TYROSINE RADICALS ARE INVOLVED IN THE PHOTOSYNTHETIC OXYGEN-EVOLVING SYSTEM
TYROSINE RADICALS ARE INVOLVED IN THE PHOTOSYNTHETIC OXYGEN-EVOLVING SYSTEM
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DOI：
10.1073/pnas.84.20.7099
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发表时间：
1987-10-01
影响因子：
11.1
通讯作者：
BABCOCK, GT
中科院分区：
文献类型：
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作者：
BARRY, BA;BABCOCK, GT
除了反应中心的叶绿素外，至少还有另外两个有机遗传辅助因子参与了光合作用放氧过程。其中一个被称为“Z”的辅因子将电子从水氧化部位转移到光系统II的反应中心。另一个物种“D”具有不确定的功能，但会产生稳定的EPR信号，称为信号II。Z.+和D+具有相同的EPR谱，通常被认为来自具有相同化学结构的物种。各种实验的结果表明，Z和D是胞二酚或胞二酚的衍生物。然而，总的来说，支持这一任务的证据是间接的。为了解决这种情况，我们开发了更直接的方法来指定Z.+/D.+自由基的结构。通过在蓝藻体内选择性地对质醌的甲基进行氢化，我们证明了来自甲基质子的超精细偶联不能解释D+EPR谱中看到的部分分辨结构。也就是说，我们通过提取和质谱学的方法验证了苯二酚在海藻中被标记，但没有观察到信号II的线形变化。考虑到D+自由基的光谱特性，酪氨酸起源是一个合理的选择。在第二个系列的实验中，我们发现酪氨酸的氢化确实使D+信号变窄。对这些培养物中的苯二酚的提取和质谱分析表明，它们不是酪氨酸标记的。这些结果排除了D+的质醌来源，相反，我们得出结论，D+和最有可能的Z+是酪氨酸自由基。
In addition to the reaction-center chlorophyll, at least two other organic genetic cofactors are involved in the photosynthetic oxygen-evolution process. One of these cofactors, called "Z," transfers electrons from the site of water oxidation to the reaction center of photosystem II. The other species, "D," has an uncertain function but gives rise to the stable EPR signal known as signal II. Z.+ and D.+ have identical EPR spectra and are generally assumed to arise from species with the same chemical structure. Results from a variety of experiments have suggested that Z and D are plastoquinones or plastoquinone derivatives. In general, however, the evidence to support this assignment is indirect. To address this situation, we have developed more direct methods to assign the structure of the Z.+/D.+ radicals. By selective in vivo deuteration of the methyl groups of plastoquinone in cyanobacteria, we show that hyperfine couplings from the methyl protons cannot be responsible for the partially resolved structure seen in the D.+ EPR spectrum. That is, we verify by extraction and mass spectrometry that quinones are labeled in algae fed deuterated methionine, but no change is observed in the line shape of signal II. Considering the spectral properties of the D.+ radical, a tyrosine origin is a reasonable alternative. In a second series of experiments, we have found that deuteration of tyrosine does indeed narrow the D.+ signal. Extraction and mass spectral analysis of the quinones in these cultures show that they are not labeled by tyrosine. These results eliminate a plastoquinone origin for D.+ we conclude instead that D.+ and most likely Z.+ are tyrosine radicals.