Folding of xylan onto cellulose fibrils in plant cell walls revealed by solid-state NMR

Abstract

Exploitation of plant lignocellulosic biomass is hampered by our ignorance of the molecular basis for its properties such as strength and digestibility. Xylan, the most prevalent non-cellulosic polysaccharide, binds to cellulose microfibrils. The nature of this interaction remains unclear, despite its importance. Here we show that the majority of xylan, which forms a threefold helical screw in solution, flattens into a twofold helical screw ribbon to bind intimately to cellulose microfibrils in the cell wall. 13C solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, supported by in silico predictions of chemical shifts, shows both two- and threefold screw xylan conformations are present in fresh Arabidopsis stems. The twofold screw xylan is spatially close to cellulose, and has similar rigidity to the cellulose microfibrils, but reverts to the threefold screw conformation in the cellulose-deficient irx3 mutant. The discovery that induced polysaccharide conformation underlies cell wall assembly provides new principles to understand biomass properties.

Regular Motifs in Xylan Modulate Molecular Flexibility and Interactions with Cellulose Surfaces

“…The conformation of XOs in solution is that of a relatively flexible 3 1 -fold screw, whereas XOs that are docked onto cellulose surfaces retain a flat, relatively rigid, 2 1 -fold screw conformation. This is in agreement with both modeling studies and NMR data showing a distinct change in conformation in xylan when there is cellulose present (Teleman et al, 2001;Simmons et al, 2016). Here, we examined these observations for XOs bound to the hydrophilic (1-10) surface and the hydrophobic (200) one, and our simulations show that the 2 1 -fold screw backbone conformation does not differ significantly between these surfaces.…”

“…These observations challenge a previous study, where it was suggested that the common structure XXA 3 XU m X, with side groups spaced at every other xylan unit, is favored, since the side groups can point out to the same side, which makes xylan fit well onto the cellulose (Busse-Wicher et al, 2016b). Orienting the side groups away from the surface could potentially allow for a closer packing of multiple xylan chains, possibly allowing the formation of a layer of single xylan molecules fully coating the cellulose surface, as is proposed for the hydrophilic (110) surface (Busse-Wicher et al, 2016b;Simmons et al, 2016). On the other hand, however, it is possible that the gain in interaction energy by having the side groups in contact with the cellulose would compensate for the loss of near-crystalline order and the stabilizing effect of neighboring chains.…”

“…Based on in silico predictions of NMR chemical shifts by density functional theory and MD simulations, this change has been identified as the transition from a 3 1 -fold helical structure to a flat 2 1 -fold screw (Busse-Wicher et al, 2016b;Simmons et al, 2016), which intuitively would facilitate close interaction between the xylan polymer and the flat cellulose surface, and even cocrystallization. Therefore, we initially placed the XOs in conformations that corresponded to the cellulose Ib structure (which constitute a 2 1 -fold screw) in an extra, hypothetical surface layer.…”

“…Solid-state NMR on never-dried Arabidopsis stems (Simmons et al, 2016) and technical pulp model systems (Larsson et al, 1999;Teleman et al, 2001) indicate that xylan undergoes a conformational change when interacting with cellulose. Based on in silico predictions of NMR chemical shifts by density functional theory and MD simulations, this change has been identified as the transition from a 3 1 -fold helical structure to a flat 2 1 -fold screw (Busse-Wicher et al, 2016b;Simmons et al, 2016), which intuitively would facilitate close interaction between the xylan polymer and the flat cellulose surface, and even cocrystallization.…”

Two Complementary Mechanisms Underpin Cell Wall Patterning during Xylem Vessel Development

“…It is plausible that xylans and/or other cell wall constituents that are deposited along the MT bands may serve this function in the absence of CSI1/POM2. Computational modeling and NMR experiments suggest a tight interplay between xylans and cellulose microfibrils (Busse-Wicher et al, 2014;Simmons et al, 2016). Such interactions could influence the direction of the CSCs and therefore cause them to successively align along the secondary wall bands.…”