【作者】 杨瑞；

【导师】 赵广华；

【作者基本信息】 中国农业大学 ， 食品科学， 2014， 博士

【摘要】 自组装(self-assembly)是一种普遍存在于生命体系中的现象。相较于DNA和多肽类物质,蛋白质大分子因其空间的结构复杂性,以及受非共价键大量性和无序性分布的影响,自组装研究水平较低。如何控制非共价键作用,进而制备蛋白质有序自组装产物是目前研究的热点和难点。铁蛋白是一类由24个亚基组成的球状壳状蛋白质,是广泛存在于动物、植物以及微生物体细胞中的一种铁贮藏蛋白。铁蛋白结构具有正八面体对称性,其每一个亚基通常都是由4个螺旋束(A、B、C、D)以及位于亚基C-末端的E螺旋结构组成,该E螺旋是形成铁蛋白四重轴通道的主要氨基酸。植物铁蛋白中成熟期大豆种子铁蛋白mature soybean seed ferritin (mSSF)是由同源性很高的分子量26.5kDa的H-1亚基与28.0kDa的H-2型亚基以1：1比例组成的。前期研究发现mSSF的H-1型亚基的E-螺旋是缺失的(H-1AE),因此形成的铁蛋白四重轴通道尺寸增大。本研究首先依据mSSF的结构特点,在体外制备了大豆种子铁蛋白reconstructed mSSF (rmSSF),并设计合成了不同聚合度的线性阳离子多聚赖氨酸Poly (a, L-lysine)(PLL).本文以rmSSF为单体,研究PLL诱导的rmSSF一维二维自组装特性；重点研究了二者相互作用机理；并从小分子多肽以及铁蛋白各自角度研究其结构对自组装的影响。该研究将拓宽铁蛋白在纳米科技领域的应用范围,为以铁蛋白为模版制备新型纳米材料和器件打下一定的研究基础。具体研究结果如下：1.实验通过分子生物学及化学手段,利用铁蛋白可逆组装性质,在pH11.4时将等比例混合的rH-2及rH-1△E蛋白变性解离为亚基,在pH7.0诱蛋白复性,体外制备了大豆种子铁蛋白nnSSF.由于H-1亚基C端末尾16个氨基酸的缺失导致了rmSSF的四重轴通道尺寸的增大,获得的rmSSF保留了铁蛋白的球形空壳四级结构、氧化沉淀活性及还原释放活性。2.依据rmSSF内部空腔在生理条件下带负电荷和四重轴通道对称性分布的特点,实验设计了在结构上与rmSSF具有电荷互补规律及尺寸契合度的15个聚合度的多聚赖氨酸PLL(PLL15).研究表明PLL15可以诱导rmSSF按照其四重轴通道方向进行有序的一维线性自组装。实验利用荧光技术、透射电镜、分子生物学、动态光散射等手段证明了PLL15可以通过静电作用力结合于rmSSF的四重轴部位,结合比例为1：1,并且PLL15在rmSSF的一维排列中起到桥梁连接作用。rmSSF自组装过程受时间、PLL添加比例、pH、离子强度、多肽电荷密度等因素的影响,通过调节作用时间和PLL添加比例可以控制rmSSF自组装产物的聚合长度。另外,研究发现线性rmSSF自组装产物还能作为模板将5nm Fe3O4铁核进行线性排列。3.研究发现尿素(10mM)能够显著增大铁蛋白铁还原释放速率U0值,即增大四重轴通道尺寸。在尿素(10.0mM)存在条件下,PLL15能诱导rmSSF按照铁蛋白四重轴通道方向进行有序自组装形成二维平面排列。在该自组装过程中,PLL15结合于rmSSF的四重轴部位,结合比例为3：1,静电作用力在二者的结合中及rmSSF二维排列中起了重要作用。rmSSF自组装过程受时间、PLL添加比例、pH、离子强度、多肽电荷密度等因素的影响。研究表明该体系同样适用于具有类似结构的杂合植物铁蛋白。另外,rmSSF内部空腔装载的三价铁矿化核也可以受PLL15的诱导进行二维点阵排列,证明rmSSF自组装产物在无机纳米粒子排列领域具有一定的应用潜力。更多还原

【Abstract】 Self-assembly is a ubiquitous phenomenon in the living system, and it is one of the most essential content in life science. The phenomenon of self-assembling of widespread molecules such as proteins, DNA, and peptides in nature is the hotspot of bio-nanotechnology research. Compared with DNA and peptides, the research level of self-assembly of protein biomolecules is much lower due to its complexity of the spatial structures, and the disorder and great amount of of non-covalent interactions. Thus, how to control the non-covalent interactions, especially the electrostatic force, to prepare ordered protein self-assembly is a hotspot of the current study. Ferritins are members of the superfamily of iron storage and detoxification proteins found throughout the animal, plant, and microbial kingdoms. All ferritins are composed of24structurally identical subunits that assemble into a very robust protein cage with octahedral (432) symmetry. The external diameter of these assembled protein cages is12nm and the internal cavity is8nm. Each subunit of the ferritin usually consists of four helix bundle (A, B, C, and D), and a short E helix located at the C-terminus. Different from mammal fertitin, phytoferritin only contains H-type subunit. Particularly, mature soybean seed ferritin (mSSF) is comprised with H-1(26.5kDa) and H-2(28.0kDa) subunits with a high homology. In mSSF, the E-helix of H-1(26.5kDa) subunit is removed during its early configuration, thereby forming a protein which has expanded4-fold channels. In view of two important properties of mSSF structure, a highly negative charge density on the inner cavity under physiological conditions and the expanded symmetrical4-fold channels, we synthesized the reconstructed mSSF (rmSSF) in vitro. In addition, based on the the structural characteristics of mSSF, we designed a linear poly (a, L-lysine)(PLL) to induce ordered self-assembly of rmSSF and explored the polymerization mechanism. The study will broaden the range of applications of ferritin nanotechnology, and provide a good template for the preparation of novel nanomaterials and devices. Main results were bbtained as follows:1. mSSF is a heteropolymer consisting of H-1ΔAE and H-2in a1:1ratio, forming a hollow and spherical structure. Through molecular biology and chemical means, using the reversible assembly property of ferritin, reconstructed mSSF (rmSSF) was prepared. The4-fold channel size of rmSSF was1.2nm in length and0.4nm in width, which was obviously larger than that of its analogue, rH-2, a homopolymer. In addition, this ferritin maintained its hollow and spherical structure and iron oxidiase activity.2. Poly (a, L-lysine)(PLL) is a good example of a water-soluble polymer with positive charges based on a naturally occurring amino acid monomer lysine. PLL with polymerization degree of15(PLL15) was designed, and rmSSF was induced into linear chains in the presence of PLL15through channel-directed electrostatic interaction, and their binding ratio was1:1(PLL15/rmSSF). Moreover, the self-assembly of rmSSF induced by PLL15could be controlled by reacton time and PLL15/ferritin ratio. The pH, ionic strength, and peptide types were also influencing factors. This study demonstrated that the electrostatic force could be controlled to fabricate the hierarchical assembly of supramolecular protein cages.3. To brodern the hierarchical assembly of ferritin cages, urea with low concentrations was used to expand the local position of the4-fold channel by taking advantage of the flexibility of protein channels. The initial rate of iron release (vo) influenced by urea was detected;10.0mM of urea was chosen to expand the4-fold channel of rmSSF. We found that rmSSF could self-assemble into2D square arrays through channel-directed electrostatic interactions with PLL15at pH7.0in the presence of urea, and the PLL15-ferritin binding ratio was3:1. Structurally, protein cages were aligned along their common4-fold symmetry axis, imposing a fixed disposition of neighboring ferritins. To explore the application of this strategy for positioning inorganic nanomaterial, reconstituted holo rmSSF_H193E/H197E with a loading of600iron atoms per shell was prepared, followed by treatment with PLL15in a3:1ratio. It revealed that the resulted Fe (Ⅲ) cores within ferritins arrayed regularly with distances of about10.0nm between centers of neighboring component, in accordance to the protein lattices. Thus, the thinking behind this strategy was that the inner cavity of apoferritin could provide an ideal, spatially restricted, and chemical reaction chamber within which nanoparticles could be accurately positioned. Such2D assembly can be utilized as a scaffold for various functionalities by manipulating three distinct interfaces (the exterior surface, the interior surface, and the interface between subunits) of each protein cage.更多还原