Quenching of the zinc-protoporphyrin triplet state as a measure of small-molecule diffusion through the structure of myoglobin.
Quenching of the zinc-protoporphyrin triplet state as a measure of small-molecule diffusion through the structure of myoglobin.
复制标题
锌-原卟啉三重态的猝灭作为小分子通过肌红蛋白结构扩散的测量。
作者：
N. Barboy;J. Feitelson
研究了小分子在肌红蛋白结构中的扩散。结果表明，在肌红蛋白中，荧光锌原卟啉很容易取代天然的非荧光铁原卟啉。用氧和取代肌红蛋白中锌原卟啉的E型延迟荧光猝灭速率来测量它们从环境溶液中通过蛋白质到配体结合部位的扩散。氧的猝灭速率常数(21℃)为Kq=(9.6+/-0.9)×10~(7)M~(-1)S~(-1)，仅比锌-血卟啉在水溶液中猝灭的速率常数小一个数量级。在2-40℃范围内的活化能为Ea=6.0+/-0.6千卡/摩尔。相应的数据为Kq=(2.1+/-0.3)×10(8)M-1 S-1，Ea=5.8+/-0.6kcal/mol。考虑到锌卟啉三重态氧猝灭所涉及的统计因素，猝灭速率非常相似。根据Norhorup和McCammon的“门控反应”理论对这些数据进行了讨论。相似的速率常数和活化能表明，蛋白质中的扩散速率是由构象变化的频率决定的，构象变化打开了猝灭剂通过蛋白质的“门”。
The diffusion of small molecules through the myoglobin structure was studied. It has been shown that the fluorescent Zn-protoporphyrin substitutes easily for the native nonfluorescent Fe-protoporphyrin in myoglobin. The quenching rate of the E-type delayed fluorescence of Zn-protoporphyrin in a substituted myoglobin by the quenchers oxygen and anthraquinonesulfonate was used to measure their diffusion from the ambient solution through the protein to the ligand binding site. The quenching rate constant (at 21 degrees C) for oxygen is kq = (9.6 +/- 0.9) X 10(7) M-1 S-1, only 1 order of magnitude less than that for Zn-hematoporphyrin quenching in aqueous solution. The activation energy in the range between 2 and 40 degrees C is Ea = 6.0 +/- 0.6 kcal/mol. The corresponding data for anthraquinonesulfonate are kq = (2.1 +/- 0.3) X 10(8) M-1 S-1 and Ea = 5.8 +/- 0.6 kcal/mol. Taking into account the statistical factor involved in the oxygen quenching of the Zn-porphyrin triplet, the quenching rates are very similar. The data are discussed in terms of the "gated reaction" theory of Northrup and McCammon. The similar rate constants and activation energies indicate that the diffusion rate in the protein is determined by the frequency of the conformational changes that open "gates" for the passage of the quencher through the protein.