STRUCTURAL BASIS OF VITAMIN B12 (COBALAMIN) DEFICIENCY

哺乳动物蛋白质转钴胺素II（TCII）负责通过血液运输维生素B12（更准确地称为钴胺素）。 维生素B12被摄入并通过肠道回肠细胞进入血流，在那里它被转钴胺素II携带到呈递特异性TCII/钴胺素表面受体的细胞。 在受体介导的内吞作用后，维生素需要进一步修饰，然后才能采用两种酶所需的活性辅因子结构，细胞质甲硫氨酸合酶（甲基-B12）和线粒体甲基丙二酰辅酶A β（5 '-脱氧腺苷-B12）。 TCII的遗传突变体导致常染色体隐性遗传疾病，在婴儿期表现为巨幼细胞性贫血。 与钴胺素结合的转钴胺素II的三维结构将使用X射线晶体学来解决，并将为人类遗传疾病提供分子解释。 肠转运蛋白，内在因子，与TCII具有高度同源性，并且也将通过同源建模或直接结构测定在结构上进行研究。具体目标：1）通过X射线衍射技术解析转钴胺素II的三维结构，并研究维生素B12转运生理中断的结构基础。 2)确定TCII和钴胺素之间的关键相互作用，并绘制与识别TCII/钴胺素复合物的特定细胞表面受体相互作用的蛋白质表面。 3)基于与TCII的高度氨基酸序列同一性，产生内因子（IF）、结合咕啉和转钴胺素I（TCI）的模型。

The mammalian protein transcobalamin II (TCII) is responsible for the transport of vitamin B12 (more accurately termed cobalamin) through the bloodstream. Vitamin B12 is ingested and passed through gut ileal cells to the bloodstream where it is carried by transcobalamin II to cells presenting a specific TCII/cobalamin surface receptor. After receptor mediated endocytosis the vitamin requires further modification before adopting an active cofactor structure required for two enzymes, cytoplasmic methionine synthase (methyl-B12) and mitochondrial methylmalonyl CoA mutase (5'-deoxyadenosyl-B12). Genetic mutants of TCII cause an autosomal recessive disorder that presents itself in infancy with megaloblastic anemia. The 3-dimensional structure of transcobalamin II bound to cobalamin will be solved using X-ray crystallography, and will provide a molecular explanation of a human genetic disorder. The intestinal transport protein, intrinsic factor, shares high homology with TCII and will also be pursued structurally, either by homology modeling or direct structure determination. Specific Aims: 1) Solve the three-dimensional structure of transcobalamin II by X-ray diffraction techniques, and investigate the structural basis for the physiological disruption of vitamin B12 transport. 2) Determine the critical interactions between TCII and cobalamin, and map the surface of the protein which interacts with the specific cell-surface receptor that recognises the TCII/cobalamin complex. 3) Produce a model of intrinsic factor (IF), haptocorrin and transcobalamin I (TCI) based on the high amino acid sequence identity with TCII.