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into the lignin polymer, these monolignols produce p-hydroxyphenyl (H),
guaiacyl (G), and syringyl (S) phenylpropane units, respectively (
Boerjan
et al., 2003
). In general, the lignin found in ferns and gymnosperms consists
mainly of G units, with a small proportion of H units, whereas the lignin of
angiosperms mainly consists of G and S units, with only traces of H units.
Lignins from grasses (monocots) incorporate G and S units at comparable
levels, but they contain more H units than eudicots (
Baucher et al., 1998
).
The three monolignols are synthesized from Phe through a number of
modifications of both the head group and the aromatic ring (
Fig. 1
). First,
phenylalanine ammonia lyase (PAL) catalyses the deamination of Phe to
form cinnamic acid, followed by a hydroxylation at the para or 4-position
of the aromatic ring producing 4-coumarate. Through a grid of reactions
4-coumarate undergoes further propene head group and aromatic ring
modifications. The carboxyl group of the C
3
-propene moiety is activated
to the CoenzymeA ester by 4-coumarate:CoA ligase (4CL) and the
CoA-thioester may be replaced reversibly by shikimate forming the
corresponding hydroxycinnamoyl-shikimate esters to allow aromatic ring
modifications. The CoA esters are also the substrate of cinnamoyl-CoA
reductase (CCR) producing the corresponding aldehydes, which are subse-
quently reduced to the alcohols by cinnamyl alcohol dehydrogenase
(CAD). Without ring modification, the sequential action of 4CL, CCR,
and CAD yields 4-coumaryl alcohol, the precursor of H-lignin units
(
Fig. 1
). Aromatic ring modifications include 3-hydroxylation followed
by methylation of the hydroxy group, the latter being catalysed by caffeic
acid O-methyltransferase (COMT) or caffeoyl-CoA O-methyltransferase
(CCOMT). After reduction of the head group, this gives rise to coniferyl
alcohol and guaiacyl-(G) lignin units. Further hydroxylation and methyla-
tion at the 5-position of the aromatic ring finally yields sinapyl alcohol and
syringyl (S)-lignin units (
Fig. 1
). For reviews on the phenylpropanoid and
monolignol pathway, see
Boerjan et al. (2003)
and
Vogt (2010)
.Itwas
originally proposed that the 3- and 5-modifications occur on the level of
the free acids. But work in the past decade, especially on the hydroxylases
involved, now suggests that the 3-hydroxylation primarily occurs at the
level of the shikimate-ester followed by 3-methoxylation at the CoA-
ester level and that 5-hydroxylation and methoxylation occurs primarily
at the level of the aldehyde or alcohol. Here we will outline our current
understanding of flow through the metabolic grid of the phenylpropanoid
pathway and discuss the flexibility of the system to employ alternative
routes. Thereby we will focus on the cytochrome P450 involved in 4-, 3-,
and 5-hydroxylations of the aromatic ring but will refer to research
performed on other enzyme of the pathway as well.
