Distance metrics for heme protein electron tunneling
Distance metrics for heme protein electron tunneling
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DOI：
10.1016/j.bbabio.2008.04.021
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发表时间：
2008-07-01
影响因子：
4.3
通讯作者：
Dutton, P. Leslie
中科院分区：
文献类型：
--
作者：
Moser, Christopher C.;Chobot, Sarah E.;Dutton, P. Leslie
毫无疑问，距离是决定蛋白质电子隧穿速率数量级的主要参数。然而，仍然有不同的方法来测量蛋白质中的电子隧穿距离。这种距离的不确定性模糊了一个问题，即介入的蛋白质介质是否是自然选择的，以加速或减缓任何特定的电子隧穿反应。对于没有金属的氧化还原辅因子，辅因子的边缘可以被定义为近似于空间的范围，包括与其隧穿电子相关的大部分波函数，超过这个边缘，波函数在空间中的衰减要明显得多。共轭卟啉环似乎是光合作用的无金属脱镁叶绿素和细菌脱镁叶绿素的合理边缘。对于含金属的氧化还原辅因子如血红素，合适的辅因子边缘更模糊。电子隧穿距离可以从共轭血红素大环边缘或从金属测量，其可以长达4.8埃。在典型的蛋白质介质中，这样的距离差通常对应于隧穿速率的类似1000倍的降低。为了解决这种模糊性，我们认为这两个天然血红素蛋白的电子转移和光激活的电子转移在benenated血红素蛋白。我们发现，共轭血红素大环的边缘提供了一个可靠的和有用的隧道距离定义与其他生物电子隧道反应一致。此外，与此距离度量，血红素轴向和边缘取向的电子转移出现类似的，同样很好地描述了一个简单的正方形势垒隧穿模型。这是在最近的金属对金属的指标，需要非常差的供体/受体耦合来解释血红素轴向电子转移的报告。(C)2008 Elsevier B. V.保留所有权利。
There is no doubt that distance is the principal parameter that sets the order of magnitude for electron-tunneling rates in proteins. However, there continue to be varying ways to measure electron-tunneling distances in proteins. This distance uncertainty blurs the issue of whether the intervening protein medium has been naturally selected to speed or slow any particular electron-tunneling reaction. For redox cofactors lacking metals, an edge of the cofactor can be defined that approximates the extent in space that includes most of the wavefunction associated with its tunneling electron. Beyond this edge, the wavefunction tails off much more dramatically in space. The conjugated porphyrin ring seems a reasonable edge for the metal-free pheophytins and bacteriopheophytins of photosynthesis. For a metal containing redox cofactor such as heme, an appropriate cofactor edge is more ambiguous. Electron-tunneling distance may be measured from the conjugated heme macrocycle edge or from the metal, which can be up to 4.8 angstrom longer. In a typical protein medium, such a distance difference normally corresponds to a similar to 1000 fold decrease in tunneling rate. To address this ambiguity, we consider both natural heme protein electron transfer and light-activated electron transfer in ruthenated heme proteins. We find that the edge of the conjugated heme macrocycle provides a reliable and useful tunneling distance definition consistent with other biological electron-tunneling reactions. Furthermore, with this distance metric, heme axially- and edge-oriented electron transfers appear similar and equally well described by a simple square barrier tunneling model. This is in contrast to recent reports for metal-to-metal metrics that require exceptionally poor donor/acceptor couplings to explain heme axially-oriented electron transfers. (C) 2008 Elsevier B.V. All rights reserved.