Lignins: Natural polymers from oxidative coupling of 4-hydroxyphenyl- propanoids

Lignins are complex natural polymers resulting from oxidative coupling of, primarily, 4-hydroxyphenylpropanoids. An understanding of their nature is evolving as a result of detailed structural studies, recently aided by the availability of lignin-biosynthetic-pathway mutants and transgenics. The currently accepted theory is that the lignin polymer is formed by combinatorial-like phenolic coupling reactions, via radicals generated by peroxidase-H2O2, under simple chemical control where monolignols react endwise with the growing polymer. As a result, the actual structure of the lignin macromolecule is not absolutely defined or determined. The ``randomness'' of linkage generation (which is not truly statistically random but governed, as is any chemical reaction, by the supply of reactants, the matrix, etc.) and the astronomical number of possible isomers of even a simple polymer structure, suggest a low probability of two lignin macromolecules being identical. A recent challenge to the currently accepted theory of chemically controlled lignification, attempting to bring lignin into line with more organized biopolymers such as proteins, is logically inconsistent with the most basic details of lignin structure. Lignins may derive in part from monomers and conjugates other than the three primary monolignols (p-coumaryl, coniferyl, and sinapyl alcohols). The plasticity of the combinatorial polymerization reactions allows monomer substitution and significant variations in final structure which, in many cases, the plant appears to tolerate. As such, lignification is seen as a marvelously evolved process allowing plants considerable flexibility in dealing with various environmental stresses, and conferring on them a striking ability to remain viable even when humans or nature alter ``required'' lignin-biosynthetic-pathway genes/enzymes. The malleability offers significant opportunities to engineer the structures of lignins beyond the limits explored to date. Abbreviations: 4CL – 4-coumarate:CoA ligase; C3H –p-coumarate 3-hydroxylase; HCT –p-hydroxycinnamoyl-CoA: quinate shikimate p-hydroxycinnamoyltransferase; CCoAOMT – caffeoyl-CoA O-methyltransferase; CCR – cinnamoyl-CoA reductase; F5H – ferulate 5-hydroxylase; CAld5H – coniferaldehyde 5-hydroxylase; COMT – caffeic acid O-methyltransferase; AldOMT – (5-hydroxyconifer)aldehyde O-methyltransferase; CAD – cinnamyl alcohol dehydrogenase; NMR – nuclear magnetic resonance (spectroscopy); DFRC – derivatization followed by reductive cleavage; TIZ – tosylation, iodination, zinc (a DFRC method); DHP – dehydrogenation polymer.