Babcock G T, Sauer K
Biochim Biophys Acta. 1975 Jul 8;396(1):48-62. doi: 10.1016/0005-2728(75)90188-7.
Two sites are distinguished for the oxidation of exogenous donors by Photosystem II in non-oxygen evolving chloroplasts. In the presence of lipophilic donors (e.g. phenylenediamine, benzidine, diphenylcarbazide), the rate for Signal IIf rereduction following a flash increases as the concentration of exogenous reductant increases. There is a decrease (20-40%) in Signal IIf magnitude accompanying donor addition at low (smaller than 10(-%) M) concentrations, but the extent of the decrease does not change further with increasing donor concentrations. Complementary polarographic experiments monitoring donor (phenylenediamine) oxidation show an increase in oxidation rate with increasing donor concentration. In the presence of the hydrophilic donor, Mn-2+, the Signal IIf decay halftime remains constant with increasing Mn-2+ concentration. However, the flash-induced Signal IIf magnitude pregressively decreases with increasing Mn-2+ concentration. These results are interpreted in terms of two competing paths for the reduction of P680+. In one path P680+ reduction is accompanied by the appearance of Signal IIf, and lipophilic donors subsequently rereduce the Signal IIf species in a series reaction. This reduction follows pseudo-first order kinetics as a function of donor concentration. In the second path Mn-2+ reduces P680+ in a parallel reaction that competes with the formation of the Signal IIf species. This results in a decrease in the magnitude of Signal IIf, but no change in its decay time.
在不进行放氧的叶绿体中，光系统II对外源供体的氧化作用存在两个不同的位点。在亲脂性供体（如苯二胺、联苯胺、二苯卡巴肼）存在的情况下，闪光后信号IIf再还原的速率会随着外源还原剂浓度的增加而提高。在低浓度（小于10⁻⁶M）的供体添加时，信号IIf的幅度会降低（20 - 40%），但随着供体浓度的增加，降低的程度不再进一步变化。监测供体（苯二胺）氧化的补充极谱实验表明，氧化速率会随着供体浓度的增加而提高。在亲水性供体Mn²⁺存在的情况下，信号IIf衰减的半衰期会随着Mn²⁺浓度的增加而保持恒定。然而，闪光诱导的信号IIf幅度会随着Mn²⁺浓度的增加而逐渐降低。这些结果可以用P680⁺还原的两条竞争途径来解释。在一条途径中，P680⁺的还原伴随着信号IIf的出现，随后亲脂性供体在一系列反应中再还原信号IIf物种。这种还原遵循假一级动力学，是供体浓度的函数。在第二条途径中，Mn²⁺在与信号IIf物种形成竞争的平行反应中还原P680⁺。这导致信号IIf的幅度降低，但其衰减时间不变。
