Ohad I, Adir N, Koike H, Kyle D J, Inoue Y
Department of Biological Chemistry, Hebrew University of Jerusalem, Israel.
J Biol Chem. 1990 Feb 5;265(4):1972-9.
The light-induced inactivation of the photochemical reaction center II (RCII) of oxygenic chloroplasts (photoinhibition) was investigated in cells and isolated thylakoids of the green alga Chlamydomonas reinhardtii. The process is resolved into a reversible conformational change followed by an irreversible modification of RCII D1 protein. The light-induced changes in vivo persisted in isolated thylakoids. The first step is characterized by (i) destabilization of the secondary acceptor semiquinone anion, Q-B, bound to the D1 protein. This is demonstrated by a reduction in the activation energy of S2,3Q-B charge recombination as measured by the thermoluminescence technique; and (ii) a rise in the intrinsic fluorescence and a decrease of the maximal fluorescence. Unoccupancy of the QB site by plastoquinone partially protected RCII against the light-induced destabilization of Q-B. The extent of charge separation (P+680Q-A) was not affected. However, the slow phase (microsecond) of P+680 dark reduction increased, and the amplitude of signal II was reduced by 20-30%, indicating that in a fraction of RCII, electron donation from Z to P+680 was impaired without losing primary photochemistry. This modification correlates with the irreversible change in D1 protein resulting in the formation of a trypsin-resistant fragment of 16 kDa detected in D1 isolated from light-exposed cells. The change in the Q-B stability could allow charge equilibration with QA and thus explain the rise in the intrinsic fluorescence level and reduction of electron flow to plastoquinone. The change in the lifetime of P+680 can account for further reduction in electron flow (photo-inhibition). The irreversible light-dependent modification of D1 may serve as the signal for its degradation and replacement by a newly synthesized molecule (turnover).
在莱茵衣藻的细胞和分离类囊体中，研究了光诱导的产氧叶绿体光化学反应中心II（RCII）失活（光抑制）过程。该过程可分为可逆的构象变化，随后是RCII D1蛋白的不可逆修饰。光诱导的体内变化在分离的类囊体中持续存在。第一步的特征是：（i）与D1蛋白结合的次级受体半醌阴离子Q-B不稳定。这通过热发光技术测量的S2,3Q-B电荷复合活化能降低得到证明；（ii）固有荧光增加，最大荧光降低。质体醌未占据QB位点可部分保护RCII免受光诱导的Q-B不稳定影响。电荷分离程度（P+680Q-A）不受影响。然而，P+680暗还原的慢相（微秒）增加，信号II的幅度降低20-30%，表明在一部分RCII中，从Z到P+680的电子供体受损，但未丧失初级光化学。这种修饰与D1蛋白的不可逆变化相关，导致在从光照细胞分离的D1中检测到16 kDa的胰蛋白酶抗性片段。Q-B稳定性的变化可能使电荷与QA平衡，从而解释固有荧光水平的升高和向质体醌的电子流减少。P+680寿命的变化可以解释电子流的进一步减少（光抑制）。D1的不可逆光依赖性修饰可能作为其降解信号，并被新合成的分子取代（周转）。
