The formation of lignin and its biochemical degradation

Abstract

The mechanism of the biogenesis of lignin has intrigued lignin chemists ever since the discovery of this complex material. The process of lignification is the transformation in certain plants whereby the aromatic lignin polymer is synthesized, ultimately from CO 2, probably by way of intermediates related to carbohydrates. Lignification is but one illustration of the more general phenomenon of aromatization, i.e. the conversion by living cells of non-aromatic precursors into compounds containing benzenoid-type rings. Fundamentally, an understanding of the biogenesis of lignin centers on knowing the enzymic pathway by which this highly polymeric aromatic compound is formed from substances pre-existing in the plant. A comprehensive solution to this problem requires consideration of two basic assumptions: First, the complex material must have its ultimate origin in certain relatively simpler units; and second, the biogenesis of lignin must involve the functioning of not one enzyme system but, rather, of several different systems, each exerting its influence in its substrate in an organized and integrated sequence, with the sum total of the reactions giving rise to a product, lignin, which is required for the existence of the mature plant. Thus, in such a biosynthetie study, both the precursors of the final product and the energetics of the reactions involved must be considered. To elucidate the mechanism of the enzymic degradation of softwood lignin by white-rot fungi, studies were conducted of the chemical properties of isolated pine and spruce lignins after decay by Polyporus versicolor, Polyporus hirsutus, Poria subacida J247 and N199, Fomes fomentarius, Fomes annosus, and Trametes pini. Determinations of the intermediate products formed from these lignins by the action of the fungi were also carried out. Products thus derived from lignin were coniferaldehyde, p-hydroxycinnamic acid, guaiacylglycerol and guaiacylglycerol-β-coniferyl ether. The results indicate the presence of the guaiacylglycerol-β-coniferyl ether unit in the lignins of pine and spruce woods. To understand the metabolism of lignin degradation products by white-rot fungi, investigations were carried out on the enzymic conversion by Polyporus versicolor and Fomes fomentarius of certain aromatic compounds structurally related to softwood lignin. Products derived in this way were vanillic acid, vanillin and dehydrodivanillin from ferulic acid and 4-hydroxy-3-methoxyphenylpyruvic acid; vanillic acid, vanillin, dehydrodivanillin and ferulic acid from coniferaldehyde and coniferyl alcohol; vanillic acid, vanillin, dehydrodivanillin and 4-droxy-3-methoxyphenylpyruvic acid from guaiacylglycerol and its β-guaiacyl ether. It is therefore suggested that 4-hydroxy-3-methoxyphenylpyruvic acid is an intermediate in the fugal decomposition of the guaiacylglycerol-β-coniferyl ether units present in softwood lignin, and that the enol form of this acid is converted to vanillic acid and vanillin. Coniferaldehyde and ferulic acid, obtained from lignin, were also converted to vanillic acid via vanillin.