Heterologous Expression of Carbohydrate-Active Enzymes in Filamentous Fungi

Efficient biomass degradation into fermentable sugars is still a major challenge in the biotechnology field. The heterogeneity and structural complexity of the fiber make enzyme action difficult. Fungi belonging to the phylum Ascomycota and Basidiomycota, as pathogenic or saprophytic organisms, are adapted to infect/obtain nutrients from various carbon and nitrogen sources, including several kinds of wood, soil, and organic waste materials. According to genomic, proteomic and biochemical data, the efficiency of fungi to break down plant cell walls is due to their capacity to produce a wide range of CAZymes. These microorganisms have a complex machinery to secrete a broad spectrum of enzymes for releasing carbon and nitrogen locked in complex substrates for nutrition. The majority of proteins secreted by filamentous fungi are glycosylated and their capacity to secrete proteins is probably faster than their synthesis. In spite of their efficiency, the production level of many proteins of interest in natural strains is too low for commercial exploitation. However, industrial strains have shown remarkable improvements in protein secretion yield after traditional mutagenesis techniques. The first part of this chapter is focused on polysaccharide structures and covers the main fungal enzymes used as a strategy for its degradation. Then, a revision on the heterologous expression of hemicellulases by filamentous fungi hosts will be presented, along with the main bottlenecks in fungal heterologous expression. Finally, we provide an extensive revision of the commercial enzymes derived from filamentous fungi systems.

Keywords: Aspergillus nidulans, Aspergillus niger, auxiliary enzymes, client proteins, cellulases, filamentous fungi, hemicellulases, heterologous expression, plant biomass, plant cell wall, Trichoderma reesei.