Fungal arabinan and l-arabinose metabolism

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.

Sorbitol dehydrogenase of Aspergillus niger, SdhA, is part of the oxido‐reductive d‐galactose pathway and essential for d‐sorbitol catabolism

“…Another hypothesis is that L-xylo-3-hexulose is reduced to D-sorbitol which is subsequently oxidised to D-fructose. This would make the pathway very similar to the fungal L-arabinose pathway in which L-arabinose goes through a similar sequence of two oxidations and two reductions to form D-xylulose [4]. Fig.…”

“…Yet another eukaryotic pathway for D-galactose catabolism was suggested to be active in filamentous fungi. It is an oxido-reductive pathway with similarities to the fungal L-arabinose pathway [4]. In Trichoderma reesei a deletion of the galactokinase gene of the Leloir pathway was not sufficient to abolish D-galactose catabolism [5].…”

Genomics Review of Holocellulose Deconstruction by Aspergilli

“…Carbohydrate side chain-hydrolyzing enzymes are enzymes that hydrolyze sugars linked to the main chain of hemicellulose. Arabinose is the second most abundant sugar in hemicellulose and pectin (171), being found in arabinoxylan and arabinan. Arabinoxylan is constituted by a ␤-1,4-linked xylopyranose backbone with heterogeneous side chains, such as L-arabinose, O-acetyl, ferulic acid, p-coumaric acid, and 4-O-methylglucuronic acid (172).…”

l-xylo-3-Hexulose Reductase Is the Missing Link in the Oxidoreductive Pathway for d-Galactose Catabolism in Filamentous Fungi

“…Seiboth and Metz (11) pointed out the possibility that the L-xylo-3-hexulose is reduced to D-sorbitol in a reaction catalyzed by an enzyme related to L-xylulose reductase or by the L-xylulose reductase itself. The complete oxidoreductive Dgalactose pathway would then have the intermediates D-galactose, galactitol, L-xylo-3-hexulose, D-sorbitol, and D-fructose (see Fig.…”