Simulations of the static and dynamic molecular conformations of xyloglucan. The role of the fucosylated sidechain in surface-specific sidechain folding.

Abstract

The hemicellulosic polysaccharide xyloglucan binds with a strong affinity to cellulosic cell wall microfibrils, the resulting heterogeneous network constituting up to 50% of the dry weight of the cell wall in dicotyledonous plants. To elucidate the molecular details of this interaction, we have performed theoretical potential energy calculations of the static and dynamic equilibrium conformations of xyloglucan using the GEGOP software. In particular, we have evaluated the preferred sidechain conformations of hexa-, octa-, deca- and heptadecasaccharide model fragments of xyloglucan for molecules with a cellulose-like, flat, glucan backbone, and a cellobiose-like, twisted, glucan backbone conformation. For the flat backbone conformation the determination of static equilibrium molecular conformations revealed a tendency for sidechains to fold onto one surface of the backbone, defined here as the H1S face, in the fucosylated region of the polymer. This folding produces a molecule that is sterically accessible on the opposite face of the backbone, the H4S face. Typically, this folding onto the H1S surface is significantly stabilized by favorable interactions between the fucosylated, trisaccharide sidechain and the backbone, with some stabilization from adjacent terminal xylosyl sidechains. In contrast, the trisaccharide sidechain folds onto the H4S face of xyloglucan fragments with a twisted backbone conformation. Preliminary NMR data on nonasaccharide fragments isolated from sycamore suspension-cultured cell walls are consistent with the hypothesis that the twisted conformation of xyloglucan represents the solution form of this molecule. Metropolis Monte Carlo (MMC) simulations were employed to assess sidechain flexibility of the heptadecasaccharide fragments. Simulations performed on the flat, rigid, backbone xyloglucan indicate that the trisaccharide sidechain is less mobile than the terminal xylosyl sidechains. MMC calculations on a fully relaxed molecule revealed a positive correlation between a specific trisaccharide sidechain orientation and the 'flatness' of the backbone glucosyl residues adjacent to this sidechain. These results suggest that the trisaccharide sidechain may play a role in the formation of nucleation sites that initiate the binding of these regions to cellulose. Based on these conformational preferences we suggest the following model for the binding of xyloglucan to cellulose. Nucleation of a binding site is initiated by the fucosylated, trisaccharide sidechain that flattens out an adjacent region of the xyloglucan backbone. Upon contacting a cellulose microfibril this region spreads by step-wise flattening of successive segments of the backbone. Self-association of xyloglucan molecules in solution may be prevented by the low frequency of formation of these nucleation sites and the geometry of the molecules in solution.

Xyloglucan sidechains modulate binding to cellulose during in vitro binding assays as predicted by conformational dynamics simulations

“…In this last conformational model ( Figure 3d) the glucan backbone is seen to be both flat and sterically accessible and therefore in the optimal conformation for cellulose-binding. In our previous conformational dynamics study (Levy et aL, 1991) we were able to identify the twisted backbone XG (Figure 3a) and the localized flattened backbone XG form ( Figure 3d). However, only the hierachical cluster analysis of the complete MMC data set reported here has shown conformational forms that could be regarded as intermediate in trisaccharide sidechain folding.…”

“…Using the ~, ~ and co values for the average conformations of the individual clusters, molecular models can be built to show the structural features of the populated backbone forms. In describing these models, we will employ nomenclature detailed previously in referring to XG backbone surfaces (Levy et aL, 1991). Thus, the H4 s face is the backbone surface defined by the location of the H4 atom on the glc s residue and consequently the H1 s face is the opposite surface, this being the surface on which the H1 atom of the glc s residue is located.…”

“…Based on the computational data it is generally assumed that binding to cellulose requires that the XG backbone adopts a flat conformation with all adjacent sidechains folded so as to render one surface of the planar backbone sterically accessible for cellulose-binding (Levy et aL, 1991). This is the conformationally optimal model that we have adopted in the absence of experimental data on glucan chain conformation on the cellulose surface and data to suggest that any significant binding occurs directly between XG sidechains and cellulose.…”

“…Our previous molecular simulation studies on the welldefined repeating oligosaccharides from pea epicotyls XXFG and XXXG (see Figure 1 for explanation of XG sidechain chemistry nomenclature) indicated that specific conformations in the fucosylated trisaccharide sidechain correlated with an enhanced occurrence of the hypothesized cellulose binding conformation (Levy et al, 1991). This is a conformation in which the glucan backbone is planar or flat in the localized region of the backbone that attaches to the trisaccharide sidechain.…”

Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth

“…An early model from Albersheim's group envisioned XG as a monolayer coating the microfibrils (Keegstra et al, 1973). Metropolis Monte Carlo computer simulations predict several probable low-energy conformations of XG in solution, and these computer modeling studies have resulted in some novel predictions about the role of the trisaccharide in XG conformation (Levy et al, 1991). The model of XG composed strictly of heptasaccharides predicts a gradual twist rather than a linear molecule, but addition of the trisaccharide sidechain theoretically restricts the molecule in solution to just two favorable conformations.…”