Carbohydrate-active enzymes that modify the cell wall of Aspergillus niger: Biochemical properties and physiological functions during autolysis and differentiation

The fungus Aspergillus niger is used for the industrial production of proteins and organic acids. Fungi like A. niger grow by producing thread-like structures which are surrounded by a cell wall. This cell wall is dynamic, carbohydrate-active enzymes produce, change and degrade the carbohydrates in the cell wall. The focus of this thesis is on the physiological roles and biochemical properties of the carbohydrate-active enzymes that are made during carbon starvation. In times of nutrient shortage, the fungus can reuse his previously stored energy and carbon – for example in the cell wall – to produce stress-resistant spores. It was investigated which genes encoding carbohydrate-active enzymes are activated during carbon starvation, and how they are regulated. Two of the identified enzymes are subsequently characterized biochemically; chitinase CfcA and α-1,3-glucanase AgnB. This chitinase was found to have a role in cell wall degradation during carbon starvation. In addition, a set of genes was found that code for carbohydrate-active enzymes that are specifically activated when the fungus produces spores. These were mainly enzymes that were predicted to be active on the cell wall carbohydrates chitin and β-1,3-glucan, such as the chitinases CfcI en CtcB. Biochemical characterization of chitinase CfcI showed that it has a for fungal chitinases new activity; CfcI breaks down short chitin chains by leaving of the sugar monomer. A clear understanding of the enzymatic mechanisms behind cell wall changes, provides a firm basis for enzyme engineering, and potentially allows improvement of industrial fermentations.