Nakai T, Tonouchi N, Konishi T, Kojima Y, Tsuchida T, Yoshinaga F, Sakai F, Hayashi T
Wood Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611, Japan.
Proc Natl Acad Sci U S A. 1999 Jan 5;96(1):14-8. doi: 10.1073/pnas.96.1.14.
Higher plants efficiently conserve energy ATP in cellulose biosynthesis by expression of sucrose synthase, in which the high free energy between glucose and fructose in sucrose can be conserved and used for the synthesis of UDP-glucose. A mixture of sucrose synthase and bacterial cellulose synthase proceeded to form UDP-glucose from sucrose plus UDP and to synthesize 1,4-beta-glucan from the sugar nucleotide. The mutant sucrose synthase, which mimics phosphorylated sucrose synthase, enhanced the reaction efficiency (Vmax/Km) on 1,4-beta-glucan synthesis, in which the incorporation of glucose from sucrose was increased at low concentrations of UDP. Because UDP formed after glucosyl transfer can be directly recycled with sucrose synthase, UDP-glucose formed appears to show high turnover with cellulose synthase in the coupled reaction. The expression of sucrose synthase in Acetobacter xylinum not only changed sucrose metabolism but also enhanced cellulose production, in which UDP-glucose was efficiently formed from sucrose. Although the level of UDP-glucose in the transformant with mutant sucrose synthase cDNA was only 1.6-fold higher than that in plasmid-free cells, the level of UDP was markedly decreased in the transformant. The results show that sucrose synthase serves to channel carbon directly from sucrose to cellulose and recycles UDP, which prevents UDP build-up in cellulose biosynthesis.
高等植物通过蔗糖合酶的表达在纤维素生物合成中高效地保存能量ATP，其中蔗糖中葡萄糖和果糖之间的高自由能可以被保存并用于合成UDP-葡萄糖。蔗糖合酶和细菌纤维素合酶的混合物从蔗糖加UDP生成UDP-葡萄糖，并从糖核苷酸合成1,4-β-葡聚糖。模拟磷酸化蔗糖合酶的突变型蔗糖合酶提高了1,4-β-葡聚糖合成的反应效率（Vmax/Km），在低浓度UDP时，蔗糖中葡萄糖的掺入量增加。由于糖基转移后形成的UDP可以直接与蔗糖合酶循环利用，在偶联反应中，形成的UDP-葡萄糖似乎与纤维素合酶具有高周转率。木醋杆菌中蔗糖合酶的表达不仅改变了蔗糖代谢，还提高了纤维素产量，其中蔗糖有效地形成了UDP-葡萄糖。虽然含有突变型蔗糖合酶cDNA的转化体中UDP-葡萄糖水平仅比无质粒细胞高1.6倍，但转化体中UDP水平显著降低。结果表明，蔗糖合酶有助于将碳直接从蔗糖导向纤维素，并循环利用UDP，这可防止UDP在纤维素生物合成中积累。
