Parodi A J
Instituto de Investigaciones Bioquímicas Fundación Campomar, Buenos Aires, Argentina.
Glycobiology. 1993 Jun;3(3):193-9. doi: 10.1093/glycob/3.3.193.
Trypanosomatid protozoa are parasites of considerable medical and economic importance in developing countries. The pathway leading to N-glycosylation in these microorganisms is characterized by the following features: (i) dolichols are composed of only 10-13 isoprene units; (ii) oligosaccharides transferred in N-glycosylation have the compositions Man(6,7,9)GlcNAc2, depending on the species; (iii) trypanosomatids are unable to synthesize dolichol-P-Glc and, in addition, some species lack certain dolichol-P-Man-dependent mannosyltransferases; (iv) the oligosaccharyltransferase does not require the presence of glucose units in the oligosaccharide in order to catalyse an efficient transfer reaction; (v) trypanosomatids have a glucosidase II-like enzyme, but lack glucosidase I; (vi) glucosidase II is required for deglucosylation of oligosaccharides glucosylated by the UDP-Glc:glycoprotein glucosyltransferase, an activity first detected in those parasites; (vii) the structures of polymannose-type compounds in these protozoa have no significant differences with those of their mammalian counterparts except for the presence, in certain species, of oligosaccharides having galactofuranose units linked to external mannose residues; (viii) biantennary complex-type oligosaccharides having in some cases terminal alpha-linked galactose units or poly-N-acetylactosamine extensions, but lacking sialic acid units, have been described in Trypanosoma brucei; (ix) complex-type oligosaccharides having alpha-linked galactose, fucose and sialic acid residues have been described in Trypanosoma cruzi. In this parasite, addition of sialic acid units to glycoproteins and glycolipids is mediated by a trans-sialidase located on the external surface of the parasite and not by an intracellular CMP-sialic acid-dependent sialyltransferase.
锥虫原生动物是发展中国家具有相当医学和经济重要性的寄生虫。这些微生物中导致N-糖基化的途径具有以下特征：（i）多萜醇仅由10 - 13个异戊二烯单元组成；（ii）N-糖基化中转移的寡糖组成取决于物种，为Man(6,7,9)GlcNAc2；（iii）锥虫无法合成多萜醇-P-葡萄糖，此外，一些物种缺乏某些依赖多萜醇-P-甘露糖的甘露糖基转移酶；（iv）寡糖基转移酶在催化高效转移反应时不需要寡糖中存在葡萄糖单元；（v）锥虫有类似葡糖苷酶II的酶，但缺乏葡糖苷酶I；（vi）葡糖苷酶II是由UDP-葡萄糖：糖蛋白葡糖基转移酶糖基化的寡糖去糖基化所必需的，这种活性最早在这些寄生虫中被检测到；（vii）除了某些物种中存在与外部甘露糖残基相连的具有呋喃半乳糖单元的寡糖外，这些原生动物中多聚甘露糖型化合物的结构与其哺乳动物对应物没有显著差异；（viii）在布氏锥虫中已描述了双天线复合型寡糖，在某些情况下具有末端α-连接的半乳糖单元或多聚N-乙酰乳糖胺延伸，但缺乏唾液酸单元；（ix）在克氏锥虫中已描述了具有α-连接的半乳糖、岩藻糖和唾液酸残基的复合型寡糖。在这种寄生虫中，向糖蛋白和糖脂中添加唾液酸单元是由位于寄生虫外表面的转唾液酸酶介导的，而不是由细胞内依赖CMP-唾液酸的唾液酸转移酶介导的。
