Heparin and heparan sulfate are a class of glycosaminoglycans for clinical anticoagulation. Heparosan N-sulfate-glucuronate 5-epimerase (C5, EC 5.1.3.17) is a critical modifying enzyme in the synthesis of heparin and heparan sulfate, and catalyzes the inversion of carboxyl group at position 5 on D-glucuronic acid (D-GlcA) of N-sulfoheparosan to form L-iduronic acid (L-IdoA). In this study, the heparin C5 epimerase gene Glce from zebrafish was expressed and molecularly modified in Escherichia coli. After comparing three expression vectors of pET-20b (+), pET-28a (+) and pCold Ⅲ, C5 activity reached the highest ((1 873.61±5.42) U/L) with the vector pCold Ⅲ. Then we fused the solution-promoting label SET2 at the N-terminal for increasing the soluble expression of C5. As a result, the soluble protein expression was increased by 50% compared with the control, and the enzyme activity reached (2 409±6.43) U/L. Based on this, site-directed mutations near the substrate binding pocket were performed through rational design, the optimal mutant (V153R) enzyme activity and specific enzyme activity were (5 804±5.63) U/L and (145.1±2.33) U/mg, respectively 2.41-fold and 2.28-fold of the original enzyme. Modification and expression optimization of heparin C5 epimerase has laid the foundation for heparin enzymatic catalytic biosynthesis.
肝素和硫酸乙酰肝素是一类应用于临床抗凝血的糖胺聚糖。肝素葡萄糖醛酸C5 异构酶(Heparosan-N-sulfate-glucuronate 5-epimerase，C5，EC 5.1.3.17) 是肝素和硫酸乙酰肝素合成过程中重要的修饰酶，催化N-硫酸化肝素前体 (N-sulfoheparosan) 的D-葡萄糖醛酸 (D-GlcA) 上5 号位羧基翻转生成L-艾杜糖醛酸(L-iduronic acid，L-IdoA)。文中以大肠杆菌Escherichia coli 为宿主对斑马鱼来源的肝素葡萄糖醛酸C5 异构酶基因Glce 进行重组表达优化与分子改造。比较了3 种不同的表达载体pET20b(+)、pET28a(+) 和pCold Ⅲ对C5 表达的差异情况，其中以嗜冷启动型载体pCold Ⅲ表达酶活最高，达到(1 873.61±5.42) U/L。为了进一步提高C5 的可溶表达量，在N 端融合促溶标签SET2 后，可溶蛋白表达量比对照提高了50%，酶活达到 (2 409.25±6.43) U/L。在此基础上，通过理性设计对底物结合口袋进行定点突变，获得最优突变体 (V153R) 的酶活和比酶活分别为(5 804.32±5.63) U/L 和(145.14±2.33) U/mg，是原始酶的2.41 倍和2.28 倍。肝素C5 异构酶改造与表达优化为酶法催化合成肝素奠定了基础。.
Keywords: glucuronic acid-C5-epimerase; heparin; heterologous expression; rational design; substrate binding pocket.