Biosynthesis of Lignin

Lignin biosynthesis begins in the cytosol,where glycosylated monolignol is synthesized from the amino acid phenylalanine.These first reactions are shared with the phenylpropanoid pathway.The attached glucose makes them water soluble and less toxic.Polymerization begins once the m-glucose is removed through the cell.Even after more than a century of research,much about its anabolic metabolism is still not understood.The polymerization step,ie radical-radical coupling, is catalyzed by oxidases.Both peroxidases and laccases are found in plant cell walls,and it is not known whether one or both of these groups are involved in polymerization.Low molecular weight oxidizing agents may also be involved.Oxidases Catalyze Free Radicals to Form Lignin Monoenzyme free radicals are generally thought to undergo uncatalyzed coupling to form lignin polymers. Another theory invokes unspecified biological control.

Biodegradation 

Lignin is less prone to degradation than other biopolymers such as proteins,DNA, and even cellulose.It is not affected by acid and base catalyzed hydrolysis.Degradability varies by species and plant tissue type.For example, syringyl (S) lignin is more susceptible to fungal decay degradation because it has fewer aryl-aryl bonds and a lower redox potential than guaiac-based units.Because it cross-links with other cell wall components,lignin minimizes the accessibility of cellulose and hemicellulose to microbial enzymes,resulting in reduced digestibility of biomass.Some ligninolytic enzymes include heme peroxidases such as lignin peroxidase,manganese peroxidase,multifunctional peroxidase and dye decolorizing peroxidase and copper-based laccases.Lignin peroxidases oxidize non-phenolic lignins,whereas manganese peroxidases oxidize only phenolic structures.Dye-depigmenting peroxidases,or DyPs,exhibit catalytic activity against a variety of lignin model compounds,but their in vivo substrates are unknown.In general, laccases oxidize phenolic substrates,but some fungal laccases have been shown to oxidize non-phenolic substrates in the presence of synthetic redox mediators.

Lignin degradation by fungi

Well-studied ligninolytic enzymes are found in Phanerochaete chrysosporium and other white-rot fungi.Some white-rot fungi,such as Ceriporipsis subvermispora,can degrade lignin in woody cells,but others lack this ability.A number of fungal laccases are also secreted that facilitate the degradation of phenolic lignin-derived compounds,although several intracellular fungal laccases have also been described.Peroxidase and other heme peroxidases.

Lignin degradation by bacteria

Bacteria lack most of the enzymes fungi use to degrade lignin, and lignin derivatives (fatty acids, furans, and dissolved phenolics) inhibit bacterial growth.However, bacterial degradation can be quite extensive,especially in aquatic systems such as lakes,rivers and streams,where inputs of terrestrial material (e.g. fallen leaves) may enter waterways.The ligninolytic activity of bacteria has not been extensively studied, although it was first described in 1930.Many bacterial DyPs have been characterized.Bacteria do not express any plant-type peroxidases (lignin peroxidase, manganese peroxidase, or multifunctional peroxidase),but three of the four classes of DyP are present only in bacteria.In contrast to fungi, most bacterial enzymes involved in lignin degradation are intracellular, including two classes of DyPs and most bacterial laccases.In the environment, lignin can be degraded biologically by bacteria or abiotically by photochemical alterations, and often the latter contributes to the former.In addition to the presence or absence of light, several environmental factors can affect lignin biodegradability including bacterial community composition, mineral incorporation, and redox state.

Pyrolysis

During wood burning or charcoal production, the pyrolysis of lignin produces a range of products, the most characteristic of which are methoxyl-substituted phenols.Among them, the most important are guaiacol and eugenol and their derivatives.Their presence can be used to trace the source of smoke to a wood fire.In cooking, lignin in the form of hardwoods is an important source of these two compounds, which give smoked foods such as barbecue their characteristic aroma and taste.The main flavor compounds of prosciutto are guaiacol and its 4- 5-and 6-methyl derivatives and 2,6-dimethylphenol.These compounds are produced by the thermal decomposition of lignin in the wood used in smokehouses.