Structural basis for the recalcitrance and molecular packing of hemicelluloses
Time: Thu 2024-09-26 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Video link: https://kth-se.zoom.us/j/68933782121
Language: English
Subject area: Biotechnology
Doctoral student: Emilia Heinonen , Wallenberg Wood Science Center, Glykovetenskap
Opponent: Professor Maija Tenkanen, University of Helsinki
Supervisor: Professor Francisco Vilaplana, Glykovetenskap, Wallenberg Wood Science Center, Fiber- och polymerteknologi, Albanova VinnExcellence Center for Protein Technology, ProNova; Docent Jakob Wohlert, Fysik, Wallenberg Wood Science Center, Biokompositer; Professor Gunnar Henriksson, Wallenberg Wood Science Center, Träkemi och massateknologi; Professor Mikael Lindström, Träkemi och massateknologi
QC 2024-08-29
Abstract
The properties of wood cell walls are determined by the composition and the molecular structures of the cell wall polymers (cellulose, hemicelluloses, pectins and lignin) and the interactions between them. In particular, hemicelluloses are an underutilized source of biopolymers that constitute around 30 % of wood. In hardwoods, the main hemicellulose is acetylated glucuronoxylan and the variation in the patterns of acetylation and glucuronidation between tree species is not fully understood. This study aimed to increase understanding of the xylan structure in particular and more generally, the interaction between the cell wall matrix polysaccharides and cellulose, which are important for the preparation of more accurate cell wall models as well as for the development of hemicellulose-based products.
The effect of matrix polysaccharides’ backbone on the assembly with cellulose fibril in water was studied using atomistic simulations. First, several setups were compared with xylo-oligosaccharides (XOs) as model hemicellulose motifs. Anti-parallel alignment together with the conformational change to a 2-fold helix as well as the formation of a distinct hydrogen bonding network were characteristics of spontaneous adsorption of XOs to cellulose. The established simulation setup was further used to expand the scope to other hemicelluloses and pectins. The parameters investigated included mobility on the cellulose surface, alignment, conformation in water and on cellulose and the interaction strength. Alignment along the fibril and conformational change upon adsorption defined the adaptability of hemicelluloses and distinguished them from the other oligosaccharides. In this sense, the mixed-linked β-glucan with a β-(1→3)-linkage was shown to be comparable to β-(1→4)-linked hemicelluloses.
To investigate the structure of hemicelluloses, sequential subcritical water (SWE) and alkaline extraction methods were applied to aspen and Eucalyptus wood. Buffered SWE released acetylated glucuronoxylan (acGX) with distinct patterns of acetylation and glucuronidation and enabled the extraction of galactosylated acGX from Eucalyptus without the need for prior delignification. In particular, a small amount of consecutive glucuronidation in both Eucalyptus and aspen xylan showed a more complex substitution pattern than previously thought, with similarity to arabinoglucuronoxylan from softwoods. The structure of GX affected its biological degradability by xylanases. Regions of low degree of glucuronidation were shown to aggregate in water, which hindered their accessibility to GH30 glucuronoxylanase. This was particularly relevant to alkali-extracted GX devoid of acetylation. In Eucalyptus, galactosylation impeded the digestion by GH30. GH10 could release galactosylated XOs but the cleavage site preference was shown to be altered. The large difference in the amount of Gal-MeGlcA from incubations with GH30 and GH10 suggests that the Gal-MeGlcA motifs may be clustered in xylan. Furthermore, the terminal galactose was released by a β- but not an α-galactose confirming the presence of galactose as a β-anomer.
The degree and the pattern of glucuronidation were further shown to affect the aggregation of glucuronoxylan at acidic pH 2.0. Separation of beech glucuronoxylan in two fractions based on their solubility at pH 2.0 was achieved by freezing and subsequent thawing. The precipitated GX had a higher DP and a lower degree of glucuronidation with an enrichment in the motifs with an even and lengthily-spaced glucuronidation pattern (X>6U) compared to the soluble GX. The fractionation of these GX populations in acidic conditions revealed the occurrence of interpolymer variability of glucuronidation, possibly corresponding to different activities of the two known glucuronyltransferases GUX1 and GUX2.
Overall, these results provide insights into the fundamentals of the molecular structure of xylans and the interaction of matrix polysaccharides with cellulose fibrils, which can increase our understanding of their biological functions and be utilized in the development of more accurate cell wall models and hemicellulose-based products.