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Kinetic studies are presented for the growth and fermentation of the yeast Pichia stipitis with xylose as the carbon source. Ethanol is produced from xylose under anaerobic as well as under oxygen-limiting conditions but only at dissolved oxygen concentrations up to 3 mumol/L Maximum yields and production rates were obtained under oxygen-limiting conditions, where the xylose metabolism may be considered to be consisted of three different components (assimilation, respiration, fermentation). The contribution of each pathway is determined by the availability of oxygen and the energy yield of each pathway. In order to describe the course of oxygen-limited fermentations, a mathematical model has been developed with the assumption that growth is coupled to the energy production. The resulting model requires only four independent parameters (Y(x/O(2) ), Y(ATP) (max), m(ATP), and P/O). These parameters were estimated on the basis of eight separate batch fermentations.

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Xylose reductase from the xylose-fermenting yeast Pichia stipitis was purified to electrophoretic homogeneity via ion-exchange, gel and affinity chromatography. At physiological pH values the thermodynamic equilibrium constant was determined to be 0.575x101° (1-mol-1). Product inhibiton studies are reported which clearly show that the kinetic mechanism of the xylose reductase is ordered-bi-bi with isomerisation of a stable enzyme form.