Kinetics of ethanol production from D‐xylose by the yeast pichia stipitis

Abstract

Pichia stipitis (CBS 5773) most efficiently ferments the sugar mixture from acid hydrolysed spent brewer's grains, whereby D‐xylose and D‐galactose (but not L‐arabinose) are consumed simultaneously, after the D‐glucose had been used up from the mixture. The key enzymes, xylose reductase (Xyl‐R) and xylitol dehydrogenase (Xit‐DH) are inducible and cannot be found in glucose‐grown cells. Purified Xyl‐R was shown to use both NADH and NADPH as coenzyme, and model simulations showed that under aerobic physiological conditions NADPH is preferred (95%), whereas under oxygen deficiency 85% of xylose are reduced by NADH. Xit‐DH seems to use only NAD⁺ as coenzyme, and the reaction is competitively inhibited by NADH. According to the equilibrium constants for Xyl‐R (5,5.10⁹ 1.Mol^‐1) and for Xit‐DH (6,9.10^‐11 Mol.1^‐1) the equlibria of both reactions are distinctly shifted to xylitol, which may be excreted into the medium, especially when in the absence of oxygen a surplus of NADH cannot be dehydrogenated.