Pathways for monolignol biosynthesis via metabolic grids: Coniferyl aldehyde 5-hydroxylase, a possible key enzyme in angiosperm syringyl lignin biosynthesis

Abstract

Lignins are aromatic polymers of methoxylated phenylpropanoids connected by both ether and carbon-carbon linkages, and classified into three major groups, guaiacyl lignin in softwoods (gymnosperms), guaiacyl-syringyl lignin in hardwoods (angiosperms) and guaiacyl-syringyl-p-hydroxyphenyl lignin in grasses (gramineae) on their monomeric units. In grass lignins p-coumaric acid (5-10%) is ester linked mostly at γ-position of the propyl side chain. Biochemical studies of lignification have elucidated that 1) lignins are formed by dehydrogenative polymerization of monolignols catalyzed by peroxidase and laccase, 2) biosynthesis of monolignols is via shikimate-cinnamate pathway, 3) syringyl lignin in hardwoods is formed from the syringyl monolignol, sinapyl alcohol, derived from 5-hydroxyguaiacyl precursors, and that 4) ferulate 5-hydroxylase, coniferyl aldehyde 5-hydroxylase and probably coniferyl alcohol 5-hydroxylase are involved in the biosynthesis of the syringyl monolignol from guaiacyl monolignol precursors in respective hardwoods.