Ferulate 5-hydroxylase Research Articles

New routes for lignin biosynthesis defined by biochemical characterization of recombinant ferulate 5-hydroxylase, a multifunctional cytochrome P450-dependent monooxygenase.

The enzymes and genes of the lignin biosynthetic pathway have been studied for several decades, but the gene encoding ferulate 5-hydroxylase (F5H) was cloned only 3 years ago by T-DNA tagging in Arabidopsis. To characterize the enzyme in detail, we have expressed F5H in yeast. According to current models of the phenylpropanoid pathway, F5H catalyzes the hydroxylation of ferulate to 5-hydroxyferulate; however, our studies indicate that the enzyme also uses coniferaldehyde and coniferyl alcohol as substrates. Unexpectedly, the K(m) values measured for the latter two substrates are three orders of magnitude lower than that measured for ferulic acid, suggesting that in lignifying tissues, syringyl monomers may be derived from their guaiacyl counterparts by hydroxylation and subsequent methylation. Thus, F5H may function later in the lignin biosynthetic pathway than was originally proposed. To further test this model, recombinant F5H was incubated together with ferulic acid, coniferaldehyde, or coniferyl alcohol in the presence of native or recombinant Arabidopsis caffeic acid/5-hydroxyferulic acid O-methyltransferase and [(14)C]S-adenosylmethionine. In all cases, the corresponding radiolabeled sinapyl derivatives were synthesized, indicating that the necessary enzymes required for this pathway are present in Arabidopsis. Taken together, these data suggest that the previously accepted pathway for lignin biosynthesis is likely to be incorrect.

Lignin monomer composition is determined by the expression of a cytochrome P450-dependent monooxygenase in Arabidopsis.

The phenylpropanoid pathway provides precursors for the biosynthesis of soluble secondary metabolites and lignin in plants. Ferulate-5-hydroxylase (F5H) catalyzes an irreversible hydroxylation step in this pathway that diverts ferulic acid away from guaiacyl lignin biosynthesis and toward sinapic acid and syringyl lignin. This fact led us to postulate that F5H was a potential regulatory step in the determination of lignin monomer composition. To test this hypothesis, we have used Arabidopsis to examine the impact of F5H overexpression. Arabidopsis is a useful model system in which to study lignification because in wild-type plants, guaiacyl and syringyl lignins are deposited in a tissue-specific fashion, while the F5H-deficient fah1 mutant accumulates only guaiacyl lignin. Here we show that ectopic overexpression of F5H in Arabidopsis abolishes tissue-specific lignin monomer accumulation. Surprisingly, overexpression of F5H under the control of the lignification-associated cinnamate-4-hydroxylase promoter, but not the commonly employed cauliflower mosaic virus 35S promoter, generates a lignin that is almost entirely comprised of syringylpropane units. These experiments demonstrate that modification of F5H expression may enable engineering of lignin monomer composition in agronomically important plant species.

Ferulate-5-hydroxylase from Arabidopsis thaliana defines a new family of cytochrome P450-dependent monooxygenases.

The fah1 mutant of Arabidopsis is defective in the accumulation of sinapic acid-derived metabolites, including the guaiacyl-syringyl lignin typical of angiosperms. Earlier results indicated that the FAH1 locus encodes ferulate-5-hydroxylase (F5H), a cytochrome P450-dependent monooxygenase (P450) of the general phenylpropanoid pathway. We have cloned the gene encoding this P450 by T-DNA tagging and have confirmed the identity of the cloned gene by complementation of the mutant phenotype. F5H shows 34% amino acid sequence identity with the avocado ripening-induced P450 CYP71A1 and 32% identity with the flavonoid-3',5'-hydroxylases of Petunia hybrida. In contrast, it shares much less homology with cinnamate-4-hydroxylase, a P450 that catalyzes the hydroxylation of cinnamic acid three steps earlier in the general phenylpropanoid pathway. Since the highest degree of identity between F5H and previously sequenced P450s is only 34%, F5H identifies a new P450 subfamily that has been designated CYP84.