L-Xylulose reductase A and B from Aspergillus niger have distinct biochemical properties and expression profiles

D-Xylose and L-arabinose are major components of plant biomass and as such also important carbon sources for most fungi and attractive compounds for biotechnology. Most fungi use the pentose catabolic pathway to convert these sugars through several redox reactions. One of the steps in this pathway is the conversion of L-xylulose to xylitol, catalyzed by L-xylulose reductase (LXR). Genetic studies in Aspergillus niger revealed the involvement of two LXR-encoding genes, lxrA and lxrB. In this study, we compared the corresponding enzymes, LxrA and LxrB, with respect to substrate specificity and kinetic properties, which revealed significant differences between them. Evaluation of these genes and their homologs from A. niger and selected other fungi revealed high diversity at the level of number of homologs per species, phylogenetic relationship and expression profiles, suggesting species-specific adaptations in fungal sugar metabolism. This study therefore not only provides more detailed insights into an ecologically and biotechnologically important fungal metabolic pathway, but also demonstrates the high diversity of sugar metabolism in fungi. This is especially relevant when knowledge from one species is transferred to another, e.g., for the engineering of fungal cell factories.