Coniferaldehyde/coniferyl alcohol

Figure 3-9. Biosynthesis of monolignols. The enzymes involved in this pathway are ( ) hydroxycinnamoyl-CoA shikimate/quinate hydroxy-cinnamoyl transferase, (b) p-coumaroyl-CoA 3 -hydroxylase (E.C. 1.14.14.1), (c) caffeoyl-CoA O-methy 1 Iranslerasc (E.C. 2.1.1.104), (d) cinnamoyl-CoA reductase (E.C. 1.2.1.44) (e) cinnamyl alcohol dehydrogenase (E.C. 1.1.1.195), (f) coniferyl aldehyde/coniferyl alcohol 5-hydroxylase (E.C. 1.14.13), (g) coniferaldehyde/coniferyl alcohol O-methyltransferase (E.C. 2.1.1.68).

Wood meal Coniferaldehyde + Coniferyl alcohol Hydroformylation 8 Nahum 1969... 

R1 =H R2 = OCRs Feruloyl-CoA Coniferaldehyde Coniferyl alcohol Coniferyl alcohol 4-O-p-glucoside (Coniferin)... 

A1 5-hydroxylase for coniferaldehyde, coniferyl alcohol Phenylpropanoids [152]... 

It has also been proposed that under the acidic conditions found ia whiskeys, ethanol reacts with lignin (qv) to reduce an alcohol-soluble form of lignin (ethanol lignin). This can be converted into coniferyl alcohol, which can be oxidized to coniferaldehyde. The partial oxidation of ethanol lignin can produce siaapic and coniferyl alcohols that can be converted to syfingaldehyde and vanillin, respectively (8). 

The three glucosides (IV-VI) are present in the cambial sap of spruce, V being by far the most abundant (49). Furthermore, spruce cambial sap contains very small amounts of coniferyl alcohol (I), coniferaldehyde (VII), the dilignols (XVII), (XIX), (XXII), and the trilignol (XXXVI) (19). Other lignols are present but in amounts too small for convenient identification. Extensive work has been done to examine the pathway of lignin formation in vivo and in vitro. 

Dehydrodiconiferaldehyde (XVIII) 25) is an unsaturated aldehyde corresponding to XVII and is formed partly by condensation of free radicals derived from coniferyl alcohol and coniferaldehyde. 

Coniferaldehyde (3.76) can undergo several fates, some of which can ultimately lead to the same end product. It can be reduced to coniferyl alcohol (3.79) by the enzyme cinnamyl alcohol dehydrogenase (CAD). Alternatively, the enzyme coniferyl aldehyde/coniferyl alcohol 5-hydroxylase (C5H), also known by its less accurate name ferulic acid 5-hydroxylase (F5H Humphreys et al., 1999) can catalyze the hydroxylation of C5 to result in 5-hydroxyconiferyl aldehyde (3.77). C5H is also able to form 5-hydroxyconiferyl alcohol (3.80) from coniferyl alcohol (3.79). This enzyme was initially identified as F5H, after analysis of the Arabidopsis ferulic acid hydroxylase 1 (fahl) mutant, which was isolated in a mutant screen based on reduced levels of the UV-fluorescent sinapoyl esters (Section 13 Chappie et al., 1992). The FAH1 gene was cloned using a T-DNA tagged mutant allele (Meyer et al., 1996), which revealed that the... 

Characterization of lignin end groups. The compounds isolated (Figure 1), which are assumed to constitute lignin end groups, include coniferyl alcohol structures (4,8, 15 and 18), coniferaldehyde structures (3,7,12 and 17), detached pyruvaldehyde enol... 

Coniferyl alcohol and coniferaldehyde structures. It has been estimated that spruce lignin contains about 6% of coniferyl alcohol units as end groups (28). In previous work, the behaviour of coniferyl alcohol structures during a mild sulfite treatment simulating the chemical pretreatment in a CTMP process was studied (75). It was found that coniferyl alcohol is liberated from spruce wood more rapidly under acidic than under neutral or alkaline conditions. The treatment time needed for complete liberation of coniferyl alcohol structures from wood lignin was, however, found to be more than 100 minutes irrespective of treatment conditions. Considering the short treatment time (5-15 min.) employed in the commercial process, it can... 

Figure 6. Relative yields of coniferaldehyde and coniferyl alcohol structures...

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,... 

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