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Figure 5. Lignin biosynthesis pathway. PAL, phenyl ammonia lyase; C4H, cinnamate 4-hydroxylase; C3H, para-coumarate 3-hydroxylase; COMT, caffeic acid O-methyltransferase; CCoAOMT, caffeoyl-CoA O-methyltransferase; 4CL, 4-coumarate:CoA ligase; 4CL??, certain species have 4CL activity toward sinapic acid; CCR, cinnamoyl-CoA reductase; CAD, cinnamyl alcohol dehydrogenase; SAD, sinapyl alcohol dehydrogenase; HCT, para-hydroxycinnamoyl-CoA:quinate shikimate para-hydroxycinnamoyltransferase; CCR? And F5H?, enzymes whose substrates have not been tested; F5H: ferulate 5-hydroxylase; ?, conversion has been demonstrated; ??, direct conversion not convincingly been demonstrated. ***, enzymatic assays in Arabidopsis have shown that the shikimate and quinate esters of para-coumaric acid are the ideal substrates for para-coumarate 3-hydroxylase (C3H). This means that in Arabidopsis, 4CL first converts the para-coumarate to para-coumaroyl-CoA, and then the C3H converts the para-coumaroyl-shikimate and para-coumaryoyl-quinate. This figure is a less-comprehensive summary of a 2003 report (Boerjan et al., 2003) and the 2002 redrawing of lignin biosynthesis pathway (Humphreys and Chapple, 2002).
