Controlled limited aeration and metabolic regulation during the production of ethanol from D-xylose by Pichia stipitis

Abstract

Under aerobic conditions Pichia stipitis CBS 5773 can grow on D-xylose with reduced specific growth rate, compared to D-glucose, but with comparable yield. In the absence of oxygen xylose is converted to ethanol and xylitol. Under conditions of controlled limited respiration ethanol production is increased whereas no xylitol is formed. With the aim of maximizing ethanol formation batch processes were performed with constant oxygen transfer rates by means of computer controlled stirrer speed. At a volumetric oxygen uptake rate (OUR) of 3.75 mmol l −1 h −1 the highest ethanol production rate was found to be 0.12 g ethanol per g cell mass and hour at a product yield, Y of 0.39. From our enzyme kinetic studies we calculate that, under aerobiosis, xylose in the first step is reduced to xylitol nearly exclusively with NADPH as the coenzyme, and that in the absence of oxygen NADH is the preferred electron donor. The activities of the two key enzymes, xylose reductase and xylitol dehydrogenase, seem to be under the control of reciprocal competitive and non-competitive inhibition by their substrates/products and the cofactors. These regulations can explain the kinetic behaviour of this yeast.