Mechanism of photosystem II photoinactivation and D1 protein degradation at low light: The role of back electron flow

Light intensities that limit electron flow induce rapid degradation of the photosystem II (PSII) reaction center D1 protein. The mechanism of this phenomenon is not known. We propose that at low excitation rates back electron flow and charge recombination between the Q(B)(•−) or Q(A)(•−) semiquinone acceptors and the oxidized S(2,3) states of the PSII donor side may cause oxidative damage via generation of active oxygen species. Therefore, damage per photochemical event should increase with decreasing rates of PSII excitation. To test this hypothesis, the effect of the dark interval between single turnover flashes on the inactivation of water oxidation, charge separation and recombination, and the degradation of D1 protein were determined in spinach thylakoids. PSII inactivation per flash increases as the dark interval between the flashes increases, and a plateau is reached at dark intervals, allowing complete charge recombination of the Q(B)(•−)/S(2,3) or Q(A)(•−)/S(2) states (about 200 and 40 s, respectively). At these excitation rates: (i) 0.7% and 0.4% of PSII is inactivated and 0.4% and 0.2% of the D1 protein is degraded per flash, respectively, and (ii) the damage per flash is about 2 orders of magnitude higher than that induced by equal amount of energy delivered by excess continuous light. No PSII damage occurs if flashes are given in anaerobic conditions. These results demonstrate that charge recombination in active PSII is promoted by low rates of excitation and may account for a the high quantum efficiency of the rapid turnover of the D1 protein induced by limiting light.

限制电子流的光强会快速诱导光系统II（Photosystem II, PSII）反应中心D1蛋白的降解。该现象的具体机制目前尚不明确。我们提出，在低激发速率条件下，Q_B(•−)或Q_A(•−)半醌受体与PSII供体侧氧化态S(2,3)状态之间的反向电子流及电荷复合反应，可能通过产生活性氧物种引发氧化损伤。因此，每光化学反应事件所造成的损伤程度应随PSII激发速率的降低而升高。为验证这一假说，我们以菠菜类囊体为实验材料，测定了单周转闪光（single turnover flashes）之间的暗间隔对水氧化失活、电荷分离与复合，以及D1蛋白降解的影响。结果显示，每单次闪光导致的PSII失活比例随闪光间暗间隔的延长而升高；当暗间隔足以使Q_B(•−)/S(2,3)或Q_A(•−)/S(2)状态完成完全电荷复合时（分别约为200秒和40秒），失活程度达到平台期。在该激发速率条件下：(i) 每单次闪光分别有0.7%和0.4%的PSII发生失活，对应0.4%和0.2%的D1蛋白发生降解；(ii) 每闪光造成的损伤程度约为等量能量通过过量连续光照射所诱导损伤的两个数量级（即约100倍）。若在厌氧条件下施加闪光，则不会观测到PSII损伤。上述结果表明，低激发速率会促进活性PSII中的电荷复合过程，这一机制或许可以解释限光条件下诱导的D1蛋白快速周转所具有的高量子效率。