Migration of small molecules through the structure of hemoglobin: evidence for gating in a protein electron-transfer reaction.
Migration of small molecules through the structure of hemoglobin: evidence for gating in a protein electron-transfer reaction.
复制标题
小分子通过血红蛋白结构的迁移：蛋白质电子转移反应中门控的证据。
DOI：
10.1021/bi00234a030
复制
发表时间：
1991
期刊：
影响因子：
2.9
通讯作者：
McLendon,G
中科院分区：
文献类型：
--
作者：
Feitelson,J;McLendon,G
耶路撒冷希伯来大学物理化学系，耶路撒冷91904，以色列；罗切斯特大学化学系，罗切斯特，纽约14627摘要：以前的研究表明，不同大小的分子通过肌红蛋白的迁移速率和活化能是非常相似的。结果被解释为蛋白质结构的构象变化，促进了不同分子在相似程度上的通过。在这里，我们想知道伴随配体（02或CO）与血红蛋白结合的四级结构变化是否会影响从溶液到蛋白质结合位点的迁移速率。作为血红蛋白R态的模型，我们使用了a亚基中的Fe原卟啉（FePP）被Zn原卟啉（ZnPP）取代，氧化血红素被CN~连接的蛋白质。血红蛋白的T状态由蛋白代表，其中四个FePP基团被ZnPP取代。氧、紫紫甲基素和蒽醌醌磺酸盐对血红蛋白内激发的ZnPP三重态的猝灭速率可作为通过蛋白质进入结合位点的迁移速率的度量。研究发现，这三种猝灭剂的活化能非常相似，与肌红蛋白的活化能非常相似，表明迁移速率仅由亚基结构决定，而四元构型的变化不影响猝灭速率。简要讨论了这些结果对蛋白质中电子转移的影响。蛋白质与小分子（例如底物）之间的反应不仅取决于特定的反应速率常数(s)，还取决于小分子通过蛋白质基质向反应位点的扩散迁移。通常衡量的是整个过程。因此，为了充分了解生物大分子内的反应，有必要单独确定上述完整反应的两个组分。通过观察结合配体的光解作用及其随后在血红素位点的再结合，人们对配体与肌红蛋白和血红蛋白的结合进行了广泛的研究（Frauenfelder & Debrunner, 1982； Ansari等，1986；Henry等，1984；Murray等，1988；Marden等，1986;Friedman, 1985）。在肌红蛋白中，研究发现配体在到达结合位点的过程中必须克服一个或多个潜在的障碍（Austin et al., 1978; Marden, 1986），才能以自身的特定活化能发生实际的结合步骤。在血红蛋白中，人们提出了一个问题，即配体结合时的四级结构变化是否会影响后者进入血红素口袋的再入率。它
Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Department of Chemistry, University of Rochester, Rochester, New York 14627 Received May 14, 1990; Revised Manuscript Received December 6, 1990 abstract: It has previously been shown that the rates and activation energies for migration molecules of different sizes through myoglobin are very similar. The results were interpreted in terms of conformational changes in the protein structure that facilitate the passage of the different molecules to a similar extent. Here we ask whether the quaternary structural changes that accompany the binding of ligands (02 or CO) to hemoglobinmight influence the migration rate from the solution into the protein’s binding site. As a model for the R state of hemoglobin, we used the protein in which the Fe protoporphyrin (FePP) in the a subunit was substituted by Zn protoporphyrin (ZnPP) and the oxidized heme was ligated by CN~. The T state of hemoglobin was represented by the protein in whichall four FePP groups were substituted by ZnPP. The quenching rate of the excited ZnPP triplet state within the hemoglobin by oxygen, methyl viologen, and anthraquinonesulfonate served as a measure of the migration rate through the protein into the binding site. It was found that the activation energies for all three quenchers were very similar and closely resembled those in myoglobin, suggesting that the migration rates are determined by the subunit structure only and that the quaternary configurational changes do not influence the quenching rates. The implications of the results for electron transfer in proteins are briefly discussed. e reaction between a protein and small molecules, for example, a substrate, depends not only on the specific reaction rate constant (s) but also on the diffusional migration of the small molecule through the protein matrix toward the reaction site. What is usually measured is the overall process. Therefore, in order to understand fully the reaction within a biological macromolecule, it is necessaryto individually de-termine thetwo components of the above complete reaction. The binding of ligands to myoglobin and to hemoglobin has been studied extensively by following the photodissociation of the bound ligand and its subsequent rebinding at the heme site (Frauenfelder & Debrunner, 1982; Ansari et al., 1986; Henry et al., 1984; Murray et al., 1988; Marden et al., 1986; Friedman, 1985). In myoglobin, it was found that the ligand must overcome one or more potential barriers on its way to-ward the binding site (Austin et al., 1978; Marden, 1986) before the actual binding step occurs with its own specific activation energy. In hemoglobin, the question has been raised whether the quaternary structural changes upon ligand binding affect the reentry rate of the latter into the heme pocket. It