Ligand binding in the ferric and ferrous states of Paramecium hemoglobin.
Ligand binding in the ferric and ferrous states of Paramecium hemoglobin.
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DOI：
10.1021/bi001681d
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发表时间：
2000-11
期刊：
影响因子：
2.9
通讯作者：
T. K. Das;R. Weber;S. Dewilde;J. Wittenberg;B. Wittenberg;K. Yamauchi;M. Van Hauwaert;L. Moens;D. Rousseau
中科院分区：
文献类型：
--
作者：
T. K. Das;R. Weber;S. Dewilde;J. Wittenberg;B. Wittenberg;K. Yamauchi;M. Van Hauwaert;L. Moens;D. Rousseau
单细胞原生动物尾草履虫（Paramecium caudatum）含有一个只有116个氨基酸残基的单体血红蛋白（Hb）。该血红蛋白与几种无脊椎动物血红蛋白同时存在远端E7谷氨酰胺和B10酪氨酸。在这里提出的研究中，我们已经使用配体结合动力学和共振拉曼光谱来表征草履虫血红蛋白的远端口袋残基在稳定血红素结合配体的效果。在铁状态下，高自旋到低自旋（水羟基）的转变发生，pK（a）约为9.0。氧亲和力（P（50）= 0.45托）与肌红蛋白相似。氧气的开关率也类似于抹香鲸肌红蛋白。共振拉曼数据表明，氢键稳定的结合氧，证明了一个相对较低的频率的Fe-OO拉伸（563厘米（-1））。我们提出氧配合物是一个氢键封闭结构和开放结构的平衡混合物。氧将优先从开放结构解离，因此，开放结构群体的分数控制氧解离的速率。在CO复合物中，Fe-CO伸缩频率在493 cm（-1）处表明存在一个开放的血红素口袋，这与CO相对于肌红蛋白的较高的结合和解离速率相一致。高配体结合率也与在220 cm（-1）处观察到的Fe-组氨酸伸缩频率一致，表明不存在显著的近端应变。我们推测草履虫血红蛋白的功能是为细胞氧化过程提供氧气。
The unicellular protozoan Paramecium caudatum contains a monomeric hemoglobin (Hb) that has only 116 amino acid residues. This Hb shares the simultaneous presence of a distal E7 glutamine and a B10 tyrosine with several invertebrate Hbs. In the study presented here, we have used ligand binding kinetics and resonance Raman spectroscopy to characterize the effect of the distal pocket residues of Paramecium Hb in stabilizing the heme-bound ligands. In the ferric state, the high-spin to low-spin (aquo-hydroxy) transition takes place with a pK(a) of approximately 9.0. The oxygen affinity (P(50) = 0.45 Torr) is similar to that of myoglobin. The oxygen on- and off-rates are also similar to those of sperm whale myoglobin. Resonance Raman data suggest hydrogen bonding stabilization of bound oxygen, evidenced by a relatively low frequency of Fe-OO stretching (563 cm(-1)). We propose that the oxy complex is an equilibrium mixture of a hydrogen-bonded closed structure and an open structure. Oxygen will dissociate preferentially from the open structure, and therefore, the fraction of open structure population controls the rate of oxygen dissociation. In the CO complex, the Fe-CO stretching frequency at 493 cm(-1) suggests an open heme pocket, which is consistent with the higher on- and off-rates for CO relative to those in myoglobin. A high rate of ligand binding is also consistent with the observation of an Fe-histidine stretching frequency at 220 cm(-1), indicating the absence of significant proximal strain. We postulate that the function of Paramecium Hb is to supply oxygen for cellular oxidative processes.