Structures and Substrate Recognitions of Animal, Botanical and Microbial α-Glucosidases and Their Molecular Evolution

α-葡萄糖苷酶(EC3.2.1.20)是一种典型的外型糖苷酶，能从底物的非还原一侧释放α-葡萄糖。我们感兴趣的是催化作用与结构之间的关系，因为底物专一性因酶的来源而有很大差异。至少有两种类型的α-葡萄糖苷酶表现出不同的底物识别能力，这表明酶可以分为两类(家族I和家族II)。然而，结构信息不足以了解属于每个基团的酶的结构是否同源。本课题对动物、植物和微生物来源的α-葡萄糖苷酶的一级结构进行了分析，发现I和II家族的酶具有不同的一级氨基酸序列，昆虫和细菌的α-葡萄糖苷酶属于I家族，其分子量约为70 kDa。有4个催化区与α-淀粉酶相似。对杂苷(蔗糖或对硝基苯基α-葡萄糖苷)的活性高于麦芽低聚糖。家族II的成员来自动物、植物和霉菌，表现出相反的底物专一性，对全苷活性高，对杂苷活性低。分子大小约为100 kDa。我们分析了属于这一类的六种α-葡萄糖苷酶的一级结构。获得的序列彼此同源，但没有观察到与家族I酶的相似性。这些发现表明，α-葡萄糖苷酶是由两种不同的祖先蛋白进化而来的。

α-Glucosidase (EC 3.2.1.20) is a typical exo-type glycosidase that releases the α-glucose from non-reducing side of substrate. We are interested in the relationship between catalytic action and the structure, since the substrate specificity differs greatly with the source of enzyme. There are at least two types of α-glucosidases which show different substrate recognitions, suggesting that enzymes can be classified into two groups (family I and family II). However, the structural information is not enough to learn that structures of enzymes belonging to each group are homologous or not. In this project, we analyzed the primary structures of α-glucosidases from animal, botanical and microbial origins, and found that the enzymes of family I and II had different primary amino acid sequences.α-Glucosidases from insect and bacteria belonged to family I, of which molecular weight was about 70 kDa. Four catalytic regions found were similar to those of α-amylase. The activity toward heteroside (sucrose or p-nitrophenyl α-glucoside) was higher than holoside (maltooligosaccharides). Members of family II were from animal, botanical and mold origins, and showed the opposite substrate specificity, high activity to holoside and low to heteroside. The molecular sizes were about 100 kDa. We analyzed the primary structures of six kinds of α-glucosidases belonging to this group. The sequences obtained were homologous each other, but no similarity was observed with family I enzymes. The findings suggest that the α-glucosidase was evolved from two different ancestral proteins.