Biology Reference
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increase in H-lignin units (
Coleman et al., 2008; Reddy et al.,2005
). In alfalfa,
both G- and S-units were severely reduced and differences in lignin unit
couplings were apparent (
Ralph et al.,2006
). This is accompanied with re-
duced recalcitrance to saccharification and in consequence positively impacts
bioconversion of lignocellulosic material to ethanol (
Chen and Dixon, 2007
).
In poplar, C3
0
H downregulation leads to reduced total lignin, but an increase
in H-lignin units is mirrored by a decrease in G-lignin units only while S-lignin
units remain largely unchanged (
Coleman et al.,2008
). Again, cell-type specific
variation in downregulation efficiency or species-specific control of fluxes into
the distinct sub-branches may explain these apparent differences.
Neither the alfalfa nor the hybrid poplar C3
0
H targeted for downregulation
has been characterized biochemically, but close orthologs of either genes have.
CYP98A44 from red clover (Trifolium pratense), which shares 96% sequence
identity with the Medicago C3
0
H, is able to convert 4-coumaroyl-shikimate
(
Sullivan and Zarnowski, 2010
). More detailed analyses have been performed
with the C3
0
Hfrombackcottonwood(Populus trichocarpa): C3
0
H3 expressed
in yeast converts 4-coumaroyl-shikimate, but not the free acid (4-coumarate);
but when PtC3
0
H was coexpressed with C4H in yeast, a drastic increase in
catalytic activity and efficiency towards 4-coumaroyl-shikimate was observed.
In this case, also a low activity with free 4-coumarate becomes detectable
(
Chen et al.,2011
). Also, most other biochemically characterized CYP98A
family members display a clear preference for 4-coumaroyl-shikimate as
a substrate (see below for details). Together with the Arabidopsis results
described above and the effects of downregulation on lignin composition in
alfalfa and poplar, this suggests that 4-coumaroyl-shikimate is the major
intermediate and substrate of the 3-hydroxylation step towards G- and
S-lignin. This has been a surprising new twist in the roadmap to lignin, because
use of the shikimate ester appears to be a metabolic detour compared to direct
3-hydroxylation of the free acid or CoA-ester. There is no obvious kinetic or
other chemical advantage to using the shikimate ester, but it demands the
activity of an additional enzyme, that is, HCT on top of C3
0
H, to yield
caffeoyl-CoA from 4-coumaroyl-CoA (
Fig. 3
). Essentially, shikimate acts
as a cofactor in the HCT/C3
0
H/HCT reaction as it is first transferred to
4-coumaroyl-CoA, and after 3-hydroxylation of the aromatic moiety, it is
recovered when CoA is transferred back to generate caffeoyl-CoA. But shiki-
mate is not only a cofactor but also a far-upstream precursor of the substrate
4-coumaroyl-CoA. It is a central intermediate and name-giver of the shikimate
pathway that connects primary carbohydrate metabolism with the biosynthe-
sis of the aromatic amino acids including Phe, which in turn is the starting
point of the general phenylpropanoid pathway leading to 4-coumaroyl-CoA.
The C3
0
H-catalysed reaction is likely the rate limiting and clearly the first
