Enzymatic modification of lignin by oxidoreductases from wood-degrading fungi
Wood-degrading fungi play an important role in many terrestrial ecosystems and the enzymes that they secrete are becoming increasingly important as catalysts in technical processes. The project is focused on enzymes that catalyze redox reactions and whose function is not well understood.
The project is financed by the Swedish Research Council.
Cellulose, hemicellulose, and lignin are the three major organic constituents of wood. In nature, many microorganisms are involved in decay of wood constituents. Initial steps are carried out by different kinds of filamentous fungi, including white-rot fungi, brown-rot fungi, and soft-rot fungi. For microorganisms it is easier to break down the polysaccharides, i.e. cellulose and hemicellulose, than to break down lignin. Although other microorganisms can be involved, mineralization of lignin to carbon dioxide is mainly carried out by white-rot fungi. The reasons for this are not well understood. The overall aim of the project is to improve understanding of the biochemical mechanisms behind the degradation of wood in general and lignin in particular.
To degrade wood, white-rot fungi secrete enzymes, i.e. biological catalysts that accelerate the process. Substances such as cellulose and lignin are too large to be directly absorbed by fungal hyphae. By secreting enzymes that break down large substances into smaller fragments, fungal cells can utilize all major wood constituents. The fungi secrete many kinds of enzymes and in some cases their functions are well known. Still, new enzymes are discovered, and their physiological roles are not always known. The project focuses on enzymes whose function is poorly known, and in particular enzymes involved in redox reactions, particularly enzymes that directly or indirectly are driven by molecular oxygen. For some of the enzymes that will be studied in the project, previous research has indicated that they are involved in the degradation of cellulose, while other studies have suggested that they instead are involved in the degradation of lignin. One of the objectives of the project is to shed light on this.
Degradation of lignin is fundamentally different from the degradation of cellulose, as complete degradation of lignin requires access to molecular oxygen. However, around a decade ago, a new enzyme was discovered that is involved in the degradation of cellulose and hemicellulose, but which can be driven by oxygen. With this discovery, it became even more difficult to distinguish clearly between degradation of cellulose, which is mainly a hydrolytic process (decomposition under uptake of water), and degradation of lignin, which is mainly an oxidative process (a redox process based directly or indirectly on the availability of oxygen from the air). Possibly this newly discovered enzyme is involved both in the degradation of cellulose and lignin. One of the objectives of the project is to shed light on this.
Some enzymes have the ability to oxidize certain lignin substructures, while other enzymes have the ability to reduce them. White-rot fungi secrete both kinds of enzymes, but it is not known why. One of the objectives of the project is therefore to clarify how these different types of enzymes affect and interact with each other. Overall, the project will provide a better understanding of enzymes that are secreted by wood-degrading fungi. Fungi that degrade wood are important for the carbon cycle in nature. According to some theories, the lack of or insufficient microbial capacity to decompose lignin is behind the large accumulation of organic matter in the earth's crust in the form of coal. Recently, enzymes from fungi involved in wood biodegradation have become important in industry. By means of recombinant DNA technology, large quantities of technically useful enzymes can be produced, e.g. for production of chemicals from renewable raw materials. These chemicals may in future replace fossil oil-based chemicals. The rapid development of this area is largely based on research into technologically interesting microorganisms and enzymes