Zheng M, Dismukes G C
Department of Chemistry, Princeton University, New Jersey 08544, USA.
Biochemistry. 1996 Jul 9;35(27):8955-63. doi: 10.1021/bi9522209.
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 and 1H ENDOR spectroscopies. The data show that the conformation of the isoprenyl chain at C beta relative to the aromatic ring differs by 90 degrees for QA- in higher plant PSII versus both types of bacterial reaction centers, Rhodobacter sphaeroides and Rhodopseudomonas viridis [containing ubiquinone (UQ) or menaquinone (MQ) at QA site, 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 C beta C gamma bond coplanar with the aromatic ring, while type I semiquinones like UQ- and MQ- place the C beta C gamma 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 QA binding 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 (C beta C gamma bond perpendicular to the ring) and excited (C beta C gamma 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 confornational 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 QA to 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.
通过电子顺磁共振（EPR）和质子电子双共振（1H ENDOR）光谱，已经确定了菠菜光系统II蛋白复合物中还原态初级电子受体（QA-，一种质体半醌-9（PQ-9-）阴离子自由基）的构象和部分电子自旋密度分布，以及溶液中的游离PQ-9-。数据表明，相对于芳香环，高等植物光系统II中QA-的Cβ位异戊二烯链的构象与两种细菌反应中心（分别在QA位点含有泛醌（UQ）或甲基萘醌（MQ）的球形红细菌和绿脓红假单胞菌）的构象相差90度。光系统II与细菌反应中心中QA-物种之间的这种构象差异恰好遵循溶液中游离的孤立半醌阴离子自由基的构象偏好；像PQ-9-这样的II型半醌的异戊二烯CβCγ键与芳香环共面，而像UQ-和MQ-这样的I型半醌则使CβCγ键垂直于环。这种构象差异源于I型半醌中存在的异戊二烯链与C6甲基之间的非键排斥，迫使形成垂直构象，但在C6处具有较小氢原子的II型半醌中不存在这种排斥。因此，高等植物光系统II和细菌反应中心中的QA结合位点都容纳了在溶液中观察到的其天然半醌的较低能量构象。据估计，UQ-和维生素K1-自由基的基态（CβCγ键垂直于环）和激发态（CβCγ键与环共面）构象之间的能量差足够大（约6千卡/摩尔），以至于在室温下这些构象的数量差异大于10倍。对于PQ-9-，估计有类似的数值。我们提出，I型和II型半醌强烈的构象偏好导致围绕QA的异戊二烯/醌头部连接的不同反应中心蛋白质结构的进化，以适应有利的低能量构象异构体。这种预测的蛋白质结构差异可以解释在高等植物光系统II和细菌反应中心分别进行的醌替代实验中观察到的II型与I型醌的低效性（高选择性）。
