Role of Calcium in Metalloenzymes: Effects of Calcium Removal on the Axial Ligation Geometry and Magnetic Properties of the Catalytic Diheme Center in MauG
Role of Calcium in Metalloenzymes: Effects of Calcium Removal on the Axial Ligation Geometry and Magnetic Properties of the Catalytic Diheme Center in MauG
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DOI：
10.1021/bi201575f
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发表时间：
2012-02-28
期刊：
影响因子：
2.9
通讯作者：
Liu, Aimin
中科院分区：
文献类型：
--
作者：
Chen, Yan;Naik, Sunil G.;Liu, Aimin
MauG是一种二血红素酶，具有一个轴向His配体的五坐标高自旋血红素和一个轴向His- tyr连接的六坐标低自旋血红素。Ca2+离子通过氢键网络连接到两个血红素上，酶的活性取决于它的存在。Ca2+的去除改变了各铁血红素的电子顺磁共振（EPR）信号，使高自旋血红素的强度降低，低自旋血红素的强度显着变宽。向样品中添加Ca2+恢复了原始的EPR信号和酶活性。通过磁共振和穆斯堡尔光谱研究了这种Ca2+依赖行为的分子基础。结果表明，在Ca2+缺失的MauG中，高自旋血红素转化为低自旋血红素，并且原来的低自旋血红素表现出其两个轴向配体的相对取向的变化。这两种血红素的性质与天然毛革中的血红素不同，现在彼此相似。无Ca2+ mag的EPR谱似乎描述了一组低自旋铁血红素信号，具有较大的g（max）和g各向异性，并且自旋弛豫特性发生了很大的变化。EPR和穆斯堡尔光谱结果表明，这两种血红素在Ca2+缺失的MauG中以不同寻常的高菱形低自旋血红素的形式存在，两个轴向配体平面之间的取向角较小。这些发现为酶活性与轴向血红素配体取向的相关性提供了见解，并描述了钙离子在维持活性所需的这种结构取向中的作用。
MauG is a diheme enzyme possessing a five-coordinate high-spin heme with an axial His ligand and a six-coordinate low-spin heme with His-Tyr axial ligation. A Ca2+ ion is linked to the two hemes via hydrogen bond networks, and the enzyme activity depends on its presence. Removal of Ca2+ altered the electron paramagnetic resonance (EPR) signals of each ferric heme such that the intensity of the high-spin heme was decreased and the low-spin heme was significantly broadened. Addition of Ca2+ back to the sample restored the original EPR signals and enzyme activity. The molecular basis for this Ca2+-dependent behavior was studied by magnetic resonance and Mossbauer spectroscopy. The results show that in the Ca2+-depleted MauG the high-spin heme was converted to a low-spin heme and the original low-spin heme exhibited a change in the relative orientations of its two axial ligands. The properties of these two hemes are each different than those of the heme in native MauG and are now similar to each other. The EPR spectrum of Ca2+-free MauG appears to describe one set of low-spin ferric heme signals with a large g(max) and g anisotropy and a greatly altered spin relaxation property. Both EPR and Mossbauer spectroscopic results show that the two hemes are present as unusual highly rhombic low-spin hemes in Ca2+-depleted MauG, with a smaller orientation angle between the two axial ligand planes. These findings provide insight into the correlation of enzyme activity with the orientation of axial heme ligands and describe a role for the calcium ion in maintaining this structural orientation that is required for activity.