Lignin, decomposition

R. Hammerschmidt, D. T. A. Lamport, and E. P. Muldoon, Cell wall hydroxypro-line enhancement and lignin decomposition as an early event in the resistance of cucumber to Cladosporium cucumerin, Physiology of Plant Pathology 24 43 (1984). 

However, there are two articles which report evidence against these relationships between biosynthesis of this secondary metabolite and lignin decomposition (43,44). For example, a mutant of P. chrysosporium, which does not produce veratryl alcohol, has ligninolytic activity (43). Another mutant, which lacks glucose oxidase, is unable to decompose lignin to CO2 and therefore is ligninase Mess. It is, however, able to produce about 30% of the amount of veratryl alcohol normally found in the fungus (44). 

Figure 1. Structure of identified lignin decomposition products...

Guaiacylglycerol-/3-coniferyl ether is a dimeric decomposition product of coniferous lignin dehydrodivanillin may be also formed by dimerization of vanillin after formation from lignin. Among the lignin decomposition products are mono-, di- and triphenol derivatives which are derived from the different types of lignin of needle trees, deciduous trees, or graminees. 

In Figure 2 the further reactions of the lignin decomposition products, which involve biological oxidation in the soil or in cultures or microorganisms, are summarized. All of the secondary reaction products could be identified. 

In order to eluddate the mechanism of the further transformations of the primary lignin decomposition products including the cleavage reactions, we synthesized some of the important primary products labelled with Cu and introduced these into the cultures of fungi or enzymes. Thus, it could be shown, for example, that the breakdown of the side chain of ferulic acid occurs at the double bond vanillic acid is found. During polymerization in the presence of phenoloxidases, in the case of carboxyl-labelled ferulic acid, about 60% of the activity is split off as Cli02. The polymers labelled in the 2 and 3 position in the side chain or in the methoxyl group contain the whole applied activity. 

Other experiments show that phenolcarboxylic acids labelled in the carboxyl group form radioactive polymers by simultaneously forming Cl4(>2. This means that not all C1402 is split off, and the polymerization does not occur only through quinones as intermediate steps. Demethylation of the lignin decomposition products is a further important reaction because o-benzoquinones can be formed under certain conditions. 

We studied the demethylation of lignin decomposition products labelled in the C atom of the methoxyl group. After demethylation the resulting 1,2-diphenols form p- or o-benzoquinones by further oxidation under mild conditions depending upon the substitution of the side chain, as shown in Figure 5. In the case of demethylation of partially methylated 1,2,3-triphenols, formation... 

The various observations and conclusions are summarized in Figure 6 where a complete scheme for lignin decomposition is proposed. It has some interesting implications for the process of humification. As Figure 6 shows, the following steps occur in the decomposition of lignin ... 

Dr. Flaig. We could show that, by oxidative polymerization of lignin decomposition products with side chains containing one or more carbon atoms, polymer-like humic substances are formed. The polymerization can be carried out with mild oxidizing agents or in the presence of phenoloxydases. 

Dr. Flaig. The mechanism of the reaction between amino acids and oxidized lignin decomposition products can be described by the scheme of Trautner, which involves oxidative deamination. 

Dr. Flaig. The polymeric products of lignin decomposition formed in the presence of phenol oxidases are not water soluble. 

W. Kaminsky, W. Schweers and H. Schwesinger, Properties and Decomposition of Lignins, Decomposition by Pyrolysis in a Fluidized Bed, Holzforschung, 34, 73-76 (1980). 

Osono T. and Takeda H. (1999) Decomposing ability of interior and surface fungal colonizers of beech leaves with reference to lignin decomposition. Euro. J. Soil Biol. 35(2), 51-56. 

Larvae do the damage extensive digestion of cellulose and hemicel-luloses lignin decomposition reported (57)... 

In the following discussion, we have assumed that wood decay is due to the concerted action of the individual enzyme systems responsible for cellulose, hemicellulose, and lignin decomposition. 

Many past studies on the enzymes of lignin decomposition focused on phenol-oxidizing enzymes, such as laccase and peroxidase, produced by white-rot fungi. It is unlikely, however, that this type of activity is important in structural decomposition (33), although the enzymes may have some other role in lignin decomposition (28). 

Scheme 1. Model compounds are being used to elucidate the specific reactions of lignin decomposition by white-rot fungi. The fate of a nonphenolic dia l-propane-tupe of model in ligninolutic cultures of Phanerochaete chrysosporium Burds is shown (113-15).

---