ABSTRACT
Lignin is an amorphous aromatic polymer, resistant to hydrolysis, which comprises some 20 to 30% dry weight of wood and thus is by far the most abundant source of the benzene nucleus on the planet. The biosynthesis of the lignin polymer from mono-, di-, tri- and oligomeric methylated lignols is believed to occur by the formation of free radicals, arising by phenolic dehydrogenations via phenol-oxidizing enzymes and their addition to quinonemethide intermediates. Most natural monomeric intermediates in lignin catabolism and the analogous ones derived from flavonoid catabolism are methoxylated and the removal of the methyl group is an essential step in preparing the molecule for ring cleavage. The simpler aromatic compounds released by ligninolytic organisms are therefore utilized immediately as growth substrates for other microorganisms or converted into humic polymers. The release of reduced aromatic alcohols from humic substances would provide an acceptable nutrient source for many soil-dwelling microorganisms.
