Periplasmic cytochrome A (PpcA) is a representative of a broad class of multiheme cytochromes functioning as protein “nanowires” for storage and extracellular transfer of multiple electrons in the δ-proteobacterium Geobacter sulfurreducens. PpcA contains three bis-His coordinated hemes held in a spatial arrangement that is highly conserved among the multiheme cytochromes c₃ and c₇ families, carries low potential hemes, and is notable for having one of the lowest number of amino acids utilized to maintain a characteristic protein fold and site-specific heme function. Low temperature X-band electron paramagnetic resonance (EPR) spectroscopy has been used to characterize the electronic configuration of the Fe(III) and the ligation mode for each heme. The three sets of EPR signals are assigned to individual hemes in the three-dimensional crystal structure. The relative energy levels of the Fe(III) 3d orbitals for individual hemes were estimated from the principal g-values. The observed g-tensor anisotropy was used as a probe of electronic structure of each heme, and differences were determined by specifics of axial ligation. To ensure unambiguous assignment of highly anisotropic low-spin (HALS) signal to individual hemes, EPR analyses of iron atom electronic configurations have been supplemented with investigation of porphyrin macrocycles by one-dimensional ¹H NMR chemical shift patterns for the methyl substituents. Within optimized geometry of hemes in PpcA, the magnetic interactions between hemes were found to be minimal, similar to the c₃ family of tetraheme cytochromes.

中文翻译：

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还原性Geobacter还原菌的三血红素细胞色素的电子顺磁共振表征

周质细胞色素A（PpcA）是一类广泛的多血红素细胞色素的代表，它们充当蛋白质“纳米线”，用于在δ变形杆菌属Geobacter sulfreducens中存储和电子转移多个电子。PpcA包含三个bis-His协调的血红素，这些血红素在空间排列中保持不变，在多血红素细胞色素c ₃和c _7中高度保守家族，具有低潜在的血红素，并且以具有用于维持特征性蛋白质折叠和位点特异性血红素功能的最低数目的氨基酸之一而著称。低温X波段电子顺磁共振（EPR）光谱已用于表征Fe（III）的电子构型和每个血红素的连接模式。将三组EPR信号分配给三维晶体结构中的各个血红素。从主要g值估算出单个血红素的Fe（III）3d轨道的相对能级。观察到的g-张量各向异性被用作每个血红素的电子结构的探针，并且差异由轴向结扎的特异性确定。为了确保将高各向异性的低自旋（HALS）信号明确分配给各个血红素，对铁原子电子构型的EPR分析通过对甲基取代基的一维¹ H NMR化学位移图进行了卟啉大环的研究，从而对此进行了补充。在PpcA中优化的血红素几何结构中，发现血红素之间的磁性相互作用极小，类似于四血红素细胞色素c ₃家族。