Lignin precursors hydroxylation

Lignin precursors p-coumaryl (p-CA), coniferyl (CA) and sinapyl (SA) alcohols, are synthesized through the shikimate and cinamic acid pathways, starting from phenylalanine which under the action of phenylalanine ammonia-lyase (PAL) is deaminated followed by hydroxylations of the aromatic ring, methyla-tions and reductions of therminal acidic group to an alcohol leading to the formation of the monolignols (Fig. 8.2), [37, 48]. 

The three major monolignols (Fig. 9.2) used to synthesize lignin polymers are formed in the cytoplasm via the shikimate pathway which produces phenylalanine as key intermediate [7-9]. Through further enzyme mediated deamination, hydroxylation, reduction and methylation reactions, the final lignin precursors are formed as depicted in Fig. 9.3. 

Fig. 1 Precursors of lignin biosynthesis. (A) / -Coumaryl alcohol (4-hydroxyl-phenyl propane) (B) coniferyl alcohol (guaiacyl) and (C) sinapyl alcohol (syringyl).

Following the earlier suggestion of Gellerstedt [101], Leary and Giampaolo [70] attribute the irreversible darkening of unbleached TMP and the absorption maximum at 420 nm to the formation of hydroxylated o-quinones. The precursors of these hydroxylated quinones are lignin groups that can be removed by peroxide or borohydride, e.g., coniferaldehyde, quinones or aryl carbonyls. 

The structures of the lignoids 1, 2 and 3 illustrate some common and some different features. All three compounds comprise two C6-C3-moieties, numbered 1 to 9 and V to 9, and hence their monomers are presumably metabolites of the shikimate pathway. Since this pathway is restricted to plants, the biosynthesis of 3 is a mystery. Could enterolactone be a bacterial degradation product of lignins ingested with plant food, or is it synthesized ex novo (133) If so, what are its precursors - a pertinent question since enterolactone lacks oxy-functions at the 4- and 4 -positions of the aryls, a structural particularity not shared with any other naturally occurring C6-C3-dimer known. Indeed free p-hydroxyls are essential for the oxidative coupling of the monomers regardless of the mechanism of the reaction. 

The building blocks of wood lignin are derived from carbohydrates metabolized via a so-called shikimic acid pathway and converted to phenylpropane amino acids (Figure 6.6). These amino acids supply precursors for the s Tithesis of lignin, plant proteins and flavonoids. The first step is the -elimination of ammonia from L-phenylalanine to form fra 5-cinnamic acid. Successive hydroxylation and methylation reactions convert cinnamic acid to />-coumaryl alcohol, caffeyl alcohol, coniferyl alcohol, 5-hydroxyconiferyl alcohol and sinapyl alcohol. Softwood mainly uses coniferyl alcohol with a small amount of p-cou-maryl alcohol. 

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