The xylose reductase/xylitol dehydrogenase/xylulokinase ratio affects product formation in recombinant xylose-utilising Saccharomyces cerevisiae

Data simulations based on a kinetic model implied that under simplified sim ulation conditions a 1:greater than or equal to 10:greater than or equal to 4 relation of the xylose reductase (XR)/xylitol dehydrogenase (XDH)/xylulok inase (XK) ratio was optimal in minimising xylitol formation during xylose utilisation in yeast. The steady-state level of the intermediary xylitol de pended also, to a great extent, on the NADH and NAD(+) concentrations. Anae robic xylose utilisation was investigated for three different recombinant. XR-, XDH- and XK-expressing Saccharomyces cerevisiae strains, TMB 3002, TMB 3003 and TMB 3004, to verify the model predictions. Overexpression of XK w as found to be necessary for ethanol formation from xylose. Furthermore, th e xylitol formation decreased with decreasing XR/XDH ratio, while the ethan ol formation increased. Of the three strains, TMB 3004, which was the strai n with a XR/XDH/XK ratio corresponding to the theoretical optimal ratio, fe rmented xylose to ethanol most efficiently. (C) 2001 Elsevier Science Inc. All rights reserved.