Marques H M, Walton T, Egan T J
Centre for Molecular Design, Department of Chemistry, University of the Witwatersrand, Johannesburg, South Africa.
J Inorg Biochem. 1995 Jan;57(1):11-21. doi: 10.1016/0162-0134(94)00009-y.
Iron release fro C-terminal monoferric transferrin at pH 5.5 and 37 degrees C was studied as a function of chloride, phosphate, and pyrophosphate concentration. The rate constant for iron release depends linearly on chloride concentration, confirming that anion binding is mandatory for iron release, not only at pH 7.4 as has been previously reported, but also at pH 5.5. The extent of iron release is relatively small (< 20% for 1.0 M chloride). Concentrations of > 0.2 M phosphate are required for complete iron removal, but millimolar concentrations of pyrophosphate effect complete removal. The observed rate constants for iron release to phosphate and pyrophosphate change from one linear dependence to another less steep linear dependence on the concentration of these ligands, providing quantitative evidence that the two-pathway mechanism that we previously proposed for iron release at pH 7.4 persists at pH 5.5. According to this model, the pathway of iron release is determined by the nature of the anion occupying a kinetically significant anion binding site on the protein. The qualitative similarity of the current data with that recently reported for iron release from the transferrin-transferrin receptor complex provides strong support for the contention that the two-pathway mechanism also persists in this complex at low pH and is hence likely to be operative in vivo.
研究了在pH 5.5和37℃条件下，C端单铁转铁蛋白中铁的释放与氯离子、磷酸根离子和焦磷酸根离子浓度的关系。铁释放的速率常数与氯离子浓度呈线性关系，这证实了阴离子结合不仅如先前报道的那样在pH 7.4时对铁释放是必需的，在pH 5.5时也是如此。铁的释放程度相对较小（1.0 M氯离子时<20%）。完全去除铁需要>0.2 M的磷酸根离子浓度，但毫摩尔浓度的焦磷酸根离子就能实现完全去除。观察到的铁释放到磷酸根离子和焦磷酸根离子的速率常数从一种线性关系转变为对这些配体浓度的另一种较平缓的线性关系，这提供了定量证据，表明我们先前提出的在pH 7.4时铁释放的双途径机制在pH 5.5时仍然存在。根据该模型，铁释放的途径由占据蛋白质上一个动力学上重要的阴离子结合位点的阴离子性质决定。当前数据与最近报道的转铁蛋白 - 转铁蛋白受体复合物中铁释放的数据在定性上的相似性，为双途径机制在低pH下也存在于该复合物中且因此可能在体内起作用的观点提供了有力支持。
