NOVEL ION SPECIFICITY OF A CARBOXYLATE CLUSTER MG(II) BINDING-SITE - STRONG CHARGE SELECTIVITY AND WEAK SIZE SELECTIVITY
NOVEL ION SPECIFICITY OF A CARBOXYLATE CLUSTER MG(II) BINDING-SITE - STRONG CHARGE SELECTIVITY AND WEAK SIZE SELECTIVITY
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DOI：
10.1021/bi00064a020
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发表时间：
1993-04-06
期刊：
影响因子：
2.9
通讯作者：
FALKE, JJ
中科院分区：
文献类型：
--
作者：
NEEDHAM, JV;CHEN, TY;FALKE, JJ
羧酸盐簇 Mg(II) 结合位点由侧链羧酸盐簇组成，通常数量为 3-4 个，部分埋藏在 Mg(II) 结合蛋白表面的浅裂隙中。这种簇通常存在于催化磷酸化学的酶的活性位点中。一个例子是大肠杆菌趋化途径的磷酸信号蛋白 CheY，它通过其磷酸化位点的三个羧酸盐簇结合 Mg(II)。本研究通过测量 Ia、IIa、IIIa 族和镧系金属离子的解离常数来定量 CheY 位点的离子电荷和尺寸特异性。这些球形阳离子为一系列底物提供了逐渐变化的电荷和半径。该位点结合二价和三价阳离子，但它有效地排除单价阳离子，包括生理离子 Na(I) 和 K(I)。这种电荷特异性与该位点明显缺乏尺寸特异性形成鲜明对比：二价和三价阳离子表现出基本上与半径无关的亲和力。将 Mg(II) 位点的离子特异性与先前表征的 Ca(II) 信号蛋白中常见的 EF-hand 类 Ca(II) 位点的特异性进行比较具有启发意义。 Mg(II) 和 Ca(II) 位点表现出相似的电荷选择性，但 Ca(II) 位点具有高度尺寸选择性，更喜欢半径与 Ca(II) 相似的二价和三价离子。 Mg(II) 和 Ca(II) 位点的结构比较表明，它们不同的尺寸特异性源于根本不同的配位方案：Ca(II) 位点用七个蛋白质氧的五角双锥阵列包围结合离子，从而固定配位数并控制底物腔的半径。相比之下，Mg(II)位点的可调节性质被认为源于其使用溶剂氧来协调结合离子的一个半球：该溶剂壳可以轻松地改变其配位数和半径以适应不同尺寸的底物离子。
Carboxylate cluster Mg(II) binding sites consist of a cluster of side-chain carboxylates, typically 3-4 in number, partially buried in a shallow cleft on the surface of a Mg(II) binding protein. Such clusters are often found in the active sites of enzymes catalyzing phosphochemistry. An example is the phospho-signaling protein CheY of the Escherichia coli chemotaxis pathway, which binds Mg(II) via a cluster of three carboxylates at its phosphorylation site. The present study quantitates both the ion charge and size specificity of the CheY site by measuring the dissociation constants of metal ions from groups Ia, IIa, IIIa, and the lanthanides; these spherical cations provide a range of substrates with incrementally varying charge and radius. The site binds divalent and trivalent cations, but it effectively excludes monovalent cations, including the physiological ions Na(I) and K(I). This charge specificity is in contrast to the site's remarkable lack of size specificity: divalent and trivalent cations exhibit affinities which are essentially independent of radius. It is revealing to compare the ion specificity of the Mg(II) site with the previously characterized specificity of the EF-hand class of Ca(II) sites commonly found in Ca(II) signaling proteins. The Mg(II) and Ca(II) sites exhibit similar charge selectivity, but the Ca(II) site is highly size-selective, preferring divalent and trivalent ions with radii similar to that of Ca(II). A structural comparison of the Mg(II) and Ca(II) sites suggests that their different size specificities stem from fundamentally different coordination schemes: the Ca(II) site surrounds the bound ion with a pentagonal bipyramidal array of seven protein oxygens, thereby fixing the coordination number and controlling the radius of the substrate cavity. In contrast, the adjustable nature of the Mg(II) site is proposed to stem from its use of solvent oxygens to coordinate one hemisphere of the bound ion: this solvent shell can easily vary its coordination number and radius to accommodate substrate ions of different size.