Release of ferulic acid dehydrodimers from plant cell walls by feruloyl esterases

Abstract

Dehydrodimers of hydroxycinnamic acids, such as ferulic and p-coumaric acids, are important structural components which serve to cross-link polymers in plant cell walls. Dehydrodiferulate oligosaccharide diesters were solubilised from wheat bran or sugar beet pulp by treatment with Driselase and release of dehydrodiferulic free acids from this substrate by purified feruloyl esterases was analysed by HPLC. We detected 5-5diFA, 8-O-4diFA and 8-5BendiFA in saponified extracts from wheat bran and sugar beet pulp. Driselase-treatment solubilised 21 and 97% of the saponifiable dehydrodimers from wheat bran and sugar beet pulp, respectively, but only as dehydrodiferulate esters, not as dehydrodiferulic acids. At low concentrations, feruloyl esterase A (FAEA) from Aspergillus niger and an esterase (XylD) from Pseudomonas fluorescens released 93% and 36% of the saponifiable 5-5diFA from solubilised wheat bran, respectively, but only 12 and 15% from solubilised sugar beet pulp. At higher concentrations, only FAEA also released 8-O-4diFA (65%) from solubilised wheat bran, but not sugar beet pulp. We could not detect release of any dehydrodiferulic acids from solubilised wheat bran or sugar beet pulp using either a cinnamoyl ester hydrolase from Piromyces equi (CEH) or from A niger (CinnAE). Our results demonstrate that FAEA and Xy1D hydrolyse dehydrodiferulate diesters to release the free acids, which means they have the potential to break the putative cross-links present in graminaceous monocot and dicot cell walls, while two other feruloyl esterases (CinnAE and CEH) did not release dehydrodiferulic acids from plant cell walls.