Negatively charged residues and hydrogen bonds tune the ligand histidine pKa values of Rieske iron-sulfur proteins
Negatively charged residues and hydrogen bonds tune the ligand histidine pKa values of Rieske iron-sulfur proteins
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DOI：
10.1021/bi0488606
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发表时间：
2004-10-05
期刊：
影响因子：
2.9
通讯作者：
Ullmann, GM
中科院分区：
文献类型：
--
作者：
Klingen, AR;Ullmann, GM
Rieske 蛋白携带氧化还原活性铁硫簇，由两个组氨酸和两个半胱氨酸侧链结合。 Rieske 蛋白的还原潜力取决于 pH 值。这种 pH 依赖性可以通过两个 pK(a) 值来描述，这两个值已分配给两个铁配位组氨酸。 Rieske 蛋白通常分为两大类：来自对苯二酚氧化细胞色素 bc 复合物的 Rieske 蛋白，其中配体组氨酸在生理 pH 范围内滴定，以及细菌铁氧还蛋白 Rieske 蛋白，其中配体组氨酸在生理 pH 范围内质子化。在这里介绍的研究中，我们使用组合密度泛函理论/连续静电学方法计算了簇配体组氨酸的 pK(a) 值。可以重现 bc 型和铁氧还蛋白 Rieske 蛋白的实验 pK(a) 值。我们可以识别这两种蛋白质之间在功能上的重要差异：朝向簇的氢键（存在于 bc 型 Rieske 蛋白质中）和带负电的残基（存在于铁氧还蛋白 Rieske 蛋白质中）。我们通过改变计算中的蛋白质来消除这些差异。突变蛋白中的 Rieske 中心具有非常相似的 pK(a) 值。因此，我们得出结论，所研究的结构差异是蛋白质不同 pH 滴定行为的主要原因。有趣的是，中和铁氧还蛋白 Rieske 蛋白中的负电荷引起的位移大于消除 bc 型 Rieske 蛋白中朝向簇的氢键引起的位移。
Rieske proteins carry a redox-active iron-sulfur cluster, which is bound by two histidine and two cysteine side chains. The reduction potential of Rieske proteins depends on pH. This pH dependence can be described by two pK(a) values, which have been assigned to the two iron-coordinating histidines. Rieske proteins are commonly grouped into two major classes: Rieske proteins from quinol-oxidizing cytochrome bc complexes, in which the ligand histidines titrate in the physiological pH range, and bacterial ferredoxin Rieske proteins, in which the ligand histidines are protonated at physiological pH. In the study presented here, we have calculated pK(a) values of the cluster ligand histidines using a combined density functional theory/continuum electrostatics approach. Experimental pK(a) values for a bc-type and a ferredoxin Rieske protein could be reproduced. We could identify functionally important differences between the two proteins: hydrogen bonds toward the cluster, which are present in bc-type Rieske proteins, and negatively charged residues, which are present in ferredoxin Rieske proteins. We removed these differences by mutating the proteins in our calculations. The Rieske centers in the mutated proteins have very similar pK(a) values. We thus conclude that the studied structural differences are the main reason for the different pH-titration behavior of the proteins. Interestingly, the shift caused by neutralizing the negative charges in ferredoxin Rieske proteins is larger than the shift caused by removing the hydrogen bonds toward the cluster in bc-type Rieske proteins.