摘要
光合系统Ⅱ(photosystemII, PSII)是光合生物类囊体膜上的色素蛋白复合体,它能够吸收光能,高效而温和地催化水的分解释放氧气,产生电子和质子。它是植物逆境条件下最为敏感的机构,对植物本身发挥着不可替代的作用。光合作用非循环式电子传递起始十PSⅡ,其中QB位点是PSⅡ电子传递的最终位点,也是大多数除草剂作用的位点,因此对QB位点电子传递的研究具有重要意义。
本研究首先从高等植物菠菜中制备了具有高活性的PSⅡ膜片段,然后筛选了一系列可以维持PSⅡ放氧活性的外源电子受体,分为疏水性和亲水性两类。测定不同疏水和亲水电子受体浓度对PSⅡ放氧活性的不同影响,并通过双倒数作图得到两个动力学参数,Vmax和Km,简单方便地估算电子受体与PSⅡ的相关结合位点及结合亲和力。疏水的醌类电子受体由于其疏水性及与内源质醌(plastoquinone, PQ)结构上的类似性,能够作为PSⅡ的外源电子受体,维持PSⅡ的放氧活性。苯醌上取代基的性质、个数和位置都会影响电子受体与PSⅡ的结合能力及接受PSⅡ电子的能力。动力学分析结果表明,苯基对苯醌(PPBQ)和四氯苯醌(chloranil)倾向于在PSⅡ上的一个位点接受电子,2,6-二氯苯醌(DCBQ)、对苯醌(BQ)和甲基对苯醌(MBQ)可以在PSⅡ的两个位点上接受电子。除此之外,亲水的铁氰化钾(K3[Fe(CN)6])和2,6-二氯酚靛酚(DCIP)也可以作为PSⅡ的外源电子受体,维持PSⅡ的放氧活性,并且亲水性的电子受体K3[Fe(CN)6]和DCIP也可以在PSⅡ的两个位点上接受电子。
通过测定PSⅡ电子传递抑制剂二氧苯基二甲基脲(DCMU)与电子受体之间的竞争关系,对电子受体的作用位点进行进一步分析,结果表明疏水电子受体可以分为两类,DCBQ、BQ和MBQ在浓度较低时倾向于从PQ位点传递电子,在高浓度时能够进入QB位点传递电子。PPBQ和chloranil倾向于从PQ位点传递电子,较难从QB位点传递电子。亲水的电子受体K3[Fe(CN)6]和DCIP在低浓度时倾向于从PQ位点传递电子,高浓度时也同时从QB位点传递电子,但它们在QB位点传递电子时对PSⅡ放氧活性的影响较小,故它们做外源电子受体时PSⅡ放氧活性较低。
Photosystemll (PSII), a pigment protein complex located in the thylakoid membranes of photosynthetic organisms, performs light-induced electron and proton transfer and efficient water-splitting reactions, leading to the formation of molecular oxygen. It is the most sensitive part of plant under stress conditions and plays an irreplaceable role. Non-cycle electron transfer of photosynthesis began with PSII. The QB site is the final sites of the electron transfer in PSII, and also is the target site of most herbicides, so it is particularly important to research the electron transfer of QB site.
We first prepared PSII membrane fragments with highly oxygen evolving activity from spinach, then screened a series of electron acceptors which can maintain the oxygen evolving activity of PSII. There are hydrophobic and hydrophilic electron acceptors. Rates of oxygen evolution were determined in the presence of various concentrations of hydrophobic and hydrophilic electron acceptors, and the two kinetic parameters, Vmax and Km, were estimated from double reciprocal plots. These results offer a simple and convenient proceture to estimate the affinities of electron acceptors to the related binding sites of PSII. Because of their hydrophobicity and structural similarities with PQ, the hydrophobic quinones can be used as exogenous electron acceptors of PSII and recover the oxygen evolution activity. The properties, number and position of substituents of benzoquinones had efffect on the ability of electron accept. Results of dynamic analysis showed that PPBQ and chloranil can accept electrons mainly on one site in PSII, while DCBQ, MBQ and BQ can accept electrons on two sites in PSII. The hydrophilic electron acceptors, K.3[Fe(CN)6] and DCIP can also act as exogenous electron acceptors of PSII and maintain the oxygen evolving activity. Results of dynamic analysis showed that the hydrophilic K3[Fe(CN)6] and DCIP can accept electrons on two sites in PSII.
Investigate the competitive relationship of DCMU and electron acceptors to further analyze the binding sites of electron acceptors. The results showed that the hydrophobic electron acceptors classified into two categories. DCBQ, BQ, MBQ can enter the QB site and accept electrons directly in their high concentrations, while at the low concentrations, they accept electrons mainly from the PQ sites. The other category, PPBQ and chloranil, mainly accept electrons from the PQ sites, and little from QB sites. The hydrophilic electron acceptors, K3[Fe(CN)6] and DCIP, they can accept electrons from both PQ and QB sites. When they act as the exogenous electron acceptors, there is little effect to oxygen evolution activity of PSII, the oxygen evolution activity is low.
引文
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