Structural basis for mammalian vitamin B12 transport by transcobalamin
Structural basis for mammalian vitamin B12 transport by transcobalamin
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DOI：
10.1073/pnas.0509099103
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发表时间：
2006-03-21
影响因子：
11.1
通讯作者：
Randaccio, L
中科院分区：
文献类型：
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作者：
Wuerges, J;Garau, G;Randaccio, L
钴胺(Cobalamin，Cb1，维生素B-12)是哺乳动物的两种基本辅因子。其肠道吸收、血浆转运和细胞摄取的途径使用细胞表面受体和三种Cbl转运蛋白，即结合角蛋白、内源性因子和转钴胺(TC)。我们对哺乳动物Cbl转运蛋白家族中的一个成员进行了结构测定。重组人和牛HALO-TCS的晶体结构显示为两个结构域，N-末端有一个α(6)-α(6)桶，C-末端有一个较小的结构域。一个碱基构象的Cbl分子被埋在结构域界面内。结合先前的结合分析，结构数据表明在Cb1结合的第一步中有结构域运动。在第二步中，添加的H2O-Cbl上轴侧的弱配位配体H2O被α(6)-α(6)桶的组跃层残基取代。对同源蛋白中TC表面氨基酸保守性的分析表明，TC受体识别位点位于a6-a6桶的延伸区域。TC结构允许定位由于人类TC基因的多态而引起的氨基酸变异的位置。结构信息被用来预测结合角蛋白和内在因子的整体折叠，并允许一个合理的方法来设计用于诊断或治疗药物输送的新的基于Cbl的生物结合物。
Cobalamin (Cbl, vitamin B-12) serves for two essential cofactors in mammals. The pathway for its intestinal absorption, plasma transport, and cellular uptake uses cell surface receptors and three Cbl-transporting proteins, haptocorrin, intrinsic factor, and transcobalamin (TC). We present the structure determination of a member of the mammalian Cbl-transporter family. The crystal structures of recombinant human and bovine holo-TCs reveal a two-domain architecture, with an N-terminal alpha(6)-alpha(6) barrel and a smaller C-terminal domain. One Cbl molecule in base-on conformation is buried inside the domain interface. Structural data combined with previous binding assays indicate a domain motion in the first step of Cbl binding. In a second step, the weakly coordinated ligand H2O at the upper axial side of added H2O-Cbl is displaced by a histicline residue of the alpha(6)-alpha(6) barrel. Analysis of amino acid conservation on TC's surface in orthologous proteins suggests the location of the TC-receptor-recognition site in an extended region on the a6-a6 barrel. The TC structure allows for the mapping of sites of amino acid variation due to polymorphisms of the human TC gene. Structural information is used to predict the overall fold of haptocorrin and intrinsic factor and permits a rational approach to the design of new Cbl-based bioconjugates for diagnostic or therapeutic drug delivery.