Progress Toward Biological Pretreatment - Understanding Plant Cell Wall Degradation by Termite and White-Rot Fungi
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In an effort developing more efficient and environmentally benign lignocellulosic biomass processing technologies, the authors’ laboratory has been studying plant cell wall degradation process in termite and white-rot fungi. This presentation summarizes the results so far obtained from various experimental investigations of the lab. First, termite guts provide both aerobic and anaerobic environments, the former is required for lignin degradation. The termite guts also have close to neutral pH range, which suggest existence of lignin degrading enzymes that are different from that of fungi. The organization of the gut system allows the conversion of biomass in termite to occur as a sequential process in which the pretreatment of the biomass occurs in the foregut and midgut, whereas the hydrolysis of the cellulose occurs mainly in the hindgut. There are synergies between termite and its symbiont in deconstructing plant cell walls. The chewing action of the termite increases the surface area for enzymatic actions, the enzymes produced by the termite tissues modifies the bonds that forms the barriers for cellulase access and symbionts produce the enzymes that are mainly responsible for cutting the cellulose polymer to sugar units. The results shows the actual changes of lignin structure as the biomass material pass through the termite guts. The lignin structure changes are more modification than degradation in nature. Equally import is the deconstruction of the association of lignin with other components of plant cell walls such as hemicelluloses that occurs in the foregut and midgut. It becomes apparent by comparing with other biological systems that the enzyme systems and their actions in biological plant cell wall degradation systems are more complex than what was previously known. A generality has also been observed in that most of the biological systems studied tended to reduce G unit from the original lignin structure. Additionally, termite is a more effective plant cell wall degradation system than white rot-fungus because of the chewing action of the termite and the better-controlled environment in the termite guts. Termite is also more effective than the cow stomach as the latter lacks capability of lignin modification. Developing an artificial system that can surpass the natural biological system in major performance criteria may offer the ultimate solutions to lignocellulosic biomass processing and utilizations.
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