MacGillivray R T, Moore S A, Chen J, Anderson B F, Baker H, Luo Y, Bewley M, Smith C A, Murphy M E, Wang Y, Mason A B, Woodworth R C, Brayer G D, Baker E N
Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, Canada.
Biochemistry. 1998 Jun 2;37(22):7919-28. doi: 10.1021/bi980355j.
The N-lobe of human serum transferrin (hTF/2N) has been expressed in baby hamster kidney cells and crystallized in both orthorhombic (P212121) and tetragonal (P41212) space groups. Both crystal forms diffract to high resolution (1.6 and 1.8 A, respectively) and have been solved by molecular replacement. Subsequent refinement resulted in final models for the structure of hTF/2N that had crystallographic R-factors of 18.1 and 19.7% for the two crystal forms, respectively; these models represent the highest-resolution transferrin structures determined to date. The hTF/2N polypeptide has a folding pattern similar to those of other transferrins, including the presence of a deep cleft that contains the metal-binding site. In contrast to other transferrins, both crystal forms of hTF/2N display disorder at the iron-binding site; model building suggests that this disorder consists of alternative conformations of the synergistically bound carbonate anion, the side chain for Arg-124, and several solvent molecules. Subsequent refinement revealed that conformation A has an occupancy of 0.63-0. 65 and corresponds to the structure of the iron-binding site found in other transferrins. The alternative conformation B has an occupancy of 0.35-0.37; in this structure, the carbonate has rotated 30 degrees relative to the iron and the side chain for Arg-124 has moved to accommodate the new carbonate position. Several water molecules appear to stabilize the carbonate anion in the two conformations. These structures are consistent with the protonation of the carbonate and resulting partial removal of the anion from the metal; these events would occur prior to cleft opening and metal release.
人血清转铁蛋白的N叶（hTF/2N）已在仓鼠肾细胞中表达，并在正交晶系（P212121）和四方晶系（P41212）空间群中结晶。两种晶体形式都能衍射到高分辨率（分别为1.6埃和1.8埃），并已通过分子置换法解析。随后的精修得到了hTF/2N结构的最终模型，两种晶体形式的晶体学R因子分别为18.1%和19.7%；这些模型代表了迄今为止确定的最高分辨率的转铁蛋白结构。hTF/2N多肽的折叠模式与其他转铁蛋白相似，包括存在一个包含金属结合位点的深裂缝。与其他转铁蛋白不同，hTF/2N的两种晶体形式在铁结合位点均表现出无序；模型构建表明，这种无序由协同结合的碳酸根阴离子、Arg-124的侧链和几个溶剂分子的替代构象组成。随后的精修显示，构象A的占有率为0.63 - 0.65，对应于其他转铁蛋白中发现的铁结合位点结构。替代构象B的占有率为0.35 - 0.37；在这种结构中，碳酸根相对于铁旋转了30度，Arg-124的侧链移动以适应新的碳酸根位置。几个水分子似乎稳定了两种构象中的碳酸根阴离子。这些结构与碳酸根的质子化以及随后阴离子从金属上的部分去除一致；这些事件将在裂缝打开和金属释放之前发生。
