Biomedical Engineering Reference
In-Depth Information
PA L
Cinnamic acid
Phenylalanine
tyrosine
C4H
CO
2
H
CO
2
H
CO
2
H
TA L
C3H
COMT
OH
OCH
3
OH
OH
OH
Coumaric acid
4CL
Caffeic acid
Ferulic acid
4CL
4CL
coscoA
coscoA
coscoA
CCoAOMT
CCH
OH
OCH
3
OH
OH
OH
Coumaroyl CoA
Caffeoyl CoA
Feruloyl CoA
CCR
CCR
CAId5H
AldOMT
5- hydroxy
coniferaldehyde
Coumaraldehyde
Coniferaldehyde
Mustard aldehyde
CAD
CAD
CAD
Coumaryl alcohol
Coniferyl alcohol
Sinapyl alcohol
Peroxidase/Laccase
G-lignin
H-lignin
S-lignin
Fig. 2.6
Biosynthetic pathway of lignin monomer [
38
].
PA L
phenylalanine ammonia lyase,
TAL
tyrosine deaminase,
Cald5H
coniferaldehyde-5-hydroxylase,
C4H
cinnamic acid-4-hydroxylase,
CCH
coumaroyl CoA-3-hydroxylase,
4CL
4-coumarate CoA ligase,
CCoAOMT
caffeoyl CoA 3-
O-methyltransferase,
C3H
coumarate-3-hydroxylase,
CCR
cinnamoyl CoA reductase,
AldOMT
5-hydroxy coniferaldehyde-O-methyltransferase,
CAD
cinnamoyl alcohol dehydrogenase
2.5.4
Lignin Synthesis
2.5.4.1
Biosynthesis of Monolignols
Biosynthesis of monolignols is conducted through the phenylpropanoid pathway.
In the shikimate pathway, glucose generated from the carbon dioxide by photosyn-
thesis first is converted to the final intermediate: shikimic acid. Then, through the
prephenic acid, the shikimic acid is converted into the final products of the shikimate
pathway: phenylalanine and tyrosine. These two amino acids, which are widely
present in plants, are starting materials for the cinnamic acid pathway. Under the
effects of various enzymes, three monomers of lignin are finally synthesized after a
set of reactions, such as deamination, hydroxylation, methylation, reduction, and so
on (Fig.
2.6
)[
37
,
38
].
Cinnamoyl CoA reductase (CCR) catalyzes the first step of the redox reaction of
lignin biosynthesis, which may be the rate-limiting step, controlling lignin synthesis
and the pathway from which carbon can go into lignin biosynthesis. Cinnamyl alco-
hol dehydrogenase (CAD) catalyzes the redox reaction of another step in the lignin
synthesis process, which may control the reduction of coniferaldehyde. Sinapyl
