CO, NO, and O2 as Vibrational Probes of Heme Protein Active Sites
CO, NO, and O2 as Vibrational Probes of Heme Protein Active Sites
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DOI：
10.1016/b978-044452839-1.50005-x
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发表时间：
2008-01-01
期刊：
影响因子：
--
通讯作者：
Wasbotten, Ingar H.
中科院分区：
文献类型：
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作者：
Spiro, Thomas G.;Ibrahim, Mohammed;Wasbotten, Ingar H.
一氧化碳是蛋白质中血红素结合位点的有用振动探针，因为 FeCO 反向键合是通过与蛋白质残基的极性相互作用以及反式配体的供体强度的变化来调节的。这种调制由 CO 和 FeC 拉伸频率敏感地监测，这些频率在红外和共振拉曼光谱中很容易检测到。两个频率呈负相关，vFeC/vCO 沿相关线的位置反映了远端极性相互作用的类型和强度。反式配体供体强度的变化将相关性移动到更高或更低的位置。回顾了 vFeC/vCO 图在带有组氨酸和硫醇盐轴向配体的蛋白质中的说明性应用。尚未发现空间拥挤对 vFeC/vCO 相关性产生显着影响，但细胞色素氧化酶的特殊情况除外，其中血红素结合的 CO 可能与附近的 Cu-B 中心相互作用。 Fe(II) 血红素蛋白的 NO 加合物也显示出 vFeN 和 vNO 频率的变化，但与 CO 加合物数据相比，数据显示出相当大的分散性。然而，无蛋白质的 Fe(II) 卟啉 NO 加合物给出了良好的反向键合相关性；新数据表明，6 配位 (6-c) 和 5 配位 (5-c) 加合物也是如此。蛋白质数据的分散反映了远端极性基团（尤其是组氨酸）引起的 FeNO 角度的变化。 Fe(III) 血红素蛋白 NO 加合物的可用数据很少，表明当近端配体是组氨酸时，vFeN/vNO 存在弱负相关性，但当近端配体是硫醇盐时，vFeN/vNO 存在正相关性。这种行为与等电子 Fe(II)CO 的行为明显不同。 DFT 模型表明，响应的改变反映了 Fe(III) 中 d(z2) 能量降低导致的前沿轨道的变化。从有限的可用数据来看，Fe(II)O-2 加合物也存在类似的模式。
Carbon monoxide is a useful vibrational probe of heme-binding sites in proteins, because FeCO backbonding is modulated by polar interactions with protein residues, and by variations in the donor strength of the trans ligand. This modulation is sensitively monitored by the CO and FeC stretching frequencies, which are readily detectable in infrared and resonance Raman spectra. The two frequencies are negatively correlated, and the vFeC/vCO position along the correlation line reflects the type and strength of distal polar interactions. Changes in the trans ligand donor strength shift the correlation to higher or lower positions. Illustrative applications of the vFeC/vCO diagram are reviewed for proteins bearing histidine and thiolate axial ligands. Steric crowding has not been found to affect the vFeC/vCO correlations significantly, except in the special case of cytochrome oxidase, where the heme-bound CO may interact with the nearby Cu-B center. NO adducts of Fe(II) heme proteins also show variations in the vFeN and vNO frequencies, but the data show considerable scatter, in contrast to the CO adduct data. However, protein-free Fe(II)porphyrin NO adducts give well-behaved backbonding correlations; new data show this to be true of 6-coordinate (6-c) as well as 5-coordinate (5-c) adducts. The scatter in the protein data is suggested to reflect changes in the FeNO angle induced by distal polar groups, especially histidine. The few data available for NO adducts of Fe(III) heme proteins suggest a weak negative vFeN/vNO correlation when the proximal ligand is histidine, but a positive correlation when the proximal ligand is thiolate. This behavior is markedly different from that of the isoelectronic Fe(II)CO. DFT modeling indicates that the altered response reflects a change in frontier orbitals resulting from the lowered d(z2) energy in Fe(III). A similar pattern is suggested for Fe(II)O-2 adducts from the limited available data.