The sugars produced by the hydrolysis of lignocellulose are glucose from cellulose, and xylose, arabinose, glucose, mannose and galactose from hemicellulose and galac — turonic acid from pectin. Saccharomyces cerevisiae can ferment glucose, mannose and galactose but not the other sugars. There are some yeasts which can metabolize xylose, but only a few can ferment xylose to ethanol. One approach was to introduce xylose catabolism into ethanol-producing yeast. Xylose-utilizing yeasts Pichia stipitis, Pachysolen tannophilus and Candida shehatae have very fastidious growth condi­tions. In S. cerevisiae, xylose is reduced to xylitol and xylitol is reduced to xylulose which is slowly metabolized. In the xylose-metabolizing yeasts, xylose is converted into xylulose by xylose reductase and xylitol dehydrogenase. In the reaction NADH and NADP are produced which need to be regenerated and can normally be carried out aerobically, but under anaerobic conditions no electron acceptors are available. A recombinant S. cerevisiae has been produced with the complete xylose pathway from P. stipitis, but the cells only metabolize xylose aerobically (Fig. 8.31) (Prasad et al., 2007), and under anaerobic conditions xylitol was produced.

Arabinose is also present in lignocellulose hydrolysates which S. cerevisiae cannot ferment. Overexpression of all structural genes of a fungal arabinose pathway pro­duced a S. cerevisiae strain capable of fermenting arabinose. However, the production was too low to be commercial.

Another approach is to expand the substrate range of ethanol-producing bacteria. One example was to insert the Z. mobilis ethanol pathway into microorganisms which can use xylose, such as Escherichia coli. This involves the insertion of pyruvate decarboxylase and alcohol dehydrogenase (Fig. 8.32).

Another group of enzymes of interest are the cellulases and genetic manipulation has been used to increase their efficiency and production. To date cellulases have been expressed in S. cerevisiae, Z. mobilis, E. coli and Klebsiella oxytoca but with limited success (Chang, 2007).