Taborsky G
J Biol Chem. 1980 Apr 10;255(7):2976-84.
With a view to the potential biological significance of iron binding by the phosphoprotein phosvitin, the interaction of these two electrostatically complementary constituents of egg yolk particles was studied by ultrafiltration, circular dichroism, and sedimentation. Ferric complexes of phosvitin are strong and stable; ferrous complexes are weak and dissociate readily. When saturated, pairs of the approximately 135 phosphate groups of a phosvitin molecule appear to bind 1 iron atom each. These findings confirm and extend previous reports regarding the ferric complex and characterize the ferrous complex for the first time. The iron binding sites are not equivalent. Contrary to previous speculations, iron binding is not accompanied by a conformational change from an unordered structure to one of the beta-type. Apparently, neutralization of negative charges, while necessary, is not a sufficient condition of this transition. Nevertheless, iron affects phosvitin structure. Above pH 2, where the protein is unordered but adjusts its average conformation to changes in its net charge as the pH of its solution is varied, iron mimics the effect of protons quantitatively. Near pH 2, where the beta-type conformation is readily acquired by the protein in the absence of iron, the consequences of iron binding upon conformation are determined by the manner in which the iron-phosvitin interaction is brought about. Either the extent of the transition to the "normal" beta-structure becomes limited or the nature of the resulting conformation becomes modified. It is noteworthy that ellipticity changes in the presence of iron do not necessarily occur in parallel at about 200 and 215 nm as they do when the transconformation is produced by pH changes alone. The binding of iron appears to be mostly intramolecular. Intermolecular cross-links become dominant only if most binding sites are filled and then only as a secondary event subsequent to binding and the initial conformational adjustment.
鉴于磷蛋白卵黄高磷蛋白结合铁的潜在生物学意义，通过超滤、圆二色性和沉降法研究了蛋黄颗粒中这两种静电互补成分之间的相互作用。卵黄高磷蛋白的铁复合物很强且稳定；亚铁复合物则较弱且容易解离。饱和时，一个卵黄高磷蛋白分子中大约135个磷酸基团似乎各结合1个铁原子。这些发现证实并扩展了先前关于铁复合物的报道，并首次对亚铁复合物进行了表征。铁结合位点并不等同。与先前的推测相反，铁结合并未伴随着从无序结构到β型结构的构象变化。显然，负电荷的中和虽然是必要的，但不是这种转变的充分条件。然而，铁会影响卵黄高磷蛋白的结构。在pH 2以上，蛋白质处于无序状态，但随着溶液pH值的变化，其平均构象会根据净电荷的变化进行调整，铁在定量上模拟了质子的作用。在接近pH 2时，蛋白质在没有铁的情况下很容易获得β型构象，铁结合对构象的影响取决于铁与卵黄高磷蛋白相互作用的方式。要么向“正常”β结构的转变程度受到限制，要么所产生构象的性质发生改变。值得注意的是，在有铁存在时椭圆率的变化并不一定像仅由pH变化引起构象转变时那样在约200和215 nm处平行发生。铁的结合似乎主要是分子内的。只有当大多数结合位点被填满时，分子间交联才会占主导地位，而且这只是在结合和初始构象调整之后的次要事件。
