Electron Paramagnetic Resonance Characterization of the Triheme Cytochrome from Geobacter sulfurreducens.
Electron Paramagnetic Resonance Characterization of the Triheme Cytochrome from Geobacter sulfurreducens.
复制标题
硫还原地杆菌三血红素细胞色素的电子顺磁共振表征。
作者：
N. Ponomarenko;J. Niklas;P. Pokkuluri;O. Poluektov;D. Tiede
周质细胞色素A （PpcA）是一类多血红素细胞色素的代表，在硫还原δ-变形杆菌中起着蛋白质“纳米线”的作用，用于储存和胞外转移多个电子。PpcA包含三个双his协调的血红素，在多血红素细胞色素c3和c7家族中高度保守，携带低电位血红素，并且具有用于维持特征蛋白折叠和位点特异性血红素功能的氨基酸数量最少的氨基酸之一。用低温x波段电子顺磁共振（EPR）谱分析了Fe（III）的电子构型和每个血红素的连接模式。这三组EPR信号被分配到三维晶体结构中的单个血红素上。根据主g值估计了单个血红素的Fe（III）三维轨道的相对能级。观察到的g张量各向异性被用作每个血红素电子结构的探针，并通过轴向结扎的具体情况来确定差异。为了确保高各向异性低自旋（HALS）信号对单个血红素的明确分配，铁原子电子构型的EPR分析已经补充了甲基取代基的一维1H NMR化学位移模式对卟啉大环的研究。在PpcA中优化的血红素几何结构中，血红素之间的磁相互作用被发现是最小的，类似于四血红素细胞色素的c3家族。
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.