Glycobiology of erythropoietin

促红细胞生成素（EPO）是红细胞生成的生理刺激物。EPO是一种高度糖基化的蛋白质，分子量为35-39 kDa;其分子量的一半是糖。其在氨基酸位置24、38和83处具有三条N-连接寡糖链，在位置126处具有一条O-连接寡糖链。这种蛋白质在成人肾脏中合成，并分泌到循环中到达造血组织。以下生物学功能可以分配给附着在蛋白质上的碳水化合物，例如在内分泌系统中起作用的EPO。首先，糖蛋白的碳水化合物是其生物合成和分泌所必需的。其次，碳水化合物赋予蛋白质稳定性，使得在其从生产位点到其靶细胞的行进期间不会从循环中消除。第三，碳水化合物参与糖蛋白表达其生物学功能。为了阐明糖链的生物学功能，研究了重组EPO中N-糖的定点去除及其对EPO生物合成和生物学特性的影响。结果表明：（1）糖影响与配体的结合亲和力，但在体外活性的表达中不起关键作用，（2）它们对于体内活性的表达是必需的，和（3）N-连接的糖对于生物合成和/或极化分泌是重要的。

Erythropoietin (EPO) is a physiological stimulator in erythropoiesis. EPO is a heavily glycosylated protein with a molecular mass of 35-39 kDa; half of its molecular weight is sugars. It has three N-linked oligosaccharide chains at amino acid positions 24, 38, and 83 and one O-linked oligosaccaride chain at position 126. This protein is synthesized in adult kidneys and secreted into the circulation to reach hemopoietic tissues. The following biological functions can be assigned to carbohydrates attached to proteins such as EPO that act in the endocrine system. First, the carbohydrates of glycoproteins are needed for their biosynthesis and secretion. Second, carbohydrates confer stability to the protein so that is not eliminated from the circulation during its travel from the production site to its target cells. Third, carbohydrates are involved in the expression by the glycoprotein of its biological function. To elucidate biological functions of sugar chains, the site-directed removal of N-linked sugars of recombinant EPO, and their effects on biosynthesis and biological properties of EPO were investigated. The results indicate that (1) the sugars affect binding affinity to the ligand but do not play a key role in expression of the in vitro activity, (2) they are essential for the expression of the in vivo activity, and (3) N-linked sugars are important for biosynthesis and/or polarized secretion.