Lignocellulosic raw materials contain much less L-arabinose than D-xylose, and solving the problem of xylose fermentation has been prioritized. The relative amounts of the sugars strongly depend on the raw material. For example, corn stover contains of 19% xylan and 3% arabinan, whereas wheat bran contains 19% xylan and 15% arabinan [58]. As a consequence, L-arabinose-utilizing strains of S. cerevisiae have been developed only re­cently. Furthermore, the conversion of L-arabinose into intermediates of the PPP requires more enzymatic reactions than the conversion of xy­lose (Fig. 2). In many bacteria, such as E. coli, L-arabinose is utilized via L-arabinose isomerase (AraA), L-ribulokinase (AraB), and L-ribulose-5- phosphate 4-epimerase (AraD) [59]. Xylulose-5-phosphate is then further metabolized via the PPP. Enzymatic activities of alternative bacterial arabi — nose and xylose utilization pathways have also been described [60-62].

The fungal arabinose utilization pathway consists of four alternating reduction-oxidation reactions (Fig. 2), where L-arabinose is converted to

D-xylitol via L-arabi(ni)tol and L-xylulose [23,63-65]. D-Xylitol is then further metabolized by XDH and XK, resulting in the PPP intermediate D-xylulose-5-phosphate. The first two complete fungal arabinose utilization pathways were recently kinetically characterized for Candida arabinofermen — tans PYCC 5603r and Pichia guilliermondii PYCC 3012 [65]. The fungal xylose and arabinose utilization pathways share the enzymes XR, XDH, and XK, since XR also reduces L-arabinose [65-68]. Indeed, all arabinose-utilizing yeast and fungi also utilize xylose, whereas not all xylose-growing yeasts uti­lize arabinose [66,69]. Similar to the fungal xylose pathway, the cofactors of the enzymes in the fungal arabinose pathway cannot be regenerated within the pathway but require oxygen or an external electron acceptor for regener­ation (Fig. 2).

3.2