Composition of pH-sensitive triad in C-lobe of human serum transferrin.: Comparison to sequences of ovotransferrin and lactoferrin provides insight into functional differences in iron release
Composition of pH-sensitive triad in C-lobe of human serum transferrin.: Comparison to sequences of ovotransferrin and lactoferrin provides insight into functional differences in iron release
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DOI：
10.1021/bi0518693
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发表时间：
2005-11-29
期刊：
影响因子：
2.9
通讯作者：
Mason, AB
中科院分区：
文献类型：
--
作者：
Halbrooks, PJ;Giannetti, AM;Mason, AB
转铁蛋白（transferrin，TF）是一个与三价铁紧密结合的双叶糖蛋白家族。每一个同源的N-和C-叶包含一个单一的铁结合位点位于一个深裂。人血清转铁蛋白（hTF）作为血液中的铁转运蛋白;循环转铁蛋白与细胞表面的受体结合，复合物通过内吞作用内化。在细胞内，pH的降低导致铁在受体依赖性过程中从hTF释放，导致每个叶中的大的构象变化。在hTF N-叶中，两个关键赖氨酸促进这种pH依赖性构象变化，允许螯合剂进入以捕获铁。在C端半段中，赖氨酸对被三个残基取代：Lys 534，Arg 632和Asp 634。先前的研究表明，这些三联体残基中的任何一个突变为丙氨酸导致在pH 7.4和pH 5.6下铁释放的显著延迟。在目前的工作中，这三个残基的作用进一步探讨转换为在卵转铁蛋白（Q-K-L）和人乳铁蛋白（K-N-N）中的等效位置观察到的残基，以及一个三联体与互换的赖氨酸和精氨酸（K534 R/R632 K）。如所预期的，所有构建体结合铁并以与野生型单铁hTF对照几乎相同的K-D与受体缔合。然而，在pH 5.6的存在和不存在的受体的情况下，观察到的取代对铁释放速率的影响的有趣的差异。此外，用KCl滴定表明，632位必须具有带正电的残基，以引起随盐增加而增强的速率加速。在这些观察的基础上，提出了从hTF C叶释放铁的模型。这些研究提供了深入了解在这些位置的氨基酸的电荷和几何形状的重要性，作为个别TF家族成员，人血清转铁蛋白，卵转铁蛋白和乳铁蛋白的行为差异的部分解释。这些研究共同强调了TF的N-和Globes共同的重要特征以及受体在铁释放中的关键作用。
The transferrins (TF) are a family of bilobal glycoproteins that tightly bind ferric iron. Each of the homologous N- and C-lobes contains a single iron-binding site situated in a deep cleft. Human serum transferrin (hTF) serves as the iron transport protein in the blood; circulating transferrin binds to receptors on the cell surface, and the complex is internalized by endocytosis. Within the cell, a reduction in pH leads to iron release from hTF in a receptor-dependent process resulting in a large conformational change in each lobe. In the hTF N-lobe, two critical lysines facilitate this pH-dependent conformational change allowing entry of a chelator to capture the iron. In the C-lobe, the lysine pair is replaced by a triad of residues: Lys534, Arg632, and Asp634. Previous studies show that mutation of any of these triad residues to alanine results in significant retardation of iron release at both pH 7.4 and pH 5.6. In the present work, the role of the three residues is probed further by conversion to the residues observed at the equivalent positions in ovotransferrin (Q-K-L) and human lactoferrin (K-N-N) as well as a triad with an interchanged lysine and arginine (K534R/R632K). As expected, all of the constructs bind iron and associate with the receptor with nearly the same K-D as the wild-type monoferric hTF control. However, interesting differences in the effect of the substitutions on the iron release rate in the presence and absence of the receptor at pH 5.6 are observed. Additionally, titration with KCl indicates that position 632 must have a positively charged residue to elicit a robust rate acceleration as a function of increasing salt. On the basis of these observations, a model for iron release from the hTF C-lobe is proposed. These studies provide insight into the importance of charge and geometry of the amino acids at these positions as a partial explanation for differences in behavior of individual TF family members, human serum transferrin, ovotransferrin, and lactoferrin. The studies collectively highlight important features common to both the N- and Globes of TF and the critical role of the receptor in iron release.