L-Rhamnose Dehydrogenase LraA of Aspergillus niger Shows High Substrate Specificity Matching Its Expression Profile

L-rhamnose is one of the main monomeric sugars of rhamnogalacturonan I and II, which are polysaccharide components of pectin. In the ascomycete fungus Aspergillus niger it is metabolized through the non-phosphorylated L-rhamnose pathway, of which the first step is catalyzed by L-rhamnose dehydrogenase (LraA), converting L-rhamnose into L-rhamnono-γ-lactone. This enzyme belongs to PFAM PF00106, unlike most of other reductases/dehydrogenases involved in fungal sugar catabolism that are typically assigned to PF00248 and PF00107. The enzymes of those families have broad substrate specificity and in some cases have been shown to be involved in multiple pathways. In this study we heterologously produced and biochemically characterized A. niger LraA and studied its expression on a set of monosaccharides. This revealed that, in contrast to other metabolic redox enzymes, LraA is highly specific for L-rhamnose and has no activity on most other substrates tested in this study. This specificity is matched by a highly specific expression profile, which only shows significant expression on L-rhamnose. It therefore can be concluded that LraA has evolved with a highly specific function in fungal sugar catabolism, unlike most other sugar reductases/dehydrogenases described so far. The high specificity of LraA also affects its biotechnological applications, as it may benefit L-rhamnose-based processes, but would be less suitable for applications involving conversion of multiple sugars.