PhD defence: A multilayered exploration of Aspergillus niger sugar metabolic enzymes

Filamentous fungi are one of the major players in sustainable biotechnology and are used in food, biofuels or enzyme production. Since plant biomass is a source of valuable compounds for various industries, including biofuels and bioplastics, the ability of fungi to break down complex plant materials is a feature that benefits science and industry. Aspergillus niger is a fungus that can produce a wide range of plant biomass-degrading enzymes, which are involved in the release of simple sugars, which are further absorbed and used by fungal cells.

To understand how A. niger processes these sugars, scientists build metabolic models using data from gene expression, proteomics, metabolomics, and growth profiling. However, this information is not sufficient - biochemical studies are needed to find out exactly how well each enzyme works and which substrate is preferred. This is especially important because A. niger often has multiple enzymes performing similar roles, thereby creating a highly flexible metabolic system.

This thesis focuses on several key reductases and dehydrogenases - enzymes that convert sugars or sugar alcohols inside A. niger cells. By studying their activity, gene expression, and evolutionary background, this work reveals how each enzyme contributes to fungal carbon metabolism.

Chapter 2 describes properties of D-xylose reductase B (XyrB). XyrB evolved separately from D-xylose reductase A (XyrA) and has high affinity to the pentose sugars D-xylose and L-arabinose. Even though XyrB can catalyse the conversion of many substrates, it is produced only when D-xylose and L-arabinose are present, indicating that its main role is to support the breakdown of these specific sugars.

Chapter 3 continues with L-xylulose reductase A and B (LxrA and LxrB). LxrA is a specialized enzyme: it mainly converts L-xylulose and is strongly expressed when L-arabinose is present in the culture medium. LxrB, on the other hand, can convert a wide range of sugars, suggesting its role in multiple metabolic pathways beyond the Pentose Catabolic Pathway.

Chapter 4 describes a case of precision in the carbon metabolism of A. niger. LraA, the first enzyme in the L-rhamnose pathway, is very specific and only acts on L-rhamnose. A. niger does not appear to have evolved multiple enzymes for this pathway, possibly because this sugar is only consumed when all others are finished.

Chapter 5 focuses on sorbitol dehydrogenase SbdA. Despite its name, SbdA is a general polyol dehydrogenase with moderate activity toward sorbitol and xylitol. The combined data suggest that SbdA is not part of the D-galactose pathway as previously assumed. Instead, it may play a broader role and help the cell cope with stress-related changes in sorbitol levels.