Acetate binding at the photosystem II oxygen evolving complex:: An S2-state multiline signal ESEEM study
Acetate binding at the photosystem II oxygen evolving complex:: An S2-state multiline signal ESEEM study
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DOI：
10.1021/ja012036c
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发表时间：
2002-09-11
影响因子：
15
通讯作者：
Britt, RD
中科院分区：
文献类型：
--
作者：
Clemens, KL;Force, DA;Britt, RD
先前，我们在甲基氢位置使用氘化的醋酸盐，我们发现醋酸盐结合在靠近Mn簇/Y-Z（.）酪氨酸双自旋配合物的位置，在醋酸盐抑制光系统II （PSII）制备中表现出由S-2-Y-Z（.）相互作用产生的“分裂”EPR信号[Force， D. A.；兰德尔，D. W.；李建平，李建平。生物化学进展[j]。通过顺磁性NO来淬灭Y-Z（.）的顺磁性，我们可以观察到氘化醋酸盐只与Mn团簇相互作用的ESEEM谱。对ESEEM数据进行了很好的拟合，发现两个H-2偶极超细耦合分别为0.097 MHz和0.190 MHz。对这些偶极相互作用进行建模，使用我们的dangler“3 + 1”模型来模拟Mn簇的s -2态，揭示了与醋酸盐直接连接到Mn簇的距离。由于乙酸抑制作用与必需的辅因子Cl-竞争，这表明Cl-直接连接到Mn簇。通过ESEEM比较NO/acetate处理的PSII和未处理的PSII中Mn-组氨酸偶联，研究了乙酸结合对Mn簇结构的影响。我们发现乙酸和NO的加入不影响组氨酸与Mn簇的连接。我们还研究了乙酸在贫锰PSII中接近Y-Z（.）的能力，这种PSII制备预计比完整的PSII更具溶剂亲和性。我们检测到Y-Z（.）和乙酸之间没有偶联。我们之前的研究表明，像甲醇这样的小醇可以很容易地连接到Mn簇上，而像2-丙醇和DMSO这样的大醇则被排除在外[Force， D. A.；兰德尔，D. W.；洛里根，g.a.；克莱门斯，k.l；布里特，r.d.j.a m。化学。社会科学学报，1998,20(2):391 - 393。我们探讨了醋酸结合对甲醇和DMSO结合Mn簇能力的影响。我们发现甲醇能够在醋酸存在的情况下与锰簇结合。我们没有检测到DMSO在醋酸存在下的结合。因此，醋酸结合不会增加Mn簇上DMSO结合的亲和力或可及性。我们还探讨了醋酸结合位点也是底物水的结合位点的可能性。通过比较50% D2O条件下未处理的PSII样品与50% D2O条件下NO/acetate处理的PSII样品的比例三脉冲ESEEM光谱，我们发现乙酸与光系统II的析氧配合物的结合取代了与Mn团簇紧密结合的氘核。
Previously, using acetate deuterated in the methyl hydrogen positions, we showed that acetate binds in close proximity to the Mn cluster/Y-Z(.) tyrosine dual spin complex in acetate-inhibited photosystem II (PSII) preparations exhibiting the "split" EPR signal arising from the S-2-Y-Z(.) interaction [Force, D. A.; Randall, D. W.; Britt, R. D. Biochemistry 1997, 36, 12062-12070]. By using paramagnetic NO to quench the paramagnetism of Y-Z(.), we are able to observe the ESEEM spectrum of deuterated acetate interacting with only the Mn cluster. A good fit of the ESEEM data indicates two H-2 dipolar hyperfine couplings of 0.097 MHz and one of 0.190 MHz. Modeling of these dipolar interactions, using our dangler" 3 + 1 model for the S-2-state of the Mn cluster, reveals distances consistent with direct ligation of acetate to the Mn cluster. As acetate inhibition is competitive with the essential cofactor Cl-, this suggests that Cl- ligates directly to the Mn cluster. The effect of acetate binding on the structure of the Mn cluster is investigated by comparing the Mn-histidine coupling in NO/acetate-treated PSII and untreated PSII using ESEEM. We find that the addition of acetate and NO does not affect the histidine ligation to the Mn cluster. We also investigate the ability of acetate to access Y-Z(.) in Mn-depleted PSII, a PSII preparation expected to be more solvent accessible than intact PSII. We detect no coupling between Y-Z(.) and acetate. We have previously shown that small alcohols such as methanol can ligate to the Mn cluster with ease, while larger alcohols such as 2-propanol, as well as DMSO, are excluded [Force, D. A.; Randall, D. W.; Lorigan, G. A.; Clemens, K. L; Britt, R. D. J. Am. Chem. Soc. 1998, 120, 13321-13333]. We probe the effect of acetate binding on the ability of methanol and DMSO to bind to the Mn cluster. We find that methanol is able to bind to the Mn cluster in the presence of acetate. We detect no DMSO binding in the presence of acetate. Thus, acetate binding does not increase the affinity or accessibility for DMSO binding at the Mn cluster. We also explore the possibility that the acetate binding site is also a binding site for substrate water. By comparing the ratioed three-pulse ESEEM spectra of a control, untreated PSII sample in 50% D2O to an NO/acetate-treated PSII sample in 50% D2O, we find that the binding of acetate to the oxygen evolving complex of photosystem II displaces deuterons bound very closely to the Mn cluster.