【作者】 成望；

【导师】 陈宇星； 周丛照；

【作者基本信息】 中国科学技术大学 ， 生物化学与分子生物学， 2013， 博士

【摘要】 1.肺炎链球菌p半乳糖苷酶BgaC的结构和功能研究肺炎链球菌BgaC是粘附在细菌表面的p半乳糖苷酶,它能特异性地水解宿主蛋白上修饰的寡糖链中β(1,3)半乳糖苷键连接的半乳糖和N乙酰葡萄糖胺。我们分别解析了1.80A的BgaC及其与半乳糖复合物1.95A的晶体结构。在pH5.5到8.0之间,BgaC都是以稳定的二体形式存在,每个亚单位都是由三个不同的结构域组成：一个由经典的TIM桶组成的催化结构域以及紧接着的两个功能未知的全部由β折叠片组成的结构域。其中活性中心是有TIMβ桶的桶壁以及从第一个ABD延伸出来的一个loop共同组成的。将结合有半乳糖的复合物结构和apo形式的BgaC进行比较,发现活性中心Trp243和Tyr455发生了显著的构象变化。通过对底物进入通道的模拟计算以及手工搭建的Galp(1,3)NAG复合物结构,我们找到了三个对BgaC的底物催化特异性可能有很关键作用的残基。随后的单点突变结合活性检测进一步确定了Trp240和Tyr455主要的贡献是稳定NAG部分,而Trp243对于半乳糖环的稳定有很重要的作用。另外,我们还推测其他的属于GH-35家族的半乳糖苷酶和BgaC一样,拥有类似的结构域组成模式,并且都有一个很保守的与底物特异性相关的芳香族残基从BgaC的第二个结构域对应的部分伸出来。2.肺炎链球菌铁色素结合蛋白PiaA的结构和功能研究铁元素营养缺陷是革兰氏阳性致病菌肺炎链球菌在侵染人体的过程中遇到众多营养缺陷的问题之一。为了保证能获得足够的铁,该菌会利用ABC转运系统Pia来吸收螯合有铁的羟戊酸类铁色素,其中底物结合蛋白PiaA为膜脂蛋白。PiaA对ferrichrome有非常高的亲和力,这使得该蛋白具有能从宿主中结合浓度非常低的铁的能力。我们分别解析了PiaA-apo形式以及ferrichrome复合形式的一对晶体结构。与其他的class Ⅲ底物结合蛋白一样,PiaA也是通过α螺旋将N端和C端的两个结构域连接起来的。一系列的保守残基将ferrichrome稳固在两个domain的之间的裂缝处。随着ferrichrome的结合,在这个裂缝入口处的两个高度柔性的区域发生了显著的构象变化,这暗示着它们可能对于ferrichrome的结合以及释放可能有很大的贡献。将PiaA的结构与来自Escherchia coli的维生素B12ABC转运系统的底物结合蛋白BtuF进行结构比较,发现两个保守的谷氨酸Glu119和Glu262可能会与跨膜蛋白的精氨酸形成盐桥。基于结构的序列比对显示PiaA对ferrichrome的结合模式在其同源蛋白中都非常保守,我们推测含有这些蛋白的细菌无论是革兰氏阳性菌还是革兰氏阴性菌也一样会对ferrichrome的衍生抗生素albomycin具有敏感性。更多还原

【Abstract】 1. Structural and functional research on β-galactosidase BgaC from Streptococcus pneumoniaeThe surface-exposed P-galactosidase BgaC from Streptococcus pneumoniae was reported to be a virulence factor because of its specific hydrolysis activity towards the p(1,3)-linked galactose and N-acetylglucosamine [Ga1β(1,3)NAG] moiety of oligosaccharides on the host molecules. Here we report the crystal structure of BgaC at1.8A and its complex with galactose at1.95A. At pH5.5to8.0, BgaC exists as a stable homodimer, each subunit of which consists of three distinct domains:a catalytic domain of a classic (α/β)8TIM barrel, followed by two all-β domains(ABDs) of unknown function. The side-walls of the TIM β-barrel and a loop extended from the first ABD constitute the active site. Superposition of the galactose-complexed structure to the apo-form revealed significant conformational changes of residues Trp243and Tyr455. Simulation of a putative substrate entrance tunnel and modeling of a complex structure with Ga1p(1,3)NAG enabled us to assign three key residues to the specific catalysis. Site-directed mutagenesis in combination with activity assays further proved that residues Trp240and Tyr455contribute to stabilizing the N-acetylglucosamine moiety, whereas Trp243is critical for fixing the galactose ring. Moreover, we propose that BgaC and other galactosidases in the GH-35family share a common domain organization and a conserved substrate-determinant aromatic residue protruding from the second domain. 2. Structural and functional research on the siderophore binding protein PiaA from Streptococcus pneumoniaeIron scarcity is one of the nutrition limitations that the Gram-positive infectious pathogens Streptococcus pneumoniae encounter in the human host. To guarantee sufficient iron supply, the ATP binding cassette (ABC) transporter Pia is employed to uptake iron chelated by hydroxamate siderophore, via the membrane-anchored substrate-binding protein PiaA. The high affinity towards ferrichrome enables PiaA to capture iron at a very low concentration in the host. We presented here the crystal structures of PiaA in both apo and ferrichrome-complexed forms at2.7and2.1A resolution, respectively. Similar to other class III substrate binding proteins, PiaA is composed of an N-terminal and a C-terminal domain bridged by an a-helix. At the inter-domain cleft, a molecule of ferrichrome is stabilized by a number of highly conserved residues. Upon ferrichrome binding, two highly flexible segments at the entrance of the cleft undergo significant conformational changes, indicating their contribution to the binding and/or release of ferrichrome. Superposition to the structure of Escherichia coli ABC transporter BtuF enabled us to define two conserved residues:Glu119and Glu262, which were proposed to form salt bridges with two arginines of the permease subunits. Further structure-based sequence alignment revealed that the ferrichrome binding pattern is highly conserved in a series of PiaA homologs encoded by both Gram-positive and negative bacteria, which were predicted to be sensitive to albomycin, a sideromycin antibiotic derived from ferrichrome.更多还原