Monolignols sinapyl

Lignin polymerization is a natural process initiated by the enzymatic oxidation of hydroxycinnamyl alcohols such as /)-coumaryl (CM), coniferyl (CF), and sinapyl alcohols (SN), which are known as monolignols (Scheme 2.20).60... 

In gymnosperms, the primary lignin precursors are the two monolignols coniferyl and /7-coumaryl alcohols, while in angiosperms sinapyl alcohol is also a participant in... 

Figure 9.43 Structure of monolignol where R[ = R2 = H p-coumaryl R[ = H, R2 = OCH3 confiferyl alcohol Rj = R2 = OCH3 sinapyl alcohol. (From de Leeuw and Largeau, 1993.)...

The alcohols formed from some cinnamic acid derivatives, namely /7-coumaryl alcohol, coniferyl alcohol (LI), and sinapyl alcohol (L2), commonly known as monolignols, undergo dimerization reactions that yield lignans such as (-l-)-pinoresinol (L3), (-l-)-sesamin (L4), (-)-matairesinol (L5), and podophyllotoxin (L6) (Fig. 13). Several thousand lignans are found to occur in nature. Lignins, the structural components of plant cell walls, are polymers of monolignols and/or lignans. 

Such products implicate the biosynthesis of sinapyl y-acetate as a precursor of Kenaf lignin (see Novel Mono-tetrahydrofurans Provide Evidence that Acylated Monolignols are True Lignin Precursors section) [101,260]. The functional role of such high levels of acetates in kenaf lignins awaits explanation. Although present at much lower levels, except in palms where they may be substantial [323], acetates on the y-position have been confirmed in aspen by a modified DFRC analysis [312]. Detailed NMR studies elucidating the nature of these minor acetate levels have not been carried out. 

Syringyl Lignin Pathways Two key enzymes (beyond the coniferyl alcohol pathway) are required for the production of sinapyl alcohol, which is the essential monolignol for production of syringyl units in lignins. Ferulate 5-hydroxylase, FH5, now perhaps more appropriately called CAld-5H to reflect coniferaldehyde as the preferred substrate [347,348], effects the 5-hydroxylation. Caffeate 0-methyltransferase,... 

Ferulate 5-hydroxylase (F5H) catalyzes the third P-450 hydroxylation step at the C5 position of the phenolic ring to ultimately afford, via the monolignol pathway, 5-hydroxyconiferyl (22) and/or sinapyl (23) alcohols. This P-450 was first detected in xylem and sclerenchyma-enriched tissues from poplar Populus x euramericana), with microsomal extracts able to catalyze hydroxylation of ferulic acid (6) to 5-hydroxyferulic acid (7) in the presence of NADPH with apparent values of 6.3 pmoll / This enzyme was thus characterized as F5H. 

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