Hemsworth Glyn R, Thompson Andrew J, Stepper Judith, Sobala Łukasz F, Coyle Travis, Larsbrink Johan, Spadiut Oliver, Goddard-Borger Ethan D, Stubbs Keith A, Brumer Harry, Davies Gideon J
Department of Chemistry, York Structural Biology Laboratory, The University of York, Heslington, York YO10 5DD, UK.
School of Chemistry and Biochemistry, The University of Western Australia, Crawley, Western Australia 6009, Australia.
Open Biol. 2016 Jul;6(7). doi: 10.1098/rsob.160142.
The human gastrointestinal tract harbours myriad bacterial species, collectively termed the microbiota, that strongly influence human health. Symbiotic members of our microbiota play a pivotal role in the digestion of complex carbohydrates that are otherwise recalcitrant to assimilation. Indeed, the intrinsic human polysaccharide-degrading enzyme repertoire is limited to various starch-based substrates; more complex polysaccharides demand microbial degradation. Select Bacteroidetes are responsible for the degradation of the ubiquitous vegetable xyloglucans (XyGs), through the concerted action of cohorts of enzymes and glycan-binding proteins encoded by specific xyloglucan utilization loci (XyGULs). Extending recent (meta)genomic, transcriptomic and biochemical analyses, significant questions remain regarding the structural biology of the molecular machinery required for XyG saccharification. Here, we reveal the three-dimensional structures of an α-xylosidase, a β-glucosidase, and two α-l-arabinofuranosidases from the Bacteroides ovatus XyGUL. Aided by bespoke ligand synthesis, our analyses highlight key adaptations in these enzymes that confer individual specificity for xyloglucan side chains and dictate concerted, stepwise disassembly of xyloglucan oligosaccharides. In harness with our recent structural characterization of the vanguard endo-xyloglucanse and cell-surface glycan-binding proteins, the present analysis provides a near-complete structural view of xyloglucan recognition and catalysis by XyGUL proteins.
人类胃肠道中栖息着无数细菌种类，统称为微生物群，它们对人类健康有着重大影响。我们微生物群中的共生成员在复杂碳水化合物的消化过程中起着关键作用，否则这些碳水化合物难以被吸收。实际上，人类固有的多糖降解酶种类仅限于各种基于淀粉的底物；更复杂的多糖需要微生物降解。特定的拟杆菌通过由特定木葡聚糖利用位点（XyGULs）编码的一系列酶和聚糖结合蛋白的协同作用，负责降解普遍存在的植物木葡聚糖（XyGs）。尽管最近进行了（宏）基因组学、转录组学和生化分析，但关于木葡聚糖糖化所需分子机制的结构生物学仍存在重大问题。在此，我们揭示了来自卵形拟杆菌XyGUL的一种α-木糖苷酶、一种β-葡萄糖苷酶和两种α-L-阿拉伯呋喃糖苷酶的三维结构。在定制配体合成的辅助下，我们的分析突出了这些酶中的关键适应性变化，这些变化赋予了对木葡聚糖侧链的个体特异性，并决定了木葡聚糖寡糖的协同、逐步拆解。结合我们最近对先锋内切木葡聚糖酶和细胞表面聚糖结合蛋白的结构表征，本分析提供了XyGUL蛋白对木葡聚糖识别和催化的近乎完整的结构视图。
