Abstract

L-Arabinose is the second most abundant pentose beside D-xylose and is found in the plant polysaccharides, hemicellulose and pectin. The need to find renewable carbon and energy sources has accelerated research to investigate the potential of L-arabinose for the development and production of biofuels and other bioproducts. Fungi produce a number of extracellular arabinanases, including α-L-arabinofuranosidases and endo-arabinanases, to specifically release L-arabinose from the plant polymers. Following uptake of L-arabinose, its intracellular catabolism follows a four-step alternating reduction and oxidation path, which is concluded by a phosphorylation, resulting in D-xylulose 5-phosphate, an intermediate of the pentose phosphate pathway. The genes and encoding enzymes L-arabinose reductase, L-arabinitol dehydrogenase, L-xylulose reductase, xylitol dehydrogenase, and xylulokinase of this pathway were mainly characterized in the two biotechnological important fungi Aspergillus niger and Trichoderma reesei. Analysis of the components of the L-arabinose pathway revealed a number of specific adaptations in the enzymatic and regulatory machinery towards the utilization of L-arabinose. Further genetic and biochemical analysis provided evidence that L-arabinose and the interconnected D-xylose pathway are also involved in the oxidoreductive degradation of the hexose D-galactose.

Degradation of different l-arabinose polymers in pectin. Different types of arabinan and arabinogalactan are present as side chains of rhamnogalacturonan I in pectin. Endo-processive arabinanases (ABN) attack randomly the backbone of arabinan, while exo-acting α-l-arabinofuranosidases (ABF) release the terminal α-l-arabinofuranoside residues

l-arabinose and d-xylose catabolism in fungi. The first step of the l-arabinose pathway is catalyzed by two different aldose reductases. In A. niger, the l-arabinose-specific reductase (LarA) is responsible for the main activity, while in T. reesei the d-xylose reductase XYL1 is responsible for the first step in l-arabinose assimilation. The last two shared steps of the two pentose catabolic pathways result in d-xylulose 5-phosphate, which is further assimilated via the pentose phosphate pathway

A second oxidoreductive pathway of d-galactose utilization. A second pathway for d-galactose uilization was recently identified in T. reesei and A. nidulans. It starts with the reduction of d-galactose to galactitol by the d-xylose reductase XYL1 (aldose reductase) in T. reesei. A hypothetical draft of the pathway for the further degradation of galactitol is summarized