Metabolism of polysaccharides by the Streptococcus mutants dexB gene product.
Metabolism of polysaccharides by the Streptococcus mutants dexB gene product.
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DOI：
10.1099/00221287-139-9-2019
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发表时间：
1993-09
期刊：
影响因子：
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通讯作者：
G. C. Whiting;I. Sutcliffe;R. Russell
中科院分区：
文献类型：
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作者：
G. C. Whiting;I. Sutcliffe;R. Russell
变形链球菌dexB基因是多种糖代谢（msm）操纵子的成员，编码细胞内葡聚糖1，6-α-葡糖苷酶，其从α-1，6-连接的异麦芽糖醛酸和葡聚糖的非还原末端释放葡萄糖。一级氨基酸序列的比较显示出与芽孢杆菌寡-1，6-葡糖苷酶的高度同源性，并且与这些酶一样，DexB能够从潘糖中的α-1，4，6-分支点释放游离葡萄糖。这表明DexB在淀粉或细胞内多糖的代谢中的作用，其中包含这样的分支点。然而，从野生型S.变异株LT 11和dexB缺陷突变株的碘染色光谱和聚合度显示，分支程度没有实质性差异。此外，放射性标记的细胞内多糖用S.变异体野生型和突变体细胞提取物在所获得的产物中没有显示出差异。使用α-淀粉酶作用于淀粉产生的α-极限糊精研究DexB在膳食淀粉代谢中的参与。这些诱导msm操纵子，包括dexB，并且DexB酶能够作用于α-极限糊精以得到进一步的可发酵底物。由msm操纵子编码的转运系统也可以转运α-极限糊精。因此，DexB在细胞外淀粉的代谢中可能是重要的。
The Streptococcus mutans dexB gene, a member of the multiple sugar metabolism (msm) operon, encodes an intracellular glucan 1,6-alpha-glucosidase which releases glucose from the non-reducing terminus of alpha-1,6-linked isomaltosaccharides and dextran. Comparison of primary amino acid sequences showed strong homology to Bacillus oligo-1,6-glucosidases and, like these enzymes, DexB was able to release free glucose from the alpha-1,4,6-branch point in panose. This suggested a role for DexB in the metabolism of either starch or intracellular polysaccharide, which contain such branch points. However, purified intracellular polysaccharide from the wild-type S. mutans strain LT11 and a mutant deficient in dexB revealed no substantial differences in the extent of branching as demonstrated by iodine staining spectra and the degree of polymerization. Furthermore, thin layer chromatography of radiolabelled intracellular polysaccharide digested with S. mutans wild-type and mutant cell extracts showed no differences in the products obtained. The involvement of DexB in dietary starch metabolism was investigated using alpha-limit dextrins produced from the action of alpha-amylase on starch. These induced the msm operon, including dexB, and the DexB enzyme was able to act on the alpha-limit dextrins to give further fermentable substrates. The transport system encoded by the msm operon can also transport alpha-limit dextrin. DexB may therefore be important in the metabolism of extracellular starch.