Lignin biosynthetic

Experimental data on hydrodynamic properties of natural and biosynthetic lignins in dilute solutions are reviewed. The results of viscometric, translational diffusion, and sedimentation studies provide evidence for the macromolecnles of lignin having a complex topological structure. The benefits and limitations of the theoretical approaches used to describe the topology of lignin macromolecular chains are discussed. The relationship between hydrodynamic characteristic and fractal properties of lignin is considered. 

Comparisons made between unpurified spruce milled-wood lignin fractions and unfractionated poljrmers made from coniferjd alcohol alone already revealed the great qualitative similarity between natural and biosynthetically duplicated lignins [see, for example, K. Freudenberg in 94), pp, 125—126]. When a purified lignin fraction is compared tvith an identical fraction made from a mixture of all three -coumaryl alcohols, the resemblance is of course much greater. 

The similarities in the structure and 3neld of the acids obtained by this degradation from natural spruce lignin and a biosynthetic "spruce lignin copolymer (mixed DHP) made in vitro from a mixture of the three -coumaryl alcohols established the general identity of the two preparations 46, 47). 

Isherwood, F. A. Bio.synthesis of Lignin. In Pridham, J. B., and T. Swain (Editors) Biosynthetic Pathways in Higher Plants. Academic Press, London-New York, 1965. 

Capsaicinoids are synthesized by the condensation of vanillylamine with a short chain branched fatty acyl CoA. A schematic of this pathway is presented in Fig. 8.4. Evidence to support this pathway includes radiotracer studies, determination of enzyme activities, and the abundance of intermediates as a function of fruit development [51, 52, 57-63], Differential expression approaches have been used to isolate cDNA forms of biosynthetic genes [64-66], As this approach worked to corroborate several steps on the pathway, Mazourek et al. [67] used Arabidopsis sequences to design primers to clone the missing steps from a cDNA library. They have expanded the schema to include the biosynthesis of the key precursors phenylalanine and leucine, valine and isoleucine. Prior to this study it was not clear how the vanillin was produced, and thus the identification of candidate transcripts on the lignin pathway for the conversion of coumarate to feruloyl-CoA and the subsequent conversion to vanillin provide key tools to further test this proposed pathway. 

Cellulose, hemicelluloses, and lignin are the main components of cell walls in woody plants. For a long time, these plant polymers have stimulated the interest of many plant botanists and biochemists in terms of their biosynthetic pathways, functional interrelationships, and anatomical distribution. 

Secondly, a biosynthetic investigation on lignin variation was undertaken using maize internodes. Maize internodes were examined in this study since (a) fewer plants were required for analysis (greater biomass) and (b) possible variations between normal and b.m-mutants (21) could be studied. Lignin contents and monomer composition were compared between internodes, both being collected at the top and the bottom of the maize stem. These plant parts were chosen because of differences in the digestibility of different internodes as documented for Timothy (Phleum pratense)... 

In a recent study (54), we showed increased activities of two enzymes of the general phenylpropanoid pathway, PAL and 4-coumarate CoA lig-ase, as well as one enzyme of the specific pathway of lignin biosynthesis, cinnamy 1-alcohol dehydrogenase (CAD), in resistant plants at the time of the hypersensitive host cell death. On the other hand, decreased activities were observed at the same time with susceptible host plants (54). Furthermore, we showed that the well known increase in peroxidase activities, which is strong in resistant and only weak in susceptible plants (55-58), is at least partly due to the increased activity of the lignin biosynthetic pathway (54,59). 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

The latter three compounds are esterified to the y-carbon of the monolignols. These compounds are found in higher quantities in certain mutants or genetically engineered plants in which the expression of specific lignin biosynthetic genes has been altered (Sederoff et al., 1999 Boeijan... 

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