Building units of lignin

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

Figure 2.10. The structure of lignin (Adler, 1977). (a) Building units of lignin, (b) Enzymatic dehydrogenation of coniferyl alcohol results in a number of resonance-stabilized phenoxyl radicals. Subsequent polymerization creates the variety of linkages between phenylpropane units that is characteristic of lignin, (c) Common substmctures and their proportions, as found in Picea abies and Betula verrucosa milled wood lignins. The proportion of individual bond types is noted beside each substructure for softwoods (S/w) and hardwoods (H/w).

Other cinnamic acids also produce aldehydes, but they have no significance as odour-active substances. By analogy, coumaralde-hyde is produced from 4-coumaric acid, caffeoyl aldehyde from caffeic acid, conifer aldehyde (also known as ferulyl aldehyde) from ferulic acid, 5-hydroxyconifer aldehyde from 5-hydroxyferulic acid and sinapyl aldehyde from sinapic acid. These aldehydes can be reduced to the corresponding alcohols, with which they play a role as the building units of lignin biosynthesis. 

The literature is devoid of any studies on the electrophoretic behavior of lignins. However, due to the presence of at least one phenolic hydroxyl group per lignin building unit, it appeared that such an investigation would be feasible. Moreover, due to the success of applying electrophoretic methods to the characterization and fractionation of proteins, a study as to the possible availability of the method to lignins was undertaken in this laboratory. 

The major polymers that make up the wall are polysaccharides and lignin. These occur together with more minor but very important constituents such as protein and lipid. Water constitutes a major and very important material of young, primary walls (2). The lignin is transported in the form of its building units (these may be present as glucosides) and is polymerized within the wall. Those polysaccharides which make up the matrix of the wall (hemicelluloses and pectin material) are polymerized in the endomembrane system and are secreted in a preformed condition to the outside of the cell. Further modifications of the polysaccharides (such as acetylation) may occur within the wall after deposition. Cellulose is polymerized at the cell surface by a complex enzyme system transported to the plasma membrane (3). 

The effect of the maleic anhydride content in the copolymerization reaction mixture on the degree of carboxylation is illustrated by the data in Table I. Carboxylation of kraft lignin and ligninsulfonates may easily reach levels of 0.7 to 0.8 maleic anhydride units per lignin-building Cg-unit, if the lignin to maleic anhydride ratio in the reactor increases to 2 1. 

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