Biology Reference
In-Depth Information
Cytochrome P450s in Lignin Biosynthesis
ANNETTE ALBER AND J
ยจ
RGEN EHLTING
1
Department of Biology & Centre for Forest Biology,
University of Victoria, Victoria, BC, Canada
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
II. Cytochrome P450 Hydroxylases in Monolignol Biosynthesis . . . . . . . . . . . . . . 117
A. Entry into the Pathway: Cinnamate 4-Hydroxylase
(C4H, CYP73A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
B. An Apparent Detour to S- and G-Lignin: The 3-Hydroxylase
of the Aromatic Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
C. Committed to S-Lignin: The 5-Hydroxylase of the Aromatic Ring . . . . 130
III. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
ABSTRACT
The phenylpropanoid metabolism channels carbon from phenylalanine to the three
monolignols and numerous other phenolic compounds. Our understanding of the
pathway has changed tremendously over the last decade, which was driven largely by
the biochemical and genetic characterization of the cytochrome P450s catalysing the
hydroxylation of the aromatic ring. The first, cinnamate 4-hydroxylase, is the rate-
limiting step into the phenylpropanoid pathway and is highly specific for cinnamate.
Blocking this step impairs the ability of plants to produce lignin. The 3-hydroxylation
occurs primarily on the level of the 4-coumaroyl-shikimate level, rather than on the
free acid or CoA-ester as previously expected, thereby linking the far upstream
shikimate pathway with the committed step towards S and G lignin. Finally, the
last hydroxylation step occurs on the level of the aldehyde or alcohol and defines flow
1
Corresponding author: E-mail: je@uvic.ca
