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光合作用机理研究的新突破
作者：佚名?来源：不详?发布时间：2007-7-27 2:21:11
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| In photosynthetic oxygen generation, photons move photo-system II through five successive oxidation states, S0 through S4. Haumann, Dau, and coworkers observed the elusive S4 state spectroscopically, but their study revealed that another state (S4') must be present as well. |
欧洲同步辐射装置(ESRF)的研究人员在-260度低温环境中，利用X-rays研究菠菜光合系统II的放氧复合体（oxygen-evolving complex）动力学，解析了Kok cycle其中一个未知步骤。研究结果发表在《Science》上。
Using sunlight to power our homes and offices is an unaccomplished dream due to the still inefficient technology for a better use of solar energy. The study of photosynthesis in plants could provide new clues by explaining how they absorb almost 100% of the sunlight reaching them, and how they transform it into other forms of energy. Researchers Michael Haumann and Holger Dau, from the Freie Universität Berlin, used the X-ray source of the European Synchrotron Radiation Facility (ESRF) to investigate the kinetics of the photosynthesis process. They have confirmed the existence of a fifth step in the catalysis process converting water into oxygen, and have published their results in Science.
Chlorophyll in plants absorbs light from the sun, which then becomes energy used by the so-called "oxygen-evolving complex" to catalyse the splitting of water into molecular oxygen. This complex contains four manganese and one calcium atom that are known to be at the centre of the catalytic reaction. Five intermediate states have been proposed in the process of photosynthesis - a cycle known as the "Kok cycle" - but only four had been proved until recently. With the help of the ESRF, scientists have been able to identify the missing state, which is particularly important because it is directly involved in the molecular oxygen formation. They suggest, furthermore, an extension of the "Kok cycle" with an additional intermediate and propose a new reaction mechanism on a molecular basis for the release of dioxygen. This gives new insight into the mechanism of photosynthesis.
In order to study this process, the use of synchrotron light was crucial: "A very intense and stable X-ray beam is necessary to perform this study on such a complex, highly diluted protein present in the investigated spinach sample", explains Pieter Glatzel, head of beamline ID26, where the experiments were carried out. The researchers measured the fluorescence from the sample that is emitted after excitation with X-rays.
They flashed the sample with a laser and registered the change using X-ray fluorescence every 10 microseconds to find out how different oxidation states developed. When carefully analysing the reaction kinetics, they observed a time delay before the O2-evolving step. This delay unambiguously proved the existence of the long-searched for intermediate state.
How far away are we then from using the sun to power our daily lives? Michael Haumann, the main author of the publication, asserts that "these are important results that will make an impact in the photosynthesis community. They help our understanding of how solar energy is used in plants and contribute to the efforts to produce more efficient solar cells for our needs".
光系统Ⅱ（ photosystem Ⅱ， PS Ⅱ） 类囊体膜上的色素蛋白复合体，由反应中心、反应中心色素分子 P680 、聚光色素复合体Ⅱ、中心天线、放氧复合体、细胞色素和多种辅助因子组成。 PS Ⅱ的生理功能是吸收光能，进行光化学反应，产生强的氧化剂，使水裂解释放氧气，并把水中的电子传至质体醌。
水氧化钟 (water oxidizing clock) 亦称 Kok 钟 (Kok clock) ， Kok 等（ 1970 ）根据一系列瞬间闪光处理叶绿体与放 O ２ 的关系提出的解释光合作用中水氧化机制的一种模型：叶绿体中的放氧复合体（根据带正电荷的多少，依次称为 S ０ 、 S １ 、 S ２ 、 S ３ 、 S ４ ）在每次闪光后积累 1 个正电荷，积累到 4 个正电荷时（ S ４ ）能裂解 2 个 H ２ O 释放 1 个 O ２ ，获取 4 个 e － ，并回到初始状态 S ０ 。此模型中，每吸收 1 个光量子推动氧化钟前进一步。
放氧复合体 (oxygen-evolving complex ， OEC) 一种含锰的蛋白，在光系统Ⅱ靠近类囊体腔的一侧，参与水的裂解和氧的释放。
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