ELECTRON-TRANSFER EVENTS NEAR THE REACTION CENTER IN O-2-EVOLVING PHOTOSYSTEM-II PREPARATIONS
ELECTRON-TRANSFER EVENTS NEAR THE REACTION CENTER IN O-2-EVOLVING PHOTOSYSTEM-II PREPARATIONS
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DOI：
10.1021/bi00416a005
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发表时间：
1988-08-09
期刊：
影响因子：
2.9
通讯作者：
BABCOCK, GT
中科院分区：
文献类型：
--
作者：
HOGANSON, CW;BABCOCK, GT
时间分辨ESR已被用来研究放氧光系统II膜碎片中的电子转移反应。外源受体二氯苯醌(DCBQ)被光系统II还原；生成的DCBQ自由基的ESR谱与内源酪氨酸自由基YD+和YZ+的ESR谱的中心重叠，而不与翅膀重叠。其中，Yz+表示参与反应中心叶绿素P680与含锰析氧复合体之间电子转移的物种，Yd+表示稳定的光系统II自由基。在DCBQ存在的情况下，利用合适的磁场，我们记录了YZ+在重复闪光条件下的动力学瞬变过程。当记录YZ+的动力学轨迹时，我们也使用1 mM的K3Fe(CN)6作为外源受体，尽管在5 mM以上的浓度下我们观察到可能来自P680+的额外信号。用DCBQ和1 mM K3Fe(CN)6得到的YZ+的动力学轨迹相似，均为两相。较慢的相具有1.2ms的半衰期，对应于Yz+被S3态还原；较快的相反映了Yz+被S1和S2还原。通过使用流动的、暗适应的PSII膜，我们分解了第一次闪光上的YZ+S1反应(T1/2=100µs)，发现它比发生在第二次闪光上的YZ+S2反应(T1/2=300µs)要快得多。利用门控积分技术获得了YZ+在放氧PSII膜中的高分辨ESR谱，该谱与Yd+和YZ+在被抑制膜中的ESR谱相似。因此，YZ+上的自旋与放氧络合物中的锰之间的磁相互作用可以忽略不计地展宽YZ+的光谱。这些结果支持这样的观点，即单个电子载体YZ在P680+和放氧络合物中的锰系综之间工作，并在所有四个S态转变中起作用。
Time-resolved ESR has been used to study electron-transfer reactions in oxygen-evolving photosystem II membrane fragments. The exogenous acceptor dichlorobenzoquinone (DCBQ) is reduced by photosystem II; the ESR spectrum of the resulting DCBQ radical overlaps the center but not the wings of the ESR spectra of the endogenous tyrosine radicals YD+ and YZ+. Here YZ+ denotes the species that is involved in electron transfer between the reaction center chlorophyll, P680, and the manganese-containing, oxygen-evolving complex, and YD+ denotes the stable photosystem II radical. By using appropriate magnetic fields, we recorded kinetic transients of YZ+ under repetitive flash conditions with DCBQ present. We also used 1 mM K3Fe(CN)6 as an exogenous acceptor when recording kinetic traces of YZ+, although at concentrations above 5 mm we observe an additional signal that could be due to P680+. The kinetic traces of YZ+ obtained with DCBQ or 1 mM K3Fe(CN)6 are similar and show two phases. The slower phase has a half-time of 1.2 ms and corresponds to the reduction of YZ+ by the S3 state; the faster phase reflects reduction of YZ+ by both S1 and S2. By using flowing, dark-adapted PSII membranes, we resolved the YZ+S1 reaction (t1/2 = 100 .mu.s) on the first flash and found it to be significantly faster than the YZ+S2 reaction (t1/2 = 300 .mu.s) which occurs on the second flash. A high-resolution ESR spectrum of YZ+ in O2-evolving PSII membranes was obtained with gated integration techniques and found to be similar to the spectra of YD+ and of YZ+ in inhibited membranes. Thus, the magnetic interaction between spins on YZ+ and the manganese in the oxygen-evolving complex broadens the YZ+ spectrum negligibly. These results support the idea that a single electron carrier, YZ, operates between P680+ and the manganese ensemble in the oxygen-evolving complex and functions on all four S-state transitions.