Selective Enzymatic Release and Gel Formation by Cross-Linking of Feruloylated Glucurono-Arabinoxylan from Corn Bran

Corn bran is a major agro-industrial byproduct from corn starch processing. The bran is particularly rich in highly substituted feruloylated glucuronoarabinoxylan (FGAX). Yet, due to its recalcitrance to biocatalytic degradation, corn bran FGAX is currently not utilized in biorefinery processes. Here, we report selective enzymatic extraction of both single, and double-stranded high-molecular-weight FGAX molecules from corn bran using a bacterial, glucuronoyl-specific glycoside hydrolase family 30 endo-1,4-β-xylanase (EC 3.2.1.8) from Dickeya chrysanthemi (DcXyn30). The enzymatic extraction using DcXyn30 was optimized with respect to temperature, pH, and time to maximize yields of high-molecular-weight polysaccharides. Examination of the enzymatically extracted FGAX using SEC, HPAEC, LC-MS, and NMR analysis (after acid or alkaline hydrolysis) revealed that both single-stranded and double-stranded FGAX were extracted, since diferulate-linkages were present in the extracted FGAX. Furthermore, the NMR-analysis indicated presence of 1,5-linked arabinan dimers suggesting that some of the xylopyranosyl residues in the extracted FGAX contained arabinofuranosyl-arabinofuranosyl substitutions in addition to a significant amount of classical doubly-arabinose substitutions. Laccase treatment of the extracted FGAX produced strong hydrogels via oxidative, covalent feruloyl-cross-linking. At pH 6.5 the Myceliophthora thermophila derived laccase produced significantly faster cross-linking kinetics than the laccase from Pleurotus ostreatus as measured rheologically. The data reveal novel insight into corn bran FGAX chemistry and provide a new direction for enzyme-assisted upgrading of corn bran for valuable functional hydrogel applications.