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arXiv 1302.6323cond-mat.softphysics.bio-phq-bio.BM
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光合膜中蛋白质的液晶共存

Coexistence between fluid and crystalline phases of proteins in photosynthetic membranes

Anna R. Schneider, Phillip L. Geissler

摘要:

光系统II(PSII)及其相关的捕光复合物II(LHCII)高度集中在光合类囊体膜的基粒堆叠区域。在膜内,PSII-LHCII超复合物可以以无序堆积、有序阵列或其混合物排列。阵列形成背后的物理驱动力是未知的,这使得确定阵列在调节光捕获或能量转换效率中的可能功能作用的尝试变得复杂。在这里,我们介绍了一个粗粒度的耦合光合膜蛋白质相互作用的模型,专注于只有两种粒子类型,具有简单的形状和结构研究激发的势能。报告模型的平衡波动的计算机模拟,我们证明了它的成功再现不同的结构特征,在实验中观察到的,包括扩展PSII-LHCII阵列。自由能计算表明,阵列的出现标志着一个相变从无序的流体状态到系统跨越晶体,这可以很容易地被逮捕的热力学约束或缓慢的动力学。流体-晶体共存的区域很宽,包括许多生理相关的参数制度。我们的研究结果表明,基粒膜位于或接近相共存,赋予显着的结构和功能的灵活性,这个密集的膜蛋白系统。

英文摘要:

Photosystem II (PSII) and its associated light-harvesting complex II (LHCII) are highly concentrated in the stacked grana regions of photosynthetic thylakoid membranes. Within the membrane, PSII-LHCII supercomplexes can be arranged in disordered packings, ordered arrays, or mixtures thereof. The physical driving forces underlying array formation are unknown, complicating attempts to determine a possible functional role for arrays in regulating light harvesting or energy conversion efficiency. Here we introduce a coarse-grained model of protein interactions in coupled photosynthetic membranes, focusing on just two particle types that feature simple shapes and potential energies motivated by structural studies. Reporting on computer simulations of the model's equilibrium fluctuations, we demonstrate its success in reproducing diverse structural features observed in experiments, including extended PSII-LHCII arrays. Free energy calculations reveal that the appearance of arrays marks a phase transition from the disordered fluid state to a system-spanning crystal, which can easily be arrested by thermodynamic constraints or slow dynamics. The region of fluid-crystal coexistence is broad, encompassing much of the physiologically relevant parameter regime. Our results suggest that grana membranes lie at or near phase coexistence, conferring significant structural and functional flexibility to this densely packed membrane protein system.

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