Structure and function of beta -1,4-galactosyltransferase.
Structure and function of beta -1,4-galactosyltransferase.
复制标题
DOI：
--
复制
发表时间：
2008
影响因子：
3.2
通讯作者：
P. Qasba;B. Ramakrishnan;E. Boeggeman
中科院分区：
文献类型：
--
作者：
P. Qasba;B. Ramakrishnan;E. Boeggeman
β-1，4-半乳糖基转移酶（beta4 Gal-T1）参与糖缀合物的Galbeta 1 -4-GlcNAc-二糖单元的合成。它是一种反式高尔基体糖基转移酶（Glyco-T），具有II型膜蛋白拓扑结构、短的N-末端胞质结构域、跨膜区以及面向反式高尔基体腔的茎和C-末端催化结构域。它的疏水跨膜区，像其他糖T，具有较短的长度相比，质膜蛋白，一个重要的特点，其保留在反式高尔基体。催化结构域有两个灵活的环，一个长的和一个小的。主要金属结合位点位于长柔性环的N-末端铰链区。当金属离子和糖-核苷酸结合时，柔性环经历显著的构象变化，从开放构象变为闭合构象。构象变化同时在柔性环的C-末端区域产生以前不存在的寡糖受体结合位点。环充当覆盖结合的供体基底的盖。糖基单元转移到受体完成后，糖产物被排出;环恢复到其天然构象以释放剩余的核苷酸部分。β 4Gal-T1的构象变化还产生了乳腺特异性蛋白α-乳白蛋白（LA）的结合位点，其改变了酶对葡萄糖的受体特异性，从而在哺乳期间合成乳糖。糖供体的特异性通常由Glyco-T的糖-核苷酸结合口袋中的几个残基决定，这些残基在来自不同物种的家族成员中是保守的。这些残基的突变使我们能够设计新的和新颖的糖基转移酶，具有更广泛的或必要的供体和受体特异性，并合成特定的复合碳水化合物以及这些酶的特异性抑制剂。
Beta-1,4-galactosylransferase (beta4Gal-T1) participates in the synthesis of Galbeta1-4-GlcNAc-disaccharide unit of glycoconjugates. It is a trans-Golgi glycosyltransferase (Glyco-T) with a type II membrane protein topology, a short N-terminal cytoplasmic domain, a membrane-spanning region, as well as a stem and a C-terminal catalytic domain facing the trans-Golgi-lumen. Its hydrophobic membrane-spanning region, like that of other Glyco-T, has a shorter length compared to plasma membrane proteins, an important feature for its retention in the trans-Golgi. The catalytic domain has two flexible loops, a long and a small one. The primary metal binding site is located at the N-terminal hinge region of the long flexible loop. Upon binding of metal ion and sugar-nucleotide, the flexible loops undergo a marked conformational change, from an open to a closed conformation. Conformational change simultaneously creates at the C-terminal region of the flexible loop an oligosaccharide acceptor binding site that did not exist before. The loop acts as a lid covering the bound donor substrate. After completion of the transfer of the glycosyl unit to the acceptor, the saccharide product is ejected; the loop reverts to its native conformation to release the remaining nucleotide moiety. The conformational change in beta4Gal-T1 also creates the binding site for a mammary gland-specific protein, alpha-lactalbumin (LA), which changes the acceptor specificity of the enzyme toward glucose to synthesize lactose during lactation. The specificity of the sugar donor is generally determined by a few residues in the sugar-nucleotide binding pocket of Glyco-T, conserved among the family members from different species. Mutation of these residues has allowed us to design new and novel glycosyltransferases, with broader or requisite donor and acceptor specificities, and to synthesize specific complex carbohydrates as well as specific inhibitors for these enzymes.