Institute of Biomedicine, Department of Medical Biochemistry and Cell Biology, University of Gothenburg, PO Box 440, SE-40530 Göteborg, Sweden.
Biochimie. 2010 May;92(5):482-90. doi: 10.1016/j.biochi.2010.02.010. Epub 2010 Feb 18.
Enterotoxigenic Escherichia coli and Vibrio cholerae are well known causative agents of severe diarrheal diseases. Both pathogens produce AB(5) toxins, with one enzymatically active A-subunit and a pentamer of receptor-binding B-subunits. The primary receptor for both B-subunits is the GM1 ganglioside (Galbeta3GalNAcbeta4(NeuAcalpha3)Galbeta4GlcbetaCer), but the B-subunits from porcine isolates of E. coli also bind neolacto-(Galbeta4GlcNAcbeta-)terminated glycoconjugates and the B-subunits from human isolates of E. coli (hLTB) have affinity for blood group A type 2-(GalNAcalpha3(Fucalpha2)Galbeta4GlcNAcbeta-)terminated glycoconjugates. A B-subunit with 73% sequence identity to the B-subunits of cholera toxin and the heat-labile toxin of E. coli is produced by certain strains of enteropathogenic E. coli and by Citrobacter freundii. This C. freundii B-subunit (CFXB) has now been expressed in V. cholerae, and isolated in high yields. Glycosphingolipid binding studies show that CFXB binds to the GM1 ganglioside with high affinity. In addition, CFXB has high affinity for both neolacto-terminated and blood group A type 2-terminated glycoconjugates. The crystal structure of the pentameric arrangement of C. freundii B-subunits display high structural similarity with related proteins from E. coli and V. cholerae and oligosaccharide binding sites can be identified on the protein surface. Small changes in the 88-95 loop connecting the GM1 and blood group A binding sites explains the minor changes in affinity seen for these two ligands. However, the enhanced affinity of CFXB for neolacto-terminated structures can be sought in the Lys34Tyr substitution affording additional hydrogen bond interactions between the tyrosyl side chain and the GlcNAcbeta3Galb4Glcbeta1 segment of neolactotetraosylceramide via bridging water molecules.
肠毒素性大肠杆菌和霍乱弧菌是众所周知的严重腹泻疾病的病原体。这两种病原体都产生 AB(5)毒素，其中一个酶活性 A 亚基和一个五聚体受体结合 B 亚基。两种 B 亚基的主要受体都是 GM1 神经节苷脂（Galβ3GalNAcβ4(NeuAcalpha3)Galβ4GlcβCer），但来自猪源大肠杆菌分离株的 B 亚基也与新乳糖（Galβ4GlcNAcβ-）末端糖缀合物结合，而来自人源大肠杆菌分离株的 B 亚基（hLTB）则与血型 A 型 2-（GalNAcα3(Fucα2)Galβ4GlcNAcβ-）末端糖缀合物结合。某些肠致病性大肠杆菌和弗氏柠檬酸杆菌产生的与霍乱毒素和大肠杆菌不耐热毒素的 B 亚基具有 73%序列同一性的 B 亚基。弗氏柠檬酸杆菌 B 亚基（CFXB）现已在霍乱弧菌中表达，并以高产量分离。糖脂结合研究表明，CFXB 与 GM1 神经节苷脂具有高亲和力。此外，CFXB 对新乳糖末端和血型 A 型 2 末端糖缀合物具有高亲和力。弗氏柠檬酸杆菌 B 亚基五聚体排列的晶体结构与大肠杆菌和霍乱弧菌的相关蛋白具有高度结构相似性，可以在蛋白质表面上识别寡糖结合位点。连接 GM1 和血型 A 结合位点的 88-95 环中的微小变化解释了这两种配体亲和力的微小变化。然而，CFXB 对新乳糖末端结构的亲和力增强可以通过 Lys34Tyr 取代来寻求，通过桥接水分子，在 CFXB 酪氨酸侧链和新乳糖四糖神经酰胺的 GlcNAcβ3Galb4Glcβ1 片段之间提供额外的氢键相互作用。
