Function and structure of glucosyltransferase of Streptococcus sobrinus

远缘链球菌（Streptococcus sobrinus）产生不溶于水的葡聚糖酶，该酶作用于蔗糖，通过沿着葡聚糖的α-1，6链在α-1，3连接的分支上钉合而形成不溶性葡聚糖。在本研究中，有证据表明远缘链球菌6715 GTF-I使用蔗糖作为唯一底物进行葡聚糖非依赖性α-1，3葡聚糖形成。该酶制剂使用了通过分子克隆方法获得的GTF-1 ^1亚片段，该亚片段在整个GTF-1肽的N-和C-末端区域都缺少一些氨基酸残基，但如果存在引物葡聚糖，则可以快速产生不溶性葡聚糖，就像GTF-1一样。当GTF-Ⅰ ^1与蔗糖共同孵育时，加入葡萄糖或黑曲霉糖，黑曲霉糖促进蔗糖裂解和葡萄糖基转移反应，而葡萄糖虽然促进了葡萄糖基转移反应，但对蔗糖裂解反应的促进作用并不大，这些实验结果表明，葡萄糖基转移可能首先发生在葡萄糖形成（glc）2或黑曲霉糖上，其又作为葡糖基受体形成（GLC）3等。

Streptococcus sobrinus produce water-insoluble glucanthase which acts on sucrose to form insoluble glucan by tacking on alpha-1,3 linked branch along the alpha-1,6 chain of dextran. In this study, evidence is presented that S.sobrinus 6715 GTF-I performs the dextran-independent alpha-1,3 glucan formation using sucrose as a sole substrate. The enzyme preparation used a subfragment of GTF-I^1 obtained by a molecular cloning method, which lacks some amino acid residues bothe in N- and C-terminal regions of whole GTF-I peptide but rapidly produces insoluble glucans as does the GTF-I if primer dextran are present. When GTF-I^1 was incubated with sucose added glucose or nigerose, nigerose enhance the sucrose splitting and glucosyl transfer reaction, but glucose did not enhance so much sucrose splitting reaction in spite of acceleration of glucosyl transfer reaction.The results these experiments suggest a possibility that the glucosyl transfer first occurs to glucose forming (glc) 2 or nigerose, which in turn serves as the glucosyl acceptor to form (glc) 3 and so on.