Ionization potentials of ferricytochrome c, ferrocytochrome c, and ferricytochrome c3
Ionization potentials of ferricytochrome c, ferrocytochrome c, and ferricytochrome c3
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铁细胞色素 c、铁细胞色素 c 和铁细胞色素 c3 的电离电位
DOI：
10.1021/ja00489a003
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发表时间：
1978
影响因子：
15
通讯作者：
H. Inokuchi
中科院分区：
文献类型：
--
作者：
K. Kimura;N. Sato;S. Hino;T. Yagi;H. Inokuchi
亚铁细胞色素c、亚铁细胞色素c和亚铁细胞色素c3的电离电势已通过真空紫外光电子能谱测定。所有样品均从水溶液中沉积。由此获得的铁细胞色素c、铁细胞色素c和铁细胞色素C3的电离电位分别为6.1、5.8和5.4eV。细胞色素 c 相当高的电离势已与其分子结构相关讨论。计算了细胞色素 c¡ 中的分子内血红素-血红素距离 d。已将铁细胞色素 c 的电离电位与铁细胞色素 c3 和四苯基卟啉锌的电离电位进行了比较。计算出的距离d为8.5 Á，几乎与四苯基卟啉锌晶体中最近邻之间的Zn-Zn距离相同。细胞色素在生物氧化还原过程中具有重要作用。分子中含有一个血红素的细胞色素 c 是植物、动物和细菌中许多不同生物体呼吸链中的电子载体。细胞色素C3是多血红素蛋白的成员，也是D. vulgaris电子传递链中的电子载体。虽然两种细胞色素的分子量几乎相同，细胞色素 c 为 12 000，细胞色素 c3 为 14 000，但它们在 pH 7.0 下对正常氢电极 (NHE) 的氧化还原电位却有很大不同，细胞色素 c1 为 + 0.255 V，细胞色素 C3 为 - 0.270 V。 3 氧化还原电位的差异是由于血红素的环境造成的。环境，换句话说，周围多肽侧链的构象，可以反映卟啉的电子状态。卟啉血红蛋白的电子能态已通过光吸收、发射、磁共振和穆斯堡尔效应等光谱方法得到广泛研究；然而，他们无法给出卟啉中电子的绝对能级。作为血红素模型化合物的气态卟啉和酞菁的几种衍生物的电子态已通过真空紫外光电子能谱4进行了研究，该光谱能够直接通过实验测定电离势。研究发现，四苯基卟啉及其金属 (II) 衍生物（如 Mg、Mn、Fe、Ni、Cu 和 Zn）的电离势在不同化合物之间没有变化。然而，尽管金属卟啉具有重要的生化作用，但电离势对金属氧化态的依赖性尚不清楚。此外，尚未确定实际细胞色素中血红素的电子能级是否与这些模型化合物中的相同。
Ionization potentials of ferricytochrome c, ferrocytochrome c, and ferricytochrome c3 have been determined from vacuum ultraviolet photoelectronspectroscopy. All samples were deposited from aqueous solution. The ionization potentials of ferricytochrome c, ferrocytochrome c, and ferricytochrome C3 thus obtained were 6.1, 5.8, and 5.4 eV, respectively. A rather high ionization potential of cytochrome c has been discussed in connection with its molecular structure. The intramolecular heme-heme distance, d, in cytochrome c¡ has been calculated. The ionization potential of ferricytochrome c has been com-pared with that of ferricytochrome c3 and of zinc tetraphenylporphyrin. The calculated distance, d, is 8.5 Á, which is almost the same as the Zn-Zn distance between the nearest neighbor in the zinc tetraphenylporphyrin crystal.Cytochromes have important roles in biological redox pro-cesses. Cytochrome c having one heme in a molecule is an electron carrier in a respiratory chain of many diverse organisms in plants, animals, and bacteria. Cytochrome C3, a member of multiheme protein, is also an electron carrier in an electron transfer chain in D. vulgaris. Though thetwo cyto-chromes have nearly the same molecular weight, 12 000 in cytochrome c and 14 000 in cytochrome c3, their redox po-tentials against normal hydrogen electrode (NHE) at pH 7.0 are considerably different,+ 0.255 V for cytochrome c1 and—0.270 V for cytochrome C3. 3 This difference in redox po-tentials is due to the environmentof a heme. The environment, in other words the conformation of the surrounding polypeptide side chain, may reflect the electronic states of porphyrin. Electronic energy states of porphyrinsin hemoproteins have been widely investigated by spectroscopic methods such as optical absorption, emission, magnetic resonance, and Mossbauer effect; however, they cannot give the absolute energy levels of electrons in porphyrin. Electronic states of several derivatives of gaseous porphyrins and phthalocyanines as a model compound of hemes have been investigated by vacuum ultraviolet photoemission spectroscopy4 which enables direct experimental determinations of ionization potentials. It was found that ionization potentials of tetraphenylporphyrin and its metal (II) derivatives such as Mg, Mn, Fe, Ni, Cu, and Zn did not change from compound to compound. In spite of biochemical importance of metal porphyrins, however, thedependence of ionization potential on the metal oxidation state is not known well. Also it has not been establishedwhether the electronic energy levels of hemes in actual cytochromes are the same as in these model com-pounds.