Lignocelluloses are defined as plant or wood cell walls in which celluloses and hemicelluloses are intimately associated with lignin. The role of lignin is to provide strength, serve as barrier against microbial attack, and act as water impermeable seal across the cell walls in the xylem tissue (Argyropoulos and Menachem 1997). Lignin is a phenylpropanoid polymer synthesized from the phenolic precursors, coniferyl, synapyl, and p-coumaryl alcohols (Sarkanen and Ludwig 1971). Free radical condensation of these precursors initiated by plant cell wall peroxidases results in the formation of a heterogeneous, amorphous, optically inactive, random, and highly branched polymer with at least 12 different types of linkages such as aryl-ether and carbon-carbon bonds connecting the aromatic nuclei. Such structural features impose unusual restrictions on its biodegradability (Hatakka 1994; Higuchi 1990; Kirk and Farrell 1987). Most biological macromolecules such as cellulose are largely linear polymers whose subunits are linked together by a repeating bond, and thus the mechanism of polymer degradation is generally centered around the common bonds. The complexity of the lignin polymer means, however, that it is not subject to enzymatic hydrolysis and the initial attack must be oxidative, nonspecific, nonhydrolytic, and extracellular (Hatakka 1994; Higuchi 1990; Kirk and Farrell 1987). Since lignin is particularly complex to biodegrade, the cellulose, hemicellulose, and other cell wall constituents with which it is intimately associated are
*Current affiliation: Technion-Israel Institute of Technology, Haifa, Israel protected from degradation and as a result their bioavail-ability is very low. Lignin degradation, therefore, plays a central role in the earth's carbon cycle, since most renewable carbon is either in lignin or in cellulose and hemicellulose. This chapter will focus on the biodegradation of lignin by fungi, with particular emphasis on white rots, which are the most efficient degraders and the only known organisms that can completely breakdown lignin to carbon dioxide (CO2) and water (H2O) (Kirk and Farrell 1987).
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