Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI, 48109, USA.
Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109, USA.
Cell Mol Life Sci. 2023 Jul 28;80(8):232. doi: 10.1007/s00018-023-04812-w.
Members of the Bacteroidetes phylum in the human colon deploy an extensive number of proteins to capture and degrade polysaccharides. Operons devoted to glycan breakdown and uptake are termed polysaccharide utilization loci or PUL. The starch utilization system (Sus) is one such PUL and was initially described in Bacteroides thetaiotaomicron (Bt). BtSus is highly conserved across many species, except for its extracellular α-amylase, SusG. In this work, we show that the Bacteroides ovatus (Bo) extracellular α-amylase, BoGH13A, is distinguished from SusG in its evolutionary origin and its domain architecture and by being the most prevalent form in Bacteroidetes Sus. BoGH13A is the founding member of both a novel subfamily in the glycoside hydrolase family 13, GH13_47, and a novel carbohydrate-binding module, CBM98. The BoGH13A CBM98-CBM48-GH13_47 architecture differs from the CBM58 embedded within the GH13_36 of SusG. These domains adopt a distinct spatial orientation and invoke a different association with the outer membrane. The BoCBM98 binding site is required for Bo growth on polysaccharides and optimal enzymatic degradation thereof. Finally, the BoGH13A structure features bound Ca and Mn ions, the latter of which is novel for an α-amylase. Little is known about the impact of Mn on gut bacterial function, much less on polysaccharide consumption, but Mn addition to Bt expressing BoGH13A specifically enhances growth on starch. Further understanding of bacterial starch degradation signatures will enable more tailored prebiotic and pharmaceutical approaches that increase starch flux to the gut.
人类结肠中的拟杆菌门成员利用大量蛋白质来捕获和降解多糖。专门用于糖链分解和摄取的操纵子被称为多糖利用基因座或 PUL。淀粉利用系统（Sus）就是这样一个 PUL，最初在拟杆菌属（Bt）中被描述。BtSus 在许多物种中高度保守，除了其细胞外的α-淀粉酶 SusG 外。在这项工作中，我们表明，卵形拟杆菌（Bo）的细胞外α-淀粉酶 BoGH13A 在进化起源、结构域架构以及在拟杆菌属 Sus 中的最普遍形式上与 SusG 不同。BoGH13A 是糖苷水解酶家族 13（GH13_47）中的一个新亚家族和一个新的碳水化合物结合模块 CBM98 的创始成员。BoGH13A 的 CBM98-CBM48-GH13_47 结构与 SusG 的 GH13_36 中嵌入的 CBM58 不同。这些结构域采用了独特的空间取向，并与外膜形成不同的关联。BoCBM98 结合位点是 Bo 在多糖上生长和最佳酶解的必要条件。最后，BoGH13A 结构具有结合的 Ca 和 Mn 离子，后者在α-淀粉酶中是新颖的。关于 Mn 对肠道细菌功能的影响知之甚少，更不用说对多糖消耗的影响了，但向表达 BoGH13A 的 Bt 添加 Mn 可以特异性增强对淀粉的生长。进一步了解细菌淀粉降解特征将能够实现更有针对性的益生元和药物方法，增加淀粉向肠道的通量。
