Levy S, York W S, Stuike-Prill R, Meyer B, Staehelin L A
Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder 80309-0347.
Plant J. 1991 Sep;1(2):195-215.
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.
半纤维素多糖木葡聚糖与纤维素细胞壁微纤丝具有很强的亲和力，由此形成的异质网络占双子叶植物细胞壁干重的50%。为了阐明这种相互作用的分子细节，我们使用GEGOP软件对木葡聚糖的静态和动态平衡构象进行了理论势能计算。特别是，我们评估了木葡聚糖的六糖、八糖、十糖和十七糖模型片段对于具有类纤维素扁平葡聚糖主链以及类纤维二糖扭曲葡聚糖主链构象的分子的优选侧链构象。对于扁平主链构象，静态平衡分子构象的测定表明，在聚合物的岩藻糖基化区域，侧链倾向于折叠到主链的一个表面上，在此定义为H1S面。这种折叠产生了一个在主链相对表面（H4S面）上空间可及的分子。通常，这种向H1S表面的折叠通过岩藻糖基化的三糖侧链与主链之间的有利相互作用而显著稳定，相邻的末端木糖基侧链也有一定的稳定作用。相比之下，三糖侧链折叠到具有扭曲主链构象的木葡聚糖片段的H4S面上。从悬铃木悬浮培养细胞壁中分离出的九糖片段的初步核磁共振数据与木葡聚糖的扭曲构象代表该分子溶液形式的假设一致。采用 metropolis 蒙特卡罗（MMC）模拟来评估十七糖片段的侧链灵活性。在扁平、刚性的主链木葡聚糖上进行的模拟表明，三糖侧链的流动性比末端木糖基侧链小。对一个完全松弛的分子进行的MMC计算揭示了特定三糖侧链取向与该侧链相邻的主链葡糖基残基的“扁平度”之间存在正相关。这些结果表明，三糖侧链可能在启动这些区域与纤维素结合的成核位点形成中起作用。基于这些构象偏好，我们提出了以下木葡聚糖与纤维素结合的模型。结合位点的成核由岩藻糖基化的三糖侧链启动，该侧链使木葡聚糖主链的相邻区域变平。与纤维素微纤丝接触时，该区域通过主链连续片段的逐步变平而扩展。溶液中木葡聚糖分子的自缔合可能由于这些成核位点形成的低频率以及溶液中分子的几何形状而受到阻止。
