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首页> 外文期刊>Biochemistry >Retardation of Proton Transfer Caused by Binding of the Transition Metal Ion to the Bacterial Reaction Center Is Due to pK_a Shifts of Key Protonatable Residues
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Retardation of Proton Transfer Caused by Binding of the Transition Metal Ion to the Bacterial Reaction Center Is Due to pK_a Shifts of Key Protonatable Residues

机译：过渡金属离子与细菌反应中心的结合导致质子转移的滞后是由于关键质子化残基的pK_a移位

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Transition metal ions bind to the reaction center (RC) protein of the photosynthetic bacterium Rhodobacter sphaeroides and slow the light-induced electron and proton transfer to the secondary quinone. QB. We studied the properties of the metal ion—RC complex by measuring the pH dependence of the dissociation constant and the stoichiometry of proton release upon ligand formation. We investigated the mechanism of inhibition by measuring the stoichiometry and kinetics of flash-induced proton binding. the transfer of (first and second) electrons to QB, and the rate of steady-state turnover of the RC in the absence and presence of Cda± and Ni2± on a wide pH range. The following results were obtained. (1) The complexation of transition metal ions Cd2± and Ni2± with the bacterial RC showed strong p1-I dependence. This observation was explained by different (pH-dependent) states of the metal—ligand cluster: the complex formation was strong when the ligand (Asp and His residues) was deprotonated and was much weaker if the ligand was partly (or fully) protonated. A direct consequence of the model was the pH-dependent proton release upon complexation. (2) The retardation of transfer of electrons and protons to QB was also strongly pH-dependent. The effect was large in the neutral pH range and decreased toward the acidic and alkaline pH values. (3) Steady-state turnover measurements indicated that the rate of the second proton transfer was much less inhibited than that of the first one, which became the rate-limiting step in continuous turnover of the RC. (4) Sodium azide partly recovered the proton transfer rate. The effect is not due to removal of the bound metal ion by azide but probably by formation of a proton-transporting azide nerxvork similarly as water molecules may build up proton pathways. (5) We argue that the inhibition comes mainlt from pKa shifts of key protonatable residues that control the proton transfer along the H-bond network to QB. The electrostatic interaction between the metal ion and these residues may result in acidic pK1 shifts between 1.5 and 2.0 that account for the observed retardation of the electron and proton transfer.
机译：过渡金属离子与光合细菌球形红球菌的反应中心（RC）蛋白结合，并减慢光诱导的电子和质子转移到仲醌的速度。 QB。我们通过测量解离常数的pH依赖性和配体形成后质子释放的化学计量来研究金属离子-RC络合物的性质。我们通过测量化学计量和闪光诱导质子结合的动力学研究了抑制的机制。在较宽的pH范围内，在不存在和存在Cda±和Ni2±的情况下，（第一和第二个）电子向QB的转移以及RC的稳态周转率。获得了以下结果。 （1）过渡金属离子Cd2±和Ni2±与细菌RC的络合表现出强烈的p1-I依赖性。该观察结果由金属-配体簇的不同（pH依赖性）状态解释：当配体（Asp和His残基）去质子化时，复合物形成很强，而如果配体部分（或全部）被质子化，则复合物形成弱得多。该模型的直接结果是络合后pH依赖的质子释放。 （2）电子和质子向QB转移的延迟也强烈依赖于pH。在中性pH范围内影响很大，而向酸性和碱性pH值降低。 （3）稳态周转测量表明，第二次质子转移的速率比第一次质子转移的速率受抑制要小得多，这成为了RC连续翻转的限速步骤。 （4）叠氮化钠部分地恢复了质子转移速率。该效果不是由于叠氮化物去除了结合的金属离子，而是由于与水分子可能建立质子途径相似，形成质子传输叠氮化物nerxvork。 （5）我们认为抑制作用主要来自控制质子沿H键网络向QB转移的关键可质子化残基的pKa移位。金属离子与这些残基之间的静电相互作用可能导致酸性pK1位移在1.5和2.0之间，这是观察到的电子和质子转移阻滞的原因。

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