胃幽門螺旋桿菌岩藻糖轉移酶之受質選擇性

Abstract

Helicobacter pylori is a major gastroduodenal pathogen to cause gastric and duodenal ulcers. The infection is associated with atrophic gastritis, a precondition to gastric cancer and lymphoma. This pathogenic bacterium expresses fucose-containing Lewis antigens as a molecular mimicry to the cell surface glycoconjugates of gastric epithelial cells in order to avoid the detection of host immune system. Such mimicry also plays an important role in mucosal adhesion, immune evasion and persistent colonization. Fucosyltransferases (FTs) are the enzymes responsible for the last steps in the biosynthesis of Lewis carbonhydrate determinants, including Lea, Leb, Lex, Ley, sLex and sLey. Expression of these antigens in H. pylori is phase variable, controlled by an on-and-off switching of specific genes involved in LPS biosynthesis. Among the enzymes involved in the biosynthesis, a1,3/4-FT catalyzes the last step of glycosylation. We have cloned the fut gene from H. pylori NCTC11639 and expressed the protein in E. coli for further studies. The enzyme was expressed at a high level but obtained in an insoluble form. Several C-terminal truncations were thus constructed to enhance the protein solubility and activity. This thesis aims to investigate the substrate specificity of H. pylori a1,3-FT, with a special emphasis on longer oligosaccharides. The results were verified by mass spectrometry. Our finding indicates that this enzyme has broad substrate specificity with sialylation and sulfation tolerance. The essential groups are consistent of those in human a1,3/4-FTs. Although H. pylori and human counterparts share low sequence similarity, they display several common features including the essential hydroxyl groups and substrate specificity. Therefore, H. pylori FT could be a useful model for studying the entire FT family. This information can be employed for the use in enzymatic synthesis of fucosylated glycoconjugates.