Abstract
Abstract Hydroxycinnamoyl shikimates were reported in 2005 to be intermediates in monolignol biosynthesis. 3-Hydroxylation of p-coumarate, originally thought to occur via coumarate 3-hydroxylase (C3H) from p-coumaric acid or its CoA thioester, was revealed to be via the action of coumaroyl shikimate 3′-hydroxylase (C3′H) utilizing p-coumaroyl shikimate as the substrate, itself derived from p-coumaroyl-CoA via hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyltransferase (HCT). The same HCT was conjectured to convert the product, caffeoyl shikimate, to caffeoyl-CoA to continue on the pathway starting with its 3-O-methylation. At least in some plants, however, a more recently discovered caffeoyl shikimate esterase (CSE) enzyme hydrolyzes caffeoyl shikimate to caffeic acid from which it must again produce its CoA thioester to continue on the monolignol biosynthetic pathway. HCT and CSE are therefore monolignol biosynthetic pathway enzymes that have provided new opportunities to misregulate lignification. To facilitate studies into the action and substrate specificity of C3H/C3′H, HCT, and CSE enzymes, as well as for metabolite authentication and for enzyme characterization, including kinetics, a source of authentic substrates and products was required. A synthetic scheme starting from commercially available shikimic acid and the four key hydroxycinnamic acids (p-coumaric, caffeic, ferulic, and sinapic acid) has been developed to provide this set of hydroxycinnamoyl shikimates for researchers.
Highlights
Lignification is the process of polymerizing phenolic monomers into lignin (Freudenberg and Neish 1968), a cell wall polymer crucial for plant growth and development
Boerjan’s group investigated other genes that appeared to coexpress with genes of the monolignol biosynthetic pathway. This led to the discovery of yet another enzyme, caffeoyl shikimate esterase (CSE), that cleaves the ester in caffeoyl shikimate 1b to produce caffeic acid 2b, at least in some plant species (Ha et al 2016; Saleme et al 2017; Vanholme et al 2013b, 2019)
To further study and engineer the role and activity of HCT, CSE, and other lignin biosynthetic pathway enzymes, and to use as standards in various biological assays to study enzyme activities, we synthesized shikimate esters 1a–d, for which chemical syntheses have not previously been reported ( bacterial synthesis has been described (Cha et al 2014))
Summary
Lignification is the process of polymerizing phenolic monomers into lignin (Freudenberg and Neish 1968), a cell wall polymer crucial for plant growth and development. Boerjan’s group investigated other genes that appeared to coexpress with genes of the monolignol biosynthetic pathway This led to the discovery of yet another enzyme, CSE (caffeoyl shikimate esterase), that cleaves the ester in caffeoyl shikimate 1b to produce caffeic acid 2b, at least in some plant species (Ha et al 2016; Saleme et al 2017; Vanholme et al 2013b, 2019). This pathway requires a further 4CL step to again produce the CoA ester, caffeoyl-CoA, for 3-O-methylation, reduction to the aldehyde, further hydroxylation and methylation if syringyl units are required, and the final reduction of the coniferaldehyde or sinapaldehyde to their monolignols, coniferyl and sinapyl alcohol, Figure 1. To further study and engineer the role and activity of HCT, CSE, and other lignin biosynthetic pathway enzymes (such as the possibility of producing feruloyl shikimate by methylating caffeoyl shikimate via COMT or another OMT), and to use as standards in various biological assays to study enzyme activities, we synthesized shikimate esters 1a–d, for which chemical syntheses have not previously been reported ( bacterial synthesis has been described (Cha et al 2014))
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