Lignin biochemistry: Biosynthesis and biodegradation

Abstract

Lignin biosynthesis via shikimate-cinnamate pathways in plants, and the biosynthetic differences of guaiacyl-and syringyl lignins between gymnosperms and angiosperms have been elucidated by tracer experiments using 14C labeled precursors and the following enzyme reactions. The formation of guaiacyl lignin but not syringyl lignin in gymnosperms was attributed to the following factors; absence of ferulate-5-hydroxylase, poor affinity of O-methyltransferase toward 5-hydroxyferulate, and lack of activation and/or reduction of sinapatc. A mechanism of lignin-carbohydrate complexes formation in wood cell walls was elucidated based on the reaction of the quinone methide of guaiacylglycerol-β-guaiacyl ether with sugars, and the analysis of DHP-polysaccharide complexes. The main cleavage mechanisms of side chains and aromatic rings of lignin model compounds and synthetic lignin (DHP) by white-rot fungi and their enzymes, lignin peroxidase and laccase have been elucidated using 2H, 13C and 18O-labeled lignin substructure dimcrs with 18O2 and H2 18O. Side chains and aromatic rings of these substrates were cleaved via aryl cation radical and phenoxy radical intermediates, in reaction mediated only by lignin peroxidase/H2O2 and laccase/O2.