Packham N K, Mueller P, Dutton P L
Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104.
Biochim Biophys Acta. 1988 Mar 30;933(1):70-84. doi: 10.1016/0005-2728(88)90057-6.
The characteristics of the photocurrent response activated by continuous illumination of planar bilayer membranes containing bacterial reaction centers have been resolved by voltage clamp methods. The photocurrent response to a long light pulse consists of an initial spike arising from the fast, quasi-synchronous electron transfer from the reaction center bacteriochlorophyll dimer, BChl2, to the primary quinone QA. This is followed by a slow relaxation of the current to that promoted by secondary, asynchronous multiple electron transfers from the reduced cytochrome c through the reaction centers to the ubiquinone-10 pool. Currents derived from cytochrome c oxidation that occurs when cytochrome c is associated with the reaction center or when limited by diffusional interaction from solution are recognized. Changes of the ionic strength and pH in the aqueous phase, and the clamped membrane potential (+/- 150 mV), affect the electron-transfer rate between cytochrome c and BChl2. In contrast, the primary light-induced charge separation between BChl2 and QA, or electron transfer between QA on the ubiquinone pool are unaffected. During illumination of reaction center membranes supplemented with cytochrome c and a ubiquinone pool, there is a small but significant steady-state current which is considered to be caused by the re-oxidation of photoreduced quinone by molecular oxygen. In the dark, after illumination of reaction centers supplemented with cytochrome c and a ubiquinone pool, there is a small amount of reverse current resulting from the movement of charges back across the membrane. This reverse current is observed maximally after 400 ms illumination while prolonged illumination diminishes the effect. The source of this current is uncertain, but it is considered to be due to the flux of anionic semiquinone within the membrane profile; this may also be the species that interacts with oxygen giving rise to the steady-state current. It is postulated that when the reaction centers are contained in an alkane-containing phospholipid membrane, in contrast to the in vivo situation, the semiquinone anion formed in the QB site is not tightly bound to the site and can, by exchange-diffusion with the membrane-quinone pool, move away from the site and accumulate in the membrane. However, in the absence, more quantitative work superoxide anion, resulting from O2 interaction with semiquinone of QA, QB or pool cannot be excluded.
通过电压钳方法解析了含有细菌反应中心的平面双层膜在持续光照下激活的光电流响应特性。对长光脉冲的光电流响应包括一个初始尖峰，它源于反应中心细菌叶绿素二聚体（BChl2）向初级醌QA的快速、准同步电子转移。随后电流缓慢弛豫至由还原型细胞色素c通过反应中心向泛醌-10池的次级、异步多电子转移所促进的电流。识别出了细胞色素c与反应中心结合时或受溶液扩散相互作用限制时发生的细胞色素c氧化所产生的电流。水相中的离子强度和pH值以及钳制的膜电位（±150 mV）会影响细胞色素c与BChl2之间的电子转移速率。相比之下，BChl2与QA之间的初级光诱导电荷分离或泛醌池上QA之间的电子转移不受影响。在补充了细胞色素c和泛醌池的反应中心膜光照期间，存在一个小但显著的稳态电流，这被认为是由分子氧对光还原醌的再氧化引起的。在黑暗中，对补充了细胞色素c和泛醌池的反应中心进行光照后，会有少量反向电流，这是由电荷反向穿过膜的移动产生的。在光照400毫秒后最大程度地观察到这种反向电流，而长时间光照会减弱这种效应。这种电流的来源尚不确定，但被认为是由于膜内轮廓中阴离子半醌的通量；这也可能是与氧相互作用产生稳态电流的物质。据推测，当反应中心包含在含烷烃的磷脂膜中时，与体内情况相反，在QB位点形成的半醌阴离子没有紧密结合在该位点，并且可以通过与膜醌池的交换扩散从该位点移开并在膜中积累。然而，在缺乏更多定量工作的情况下，不能排除由QA、QB或池的半醌与O2相互作用产生超氧阴离子的可能性。
