The conformation of the isoprenyl chain relative to the semiquinone head in the primary electron acceptor (QA) of higher plant PSII (plastosemiquinone) differs from that in bacterial reaction centers (ubisemiquinone or menasemiquinone) by ca. 90 degrees.
The conformation of the isoprenyl chain relative to the semiquinone head in the primary electron acceptor (QA) of higher plant PSII (plastosemiquinone) differs from that in bacterial reaction centers (ubisemiquinone or menasemiquinone) by ca. 90 degrees.
复制标题
高等植物 PSII（质体半醌）的初级电子受体（QA）中异戊二烯基链相对于半醌头的构象与细菌反应中心（泛半醌或甲那半醌）中的构象不同。
DOI：
10.1021/bi9522209
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发表时间：
1996
期刊：
影响因子：
2.9
通讯作者：
Dismukes,GC
中科院分区：
文献类型：
--
作者：
Zheng,M;Dismukes,GC
用EPR和~ 1H ENDOR方法测定了菠菜光系统Ⅱ蛋白质复合物中还原的初级电子受体（QA-）和溶液中游离的PQ-9-的构象和部分电子自旋密度分布。结果表明，在高等植物PSII中的QA-与两种类型的细菌反应中心Rhodopyrusphaeroides和Rhodopyrusviridis [在QAsite上分别含有泛醌（UQ）或甲萘醌（MQ）]相比，异戊二烯基链Cβ相对于芳环的构象相差90°。PSII与细菌RC中QA-物种之间的构象差异精确地遵循了溶液中游离的孤立半醌阴离子自由基的构象偏好; II型半醌如PQ-9-具有与芳环共面的异戊二烯基CβCγ键，而I型半醌如UQ-和MQ-将CβCγ键垂直于环放置。这种构象差异起源于异戊二烯基链和C6甲基之间的非键合排斥存在于I型半醌中，迫使垂直构象，但在II型半醌中不存在，在C6处具有较小的H原子。因此，在高等植物PSII和细菌的反应中心的QA结合位点容纳较低的能量构象的本地semiquinones在溶液中观察到。UQ-和维生素K1-自由基的基态（C β C γ键垂直于环）和激发态（CβCγ键与环共面）构象之间的能量差估计足够大（约为1000）。6千卡/摩尔），以在室温下产生大于10倍的这些构象的群体差异。对于PQ-9-，估计了类似的数量。我们建议，I型和II型semiquinones强的构象偏好导致不同的反应中心蛋白质结构的进化周围的异戊二烯基/醌头连接的QA，以适应有利的低能量构象。这种预测的蛋白质结构的差异可以解释低效率（高选择性）的醌替代实验中观察到的II型与I型醌在高等植物PSII和细菌反应中心，分别。
The conformation and partial electron spin density distribution of the reduced primary electron acceptor (QA-), a plastosemiquinone-9 (PQ-9-) anion radical, in photosystem II protein complexes from spinach as well as free PQ-9-in solution have been determined by EPR and1H ENDOR spectroscopies. The data show that the conformation of the isoprenyl chain at Cβ relative to the aromatic ring differs by 90° for QA-in higher plant PSII versus both types of bacterial reaction centers,RhodobactersphaeroidesandRhodopseudomonas viridis[containing ubiquinone (UQ) or menaquinone (MQ) at QAsite, respectively]. This conformational distinction between the QA-species in PSII vs bacterial RCs follows precisely the conformational preferences of the isolated semiquinone anion radicals free in solution; type II semiquinones like PQ-9-have the isoprenyl CβCγ bond coplanar with the aromatic ring, while type I semiquinones like UQ-and MQ-place the CβCγ bond perpendicular to the ring. This conformational difference originates from nonbonded repulsions between the isoprenyl chain and the C6 methyl group present in type I semiquinones, forcing the perpendicular conformation, but absent in type II semiquinones having the smaller H atom at C6. Thus, the QAbinding site in both higher plant PSII and bacterial reaction centers accommodates the lower energy conformation of their native semiquinones observed in solution. The energy difference between ground (CβCγ bond perpendicular to the ring) and excited (CβCγ bond coplanar with the ring) conformations of UQ-and vitamin K1-radicals is estimated to be sufficiently large (ca. 6 kcal/mol) to produce greater than a 10-fold difference in populations of these conformations at room temperature. For PQ-9-, a similar number is estimated. We propose that the strong conformational preferences of type I and type II semiquinones has lead to the evolution of different reaction center protein structures surrounding the isoprenyl/quinone head junction of QAto accommodate the favored low energy conformers. This predicted difference in protein structures could explain the low effectiveness (high selectivities) observed in quinone replacement experiments for type II vs type I quinones seen in higher plant PSII and bacterial reaction centers, respectively.