Ethanol utilization, pathway, yeast

Phosphatidylethanolamine synthesis begins with phosphorylation of ethanol-amine to form phosphoethanolamine (Figure 25.19). The next reaction involves transfer of a cytidylyl group from CTP to form CDP-ethanolamine and pyrophosphate. As always, PP, hydrolysis drives this reaction forward. A specific phosphoethanolamine transferase then links phosphoethanolamine to the diacylglycerol backbone. Biosynthesis of phosphatidylcholine is entirely analogous because animals synthesize it directly. All of the choline utilized in this pathway must be acquired from the diet. Yeast, certain bacteria, and animal livers, however, can convert phosphatidylethanolamine to phosphatidylcholine by methylation reactions involving S-adenosylmethionine (see Chapter 26). 

As previously mentioned and in the earlier discussion of fermentation methanol, bacteria of the genus Zymomonas such as Z. mobilis are known to convert hexoses to ethanol at high yields and short residence times. These bacteria are facultative anaerobes that have fermentative capacity and convert only glucose, fructose, and sucrose to equimolar quantities of ethanol and CO2 the pentoses are not converted. The Entner-Doudoroff pathway is utilized instead of the Embden-Meyerhof pathway, and a net yield of 1 mol of ATP is generated, not 2 mol as in bakers yeast. But pyruvate is the same key intermediate. In Z. mobilis, it is decarboxylated by pyruvate decarboxylase to yield acetaldehyde which is then reduced to ethanol by alcohol dehydrogenase. 

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. 

Dekkera/Brettanomyces are unique with regard to carbohydrate utilization. Like other yeasts, Dekkera/Brettanomyces possess the glycolytic pathway and so will produce ethanol from acetaldehyde along with NAD, the latter being reused for the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate (Fig. 1.10). However, carbohydrates can also be oxidized to acetate (acetic acid) through acetaldehyde as follows ... 

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]. 

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