Department of Biotechnology, Delft University of Technology, Building 58, Van der Maasweg 9, 2629 HZ Delft, The Netherlands.
Instituto de Tecnologia Química e Biológica António Xavier (ITQB-NOVA), Universidade Nova de Lisboa, 2780-157 Oeiras, Portugal.
Int J Mol Sci. 2023 Aug 12;24(16):12713. doi: 10.3390/ijms241612713.
Distances between Fe ions in multiheme cytochromes are sufficiently short to make the intramolecular dipole-dipole interaction between hemes probable. In the analysis of EPR data from cytochromes, this interaction has thus far been ignored under the assumption that spectra are the simple sum of non-interacting components. Here, we use a recently developed low-frequency broadband EPR spectrometer to establish the extent of dipolar interaction in the example cytochromes, characterize its spectral signatures, and identify present limitations in the analysis. Broadband EPR spectra of MR-1 small tetraheme cytochrome (STC) have been collected over the frequency range of 0.45 to 13.11 GHz, and they have been compared to similar data from Hildenborough cytochrome . The two cases are representative examples of two very different heme topologies and corresponding electron-transfer properties in tetraheme proteins. While in cytochrome , the six Fe-Fe distances can be sorted into two well-separated groups, those in STC are diffuse. Since the onset of dipolar interaction between Fe-Fe pairs is already observed in the X-band, the values are determined in the simulation of the 13.11 GHz spectrum. Low-frequency spectra are analyzed with the inclusion of dipolar interaction based on available structural data on mutual distances and orientations between all hemes. In this procedure, all 24 possible assignments of individual heme spectra to heme topologies are sampled. The 24 configurations can be reduced to a few, but inspection falls short of a unique assignment, due to a remaining lack of understanding of the fine details of these complex spectra. In general, the EPR analysis suggests the four-heme system in to be more rigid than that in STC, which is proposed to be related to different physiological roles in electron transfer.
多血红素细胞色素中的 Fe 离子之间的距离足够近，使得血红素之间的分子内偶极-偶极相互作用成为可能。在细胞色素的 EPR 数据分析中，迄今为止，由于假定光谱是互不作用的成分的简单总和，因此忽略了这种相互作用。在这里，我们使用最近开发的低频宽带 EPR 光谱仪来确定示例细胞色素中偶极相互作用的程度，表征其光谱特征，并确定分析中的现有局限性。在 0.45 至 13.11 GHz 的频率范围内收集了 MR-1 小四血红素细胞色素 (STC) 的宽带 EPR 光谱，并将其与来自 Hildenborough 细胞色素的类似数据进行了比较。这两种情况是四血红素蛋白中两种非常不同的血红素拓扑结构和相应电子转移性质的代表性例子。虽然在细胞色素中，六个 Fe-Fe 距离可以分为两个明显分开的组，但在 STC 中则是弥散的。由于在 X 波段已经观察到 Fe-Fe 对之间偶极相互作用的开始，因此在模拟 13.11 GHz 光谱时确定了 值。基于所有血红素之间的相互距离和取向的可用结构数据，在包含偶极相互作用的情况下分析低频光谱。在此过程中，对个别血红素光谱到血红素拓扑结构的所有 24 种可能分配进行采样。可以将 24 种配置减少到几种，但由于对这些复杂光谱的细微细节缺乏了解，因此检查结果仍然无法给出唯一的分配。总体而言，EPR 分析表明，与 STC 相比， 中的四血红素系统更具刚性，这被认为与电子转移中不同的生理作用有关。
