Xylitol carbon sources

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. 

The xylitol producing yeast C. guilliermondii (ATTC No. 201935) was cultivated in a semisynthetic basal salt medium (BSM) for experiments investigating cell growth and xylitol production. This medium consisted of 40 g/1 xylose (EMD Chemicals, Darmstadt, Germany) as the carbon source, 5 g/1 ammonium sulfate as the nitrogen source, 0.5 g/1 magnesium sulfate, 0.1 g/1 calcium chloride, 1 g/1 potassium sulfate, and 1 g/1 yeast extract (Fisher Scientific, Rochester, NY). The medium was prepared separately for each experiment and autoclaved for 20 min at 121 °C to assure sterility. 

As shown in Figure 18.2, a sole carbon source of xylose is metabolized into xylitol, ATP, NAD(P)H, and some metabolites through the pentose phosphate pathway. 

Host strain Genetic manipulation Culture type Initial carbon source Leeding solution Xylitol concentration (g/L) Xylitol productivity (g/L-h) Xylitol yield (g/g. %) Reference... 

Candida of the monosaccharides D-galactose, L-sorbose, and D-xylose, as well as that of a number of alditols, namely, erythritol, D- and l-arabinitol, ribitol, xylitol, D-glucitol, and D-mannitol. The yeast was grown on succinate as the sole source of carbon, and so the carriers could be considered non-induced. The authors reported214 that the substrates enter by active transport by means of four carriers (i) the first has a high affinity for alditols (ii) the second has a high affinity for the monosaccharides (in) the third has a low affinity both for alditols and monosaccharides and (iv) the fourth is specific to erythritol and D-ribose, for both of which it has a high affinity. 

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