BioC: Biochemical characterization of fungal subgroup C-chitinases

Chitinases (E.C. 3.2.1.14) hydrolyze the linear biopolymer chitin, a polysaccharide of ß-(1, 4) linked N- acetylglucosamine units. Glycoside hydrolase (GH) family 18 represents an ancient chitinase family found in all kingdoms of life. It is also the largest GH family of carbohydrate-active enzymes in fungi that are able to parasitize chitin-containing organisms, i.e. parasites of invertebrates and mycoparasites (fungi that parasitize other fungi). In addition to the roles of chitinases during various forms of exogenous chitin degradation, they are also involved in remodeling and recycling of the fungus` own cell wall during growth and development. It is not yet understood how the fungus differentiates between self- and non-self-fungal cell wall degradation. Fungal chitinases can be divided into three different subgroups, A, B and C. Subgroup C chitinases have several conspicuous properties that distinguish them from other fungal chitinases. They contain several carbohydrate binding modules in addition to their catalytic domain and genome data indicate that they are three to five times larger than other fungal chitinases. None of these proteins has been biochemically characterized yet, although e.g. the mycoparasite Trichoderma virens has 15 subgroup C chitinase genes. A more detailed understanding of the mature protein forms and substrate specificities of subgroup C chitinases will be important toward enhancing our knowledge of GH family 18 chitinases. This project therefore aims at analyzing the properties of the mature subgroup C chitinase proteins, their biochemical characteristics and their subcellular localization. The results from this project will answer important biochemical questions for this group of proteins and will significantly advance our understanding of the complex chitinolytic system of filamentous fungi and of GH family 18 chitinases in general.

Chitin is the second most abundant polymer in nature after cellulose. It is a sustainable natural resource that is ubiquitous but still relatively under-exploited commercially, although products based on chitin-derivatives find numerous applications in agriculture, cosmetics, water treatment and medicine. Chitin is a structural polysaccharide found in the cell walls of fungi, the cuticle of insects, shells and the exoskeleton of crustaceans. It does not visibly accumulate in nature, which shows that these huge amounts of annually produced chitin also are degraded each year. The main degraders of chitin are microorganisms (bacteria and fungi), which are able to use chitin as a nutrient source. However, details about the chitin turnover cycle in nature are not known yet. Based on data from genome sequencing programmes, we were only in the last few years able to begin to understand the large variety of chitin-degrading enzymes and chitin-binding proteins that can be found in fungi. The classification of fungal chitinases into different subgroups, based on our work, is now widely accepted. Ongoing efforts focus on the biochemical characterization of chitin-degrading enzymes and chitin-binding proteins and their gene expression patterns in order to elucidate their detailed biological functions as well as their biotechnological application potential. Particularly the biochemical analysis of chitin-binding proteins revealed recently not only unexpected, new features such as surface-activity altering properties and different carbohydrate-binding specificities, but phylogenetic analyses showed also fungal-specific amino acid sequence features which opens up new potential biological roles as well as application possibilities for these proteins.