the E. coli xylB improves ethanol production and xylose consumption [85]. Since this initial report, a variety of other studies have reported strategies for increasing xylulokinase activity that also improve xylose utilization [86, 87], including those implemented in the thermotolerant yeast Hansenula polymorpha [88]. 

Different pathways are available in nature for metabolism of arabinose and xylose which are converted to xylulose 5-phosphate (intermediate com-poimd) 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-phos-phate 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). 

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