Understanding how noncatalytic carbohydrate binding modules can display specificity for xyloglucan.
Understanding how noncatalytic carbohydrate binding modules can display specificity for xyloglucan.
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DOI：
10.1074/jbc.m112.432781
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发表时间：
2013-02-15
期刊：
影响因子：
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通讯作者：
Gilbert HJ
中科院分区：
文献类型：
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作者：
Luís AS;Venditto I;Temple MJ;Rogowski A;Baslé A;Xue J;Knox JP;Prates JA;Ferreira LM;Fontes CM;Najmudin S;Gilbert HJ
背景：碳水化合物结合模块（CBM）有助于复杂多糖结构的酶促降解。结果如下：新的CBM通过与主链和侧链结构相互作用的广泛的疏水平台显示对修饰的葡聚糖的特异性。结论：与复合β-葡聚糖结合的CBM利用这些配体的不同组分作为特异性决定簇。意义：CBM可以利用修饰葡聚糖的侧链作为特异性决定簇。植物生物质是碳循环和以生物燃料部门为代表的环境可持续工业的核心。植物细胞壁降解酶通常含有非催化性碳水化合物结合模块（CBM），其实现靶向功能，这增强了催化作用。结合β-葡聚糖链的CBM通常显示出广泛的特异性，通过靶向三种多糖共有的结构识别β 1，4-葡聚糖（纤维素）、β 1，3-β 1，4-混合连接的葡聚糖和木葡聚糖（一种用α 1，6-木糖残基修饰的β 1，4-葡聚糖）。因此，识别木葡聚糖的CBM靶向β 1，4-葡聚糖主链并且仅容纳木糖装饰。在这里，我们表明，两个密切相关的CBM，CBM 65 A和CBM 65 B，来自EcCel 5A，一种溶纤维真杆菌内切葡聚糖酶，结合到一系列的β-葡聚糖，但独特地，显示出显着的偏好木葡聚糖。两种CBM的结构均为β-夹心结构。配体结合位点包括形成蛋白质凹面的β折叠。与β-葡聚糖主链的结合主要由五个芳族残基介导，所述芳族残基也与木葡聚糖的木糖侧链进行疏水相互作用，从而赋予CBM对修饰的多糖的独特特异性。值得注意的是，与识别β-葡聚糖的其他CBM相反，CBM 65 A利用不同的极性残基结合纤维素和混合连接的葡聚糖。因此，Gln 106是纤维素识别的核心，但不是结合混合连接葡聚糖所必需的。该报告揭示了β-葡聚糖特异性CBM可以区分线性和混合连接葡聚糖的机制，并显示了这些CBM如何利用广泛的疏水平台来靶向修饰的β-葡聚糖的侧链。
Background: Carbohydrate binding modules (CBMs) contribute to the enzymatic degradation of complex polysaccharide structures. Results: New CBMs display specificity for decorated glucans through an extensive hydrophobic platform that interacts with both backbone and side chain structures. Conclusion: CBMs that bind to complex β-glucans exploit different components of these ligands as specificity determinants. Significance: CBMs can utilize the side chains of decorated glucans as specificity determinants. Plant biomass is central to the carbon cycle and to environmentally sustainable industries exemplified by the biofuel sector. Plant cell wall degrading enzymes generally contain noncatalytic carbohydrate binding modules (CBMs) that fulfil a targeting function, which enhances catalysis. CBMs that bind β-glucan chains often display broad specificity recognizing β1,4-glucans (cellulose), β1,3-β1,4-mixed linked glucans and xyloglucan, a β1,4-glucan decorated with α1,6-xylose residues, by targeting structures common to the three polysaccharides. Thus, CBMs that recognize xyloglucan target the β1,4-glucan backbone and only accommodate the xylose decorations. Here we show that two closely related CBMs, CBM65A and CBM65B, derived from EcCel5A, a Eubacterium cellulosolvens endoglucanase, bind to a range of β-glucans but, uniquely, display significant preference for xyloglucan. The structures of the two CBMs reveal a β-sandwich fold. The ligand binding site comprises the β-sheet that forms the concave surface of the proteins. Binding to the backbone chains of β-glucans is mediated primarily by five aromatic residues that also make hydrophobic interactions with the xylose side chains of xyloglucan, conferring the distinctive specificity of the CBMs for the decorated polysaccharide. Significantly, and in contrast to other CBMs that recognize β-glucans, CBM65A utilizes different polar residues to bind cellulose and mixed linked glucans. Thus, Gln106 is central to cellulose recognition, but is not required for binding to mixed linked glucans. This report reveals the mechanism by which β-glucan-specific CBMs can distinguish between linear and mixed linked glucans, and show how these CBMs can exploit an extensive hydrophobic platform to target the side chains of decorated β-glucans.