Pentose-Utilizing Yeasts

The xylose transport across cell membrane is the initial step of the xylose utilization by tee yeasts. Pentose-fermenting yeasts generally posses multiple xylose uptake systems, which can be classified as low- or hi -affinity systems. The types of transport system vaiy between the yeasts and depend on nutritional conditions (82, i, 4, J). In batch processes, the increase in sugar concentration... 

Candida utilis is grown on sulfite waste Hquor in Western Europe and North America, on sugar cane molasses in Cuba and Taiwan and on ceUulose acid hydrolysates in Eastern Europe and the former Soviet Union. C. ///i/if utilizes hexoses, pentoses, and many organic acids. Sulfite Hquor from hardwoods contains 2—3% fermentable sugars of which 20% are hexoses and 80% pentoses in softwood Hquors the proportions are reversed. The SO2 must be stripped out to allow yeast growth, which is carried out in large, highly-aerated fermentors. Eor continuous fermentations, carried out at pH 4 and 30°C, the dilution rate is 0.27—0.30 (34). 

The transaldolase (EC 2.2.1.2) is an ubiquitous enzyme that is involved in the pentose phosphate pathway of carbohydrate metabolism. The class I lyase, which has been cloned from human [382] and microbial sources [383], transfers a dihydroxyacetone unit between several phosphorylated metabolites. Although yeast transaldolase is commercially available and several unphosphorylated aldehydes have been shown to be able to replace the acceptor component, preparative utilization has mostly been limited to microscale studies [384,385] because of the high enzyme costs and because of the fact that the equilibria usually are close to unity. Also, the stereochemistry of transaldolase products (e.g. 38, 40) [386] matches that of the products from the FruA-type DHAP aldolase which are more effortlessly obtained. 

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. 

Some eukaryotes are unable to assimilate pentoses efficiently. For instance, the yeast Saccha-romyces cerevisiae cannot metabolize D-xylose, although four strains were found to utilize... 

It is D-fructofuranose, not D-fructopyranose, that is utilized, at least by bakers yeast.303,304 As with D-glucose, the initial step in the intracellular utilization of either D-fructofuranose305 or D-mannose is phosphorylation by the constitutive hexokinase306 (see Ref. 307 for a review). The D-fructose 6-phosphate formed is an intermediate of both the glycolytic pathway and the pentose cycle. D-Mannose phosphate isomerase (EC 5.3.1.8) effects the conversion of D-mannose 6-phosphate into D-fructose 6-phosphate,308,309 or D-mannose 6-phosphate is epimerized to D-glucose 6-phosphate.308... 

There are various, interrelated reasons why the abilities of yeasts to utilize pentoses and alditols should be associated. (i) Pentoses and pentitols may be reversibly interconverted by dehydrogenases (it) catabolic routes may be shared and (Hi) many of the alditol dehydrogenases have particularly wide substrate-specificity, so that a single enzyme may act on pentitols and pentoses, as well as on hexi-tols and hexoses. 

Abilities of 497 Yeast Strains to Utilize Pairs of Substrates Pentoses, Alditols, D-Calactose, and L-r(/io-Hexulose ... 

Yeasts that Naturally Utilize Xylose and Other Pentoses 681 Conclusions 681 C onflict of Intere st 682 References 682... 

Yeasts that Naturally Utilize Xylose and Other Pentoses... 

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