Oxido-reductive metabolism of L-arabinose and D-galactose in filamentous fungi: Metabolic crosstalk versus specific enzymes

L-arabinose, the second most abundant pentose sugar, is used as a carbon source by a variety of microorganisms living on decaying plant material. Fungal microorganisms catabolize L-arabinose through an oxido-reductive pathway. We have identified two missing links in the pathway, L-arabinose and L-xylulose reductases in A.niger. D-galactose is a relatively rare hexose sugar in the plant cell wall mainly found in galactoglucomannan. There are three pathways indentified in fungi for D-galactose degradation; 1) the Leloir pathway in which
D-galactose is phosphorylated, 2) the oxidative pathway which starts by an extracellular galactose oxidase reaction, and 3) a recently proposed oxido-reductive pathway which resembles the pathway for L-arabinose catabolism. It has been suggested in T. reesei and A. nidulans the oxido-reductive D-galactose pathway employs the enzymes from the L-arabinose pathway. It starts with the conversion of D-galactose to D-galactitol, probably carried by the xylose/arabinose reductase. The second step is catalyzed by L-arabitol dehydrogenase and the product of the reaction is an unusual sugar L-xylo-3-hexulose. We have identified the L-xylulose reductase possesses the activity
with this intermediate which is converted to D-sorbitol. Finally, D-sorbitol is oxidized to D-fructose, which enters glycolysis. We have studied the pathway in A. niger and uncovered a more complex picture. Apart from showing the possible involvement of the L-arabinose pathway enzymes, we identified two dehydrogenases specifically induced on D-galactose, suggesting that A. niger might have specific genes for catabolism of D-galactose rather than using metabolic crosstalk suggested for T. reesei and A. nidulans.