Alcoholic Fermentation of d-Xylose by Yeasts

The NADP(H) redox couple in yeast metabolism

Theoretical calculations of the NADPH requirement for biomass formation indicate that in yeast this parameter is strongly dependent on the carbon and nitrogen sources used for growth. Enzyme surveys of NADPH-generating metabolic pathways and radiorespirometric studies demonstrate that in yeast the HMP pathway is the major source of NADPH. Furthermore, radiorespirometric data suggest that in yeasts the HMP pathway activities are close to the theoretical minimum. It may be concluded that the mitochondrial NADPH oxidation, which in yeasts may yield ATP, is quantitatively not an important process. The inability of C. utilis to utilize the NADH produced in formate oxidation as an extra source of NADPH strongly suggests that transhydrogenase activity is absent. Furthermore, the absence of xylose utilization under anaerobic conditions in most facultatively fermentative yeasts indicates that also in these organisms transhydrogenase activity is absent. This conclusion is supported by the observation that anaerobic xylose utilization is observed only in those yeasts which possess a high activity of an NADH-linked xylose reductase. Hence in these organisms the redox-neutral conversion of xylose to ethanol is possible, since the second step in xylose metabolism is mediated by an NAD+-linked xylitol dehydrogenase.

Aerobic Fermentation of D-Xylose to Ethanol by Clavispora sp

Eleven strains of an undescribed species of Clavispora fermented D-xylose directly to ethanol under aerobic conditions. Strain UWO(PS)83-877-1 was grown in a medium containing 2% D-xylose and 0.5% yeast extract, and the following results were obtained: ethanol yield coefficient (ethanol/D-xylose), 0.29 g g −1 (57.4% of theoretical); cell yield coefficient (dry biomass/D-xylose), 0.25 g g −1 ; maximum ethanol concentration, 5.9 g liter −1 ; maximum volumetric ethanol productivity, 0.11 g liter −1 h −1 . With initial D-xylose concentrations of 40, 60, and 80 g liter −1 , maximum ethanol concentrations of 8.8, 10.9, and 9.8 g liter −1 were obtained, respectively (57.2, 57.1, and 48.3% of theoretical). Ethanol was found to inhibit the fermentation of D-xylose ( K p = 0.58 g liter −1 ) more than the fermentation of glucose ( K p = 6.5 g liter −1 ). The performance of this yeast compared favorably with that reported for some other D-xylose-fermenting yeasts.

Properties of the NAD(P)H-dependent xylose reductase from the xylose-fermenting yeast Pichia stipitis

Xylose reductase from the xylose-fermenting yeast Pichia stipitis was purified to electrophoretic and spectral homogeneity via ion-exchange, affinity and high-performance gel chromatography. The enzyme was active with various aldose substrates, such as DL-glyceraldehyde, L-arabinose, D-xylose, D-ribose, D-galactose and D-glucose. Hence the xylose reductase of Pichia stipitis is an aldose reductase (EC 1.1.1.21). Unlike all aldose reductases characterized so far, the enzyme from this yeast was active with both NADPH and NADH as coenzyme. The activity with NADH was approx. 70% of that with NADPH for the various aldose substrates. NADP+ was a potent inhibitor of both the NADPH- and NADH-linked xylose reduction, whereas NAD+ showed strong inhibition only with the NADH-linked reaction. These results are discussed in the context of the possible use of Pichia stipitis and similar yeasts for the anaerobic conversion of xylose into ethanol.

The temperature profiles of growth, thermal death and ethanol tolerance of the xylose-fermenting yeast Candida shehatae

The temperature profile of growth and thermal death of the xylose-fermenting yeast Candida shehatae was dissociative. The ARRHENIUS plot of growth lacked a descending supraoptimal branch and the specific growth rate at the maximum temperature for growth (around 31 degrees C) was not significantly different from its values at the other temperatures studied (down to 20 degrees C). Ethanol enhanced thermal death by increasing its entropy of activation (entropy coefficient 16.1 entropy units mol-1]-1). The temperature profile of ethanol tolerance with respect to growth displayed a temperature plateau (10-17.5 degrees C) of maximum ethanol tolerance (limit 6% v/v of ethanol) while the toxic effects of ethanol increased on either side of the plateau depressing the maximum temperature for growth from 31 to 17.5 degrees C and increasing the minimum temperature for growth from 2.5 to 10 degrees C.