Tonetti M, Sturla L, Bisso A, Zanardi D, Benatti U, De Flora A
Institute of Biochemistry, University of Genova, Italy.
Biochimie. 1998 Nov;80(11):923-31. doi: 10.1016/s0300-9084(00)88889-6.
L-fucose and L-rhamnose are two 6-deoxyhexoses naturally occurring in several complex carbohydrates. In prokaryotes both of them are found in polysaccharides of the cell wall, while in animals only L-fucose has been described, which mainly participates to the structure of glycoconjugates, either in the cell membrane or secreted in biological fluids, such as ABH blood groups and Lewis system antigens. L-fucose and L-rhamnose are synthesized by two de novo biosynthetic pathways starting from GDP-D-mannose and dTDP-D-glucose, respectively, which share several common features. The first step for both pathways is a dehydration reaction catalyzed by specific nucleotide-sugar dehydratases. This leads to the formation of unstable 4-keto-6-deoxy intermediates, which undergo a subsequent epimerization reaction responsible for the change from D- to L-conformation, and then a NADPH-dependent reduction of the 4-keto group, with the consequent formation of either GDP-L-fucose or dTDP-L-rhamnose. These compounds are then the substrates of specific glycosyltransferases which are responsible for insertion of either L-fucose or L-rhamnose in the corresponding glycoconjugates. The enzyme involved in the first step of GDP-L-fucose biosynthesis in E. coli, i.e., GDP-D-mannose 4,6 dehydratase, has been recently expressed as recombinant protein and characterized in our laboratory. We have also cloned and fully characterized a human protein, formerly named FX, and an E. coli protein, WcaG, which display both the epimerase and the reductase activities, thus indicating that only two enzymes are required for GDP-L-fucose production. Fucosylated complex glycoconjugates at the cell surface can then be recognized by specific counter-receptors in interacting cells, these mechanisms initiating important processes including inflammation and metastasis. The second pathway starting from dTDP-D-glucose leads to the synthesis of antibiotic glycosides or, alternatively, to the production of dTDP-L-rhamnose. While several sets of data are available on the first enzyme of the pathway, i.e., dTDP-D-glucose dehydratase, the enzymes involved in the following steps still need to be identified and characterized.
L-岩藻糖和L-鼠李糖是两种天然存在于多种复合碳水化合物中的6-脱氧己糖。在原核生物中，它们都存在于细胞壁的多糖中，而在动物中，仅发现了L-岩藻糖，它主要参与糖缀合物的结构，存在于细胞膜中或分泌到生物体液中，如ABH血型和Lewis系统抗原。L-岩藻糖和L-鼠李糖分别由两条从头合成途径合成，起始于GDP-D-甘露糖和dTDP-D-葡萄糖，这两条途径有几个共同特征。两条途径的第一步都是由特定的核苷酸糖脱水酶催化的脱水反应。这导致形成不稳定的4-酮基-6-脱氧中间体，该中间体随后经历差向异构化反应，导致从D-构象转变为L-构象，然后4-酮基被NADPH依赖性还原，从而形成GDP-L-岩藻糖或dTDP-L-鼠李糖。这些化合物随后是特定糖基转移酶的底物，这些糖基转移酶负责将L-岩藻糖或L-鼠李糖插入相应的糖缀合物中。大肠杆菌中GDP-L-岩藻糖生物合成第一步所涉及的酶，即GDP-D-甘露糖4,6-脱水酶，最近已作为重组蛋白表达并在我们实验室中进行了表征。我们还克隆并全面表征了一种以前称为FX的人类蛋白和一种大肠杆菌蛋白WcaG，它们同时具有差向异构酶和还原酶活性，因此表明GDP-L-岩藻糖的产生仅需要两种酶。细胞表面的岩藻糖基化复合糖缀合物随后可被相互作用细胞中的特定反受体识别，这些机制引发包括炎症和转移在内的重要过程。从dTDP-D-葡萄糖开始的第二条途径导致抗生素糖苷的合成，或者导致dTDP-L-鼠李糖的产生。虽然关于该途径的第一种酶，即dTDP-D-葡萄糖脱水酶，已有多组数据，但后续步骤所涉及的酶仍有待鉴定和表征。
