Sucrose-hydrolyzing enzymes are largely divided into β-fructofuranosidase and sucrose α-glucosidase. The domestic silkworm Bombyx mori possesses both enzymes, BmSUC1 and BmSUH, belonging to the glycoside hydrolase family 32 (GH32) and GH13, respectively. BmSUC1 was presumed to be acquired by horizontal gene transfer from bacteria based on phylogenetic analysis and related to tolerance to sugar-mimic alkaloids contained in mulberry latex. Here we investigated the substrate specificity of recombinant BmSUC1 that can hydrolyze not only sucrose but also fructooligosaccharides and fructans, and revealed that the enzyme was competitively inhibited by 1,4-dideoxy-1,4-imino-D-arabinitol, one of the alkaloids. Moreover, the crystal structures of BmSUC1 in apo form and complex with sucrose were determined, and the active site pocket was shallow and suitable for shorter substrates but was related to more relaxed substrate specificity than the strict sucrose α-glucosidase BmSUH. Considering together with the distribution of BmSUC1-orthologous genes in many lepidopterans, our results suggest that BmSUC1 contributes to the digestion of fructooligosaccharides and fructans derived from feed plants.

蔗糖水解酶主要分为β-果糖呋喃糖苷酶和蔗糖α-葡萄糖苷酶。国内蚕家蚕拥有两种酶BmSUC1和BmSUH，分别属于糖苷水解酶家族32（GH32）和GH13。据推测，BmSUC1是通过基于系统发育分析从细菌中进行水平基因转移而获得的，并且与桑树乳胶中所含糖模拟生物碱的耐受性有关。在这里，我们研究了重组BmSUC1的底物特异性，该BmSUC1不仅可以水解蔗糖，还可以水解低聚果糖和果聚糖，并揭示了该酶被1,4-dideoxy-1,4-imino-D-arabinitol（一种生物碱）竞争性抑制。 。此外，确定了载脂蛋白形式和与蔗糖复合的BmSUC1的晶体结构，并且活性位点袋浅且适合于较短的底物，但与严格的蔗糖α-葡萄糖苷酶BmSUH相比，其底物特异性更高。