ULTRAFAST TIME-RESOLVED CRYSTALLOGRAPHY ON SCAPHARCA DIMERIC AND TETRAMERIC H

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 文蛤Scapharca inaequvalvis拥有两种血红蛋白，这两种血红蛋白代表了蛋白质变构研究的特殊模型系统。这两种血红蛋白通过一种与研究更充分的人类血红蛋白非常不同的结构机制协同结合氧气。二聚体血红蛋白，称为HBI，是具有两个相同亚基的变构最简单的可能的模型系统。这种血红蛋白的时间分辨结晶学分析首次实时提供了变构变化的初步结构描述(Knapp et.艾尔2006年，PNAS 103 7649-7654)。尽管这些实验总体上取得了成功，但一个主要缺点是晶体中的双酯重新结合水平非常高，这在变构转变期间大大减少了信号。我们对配体迁移的分析，包括时间分辨结晶学实验、溶液实验和计算分析(Knapp等人)。2009，Structure 17，In Press)强烈表明，晶格通过抑制通过远端组氨酸门退出所需的瞬时亚基旋转来限制配体的退出。这些实验还揭示了一条潜在的替代出口路线--通过“后门”渠道。我们正在生产突变体，它将允许配体在晶格的严格限制内通过这个后门退出。四聚体血红蛋白，称为HbII，由两个异二聚体形成，每个异二聚体都有类似于HBI的组装。两个不同亚基的存在将允许研究一个亚基如何影响第二个亚基，这在双重对称HBI中是不可能的。因此，我们建议使用时间分辨X射线衍射实验来阐明四聚体HbII及其特定突变体中的动力学结构途径。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The clam, Scapharca inaequivalvis, possesses two hemoglobins that represent exceptional model systems for the investigation of protein allostery. Both hemoglobins bind oxygen cooperatively using a structural mechanism that is very different from the more well studied human hemoglobin. The dimeric hemoglobin, termed HbI, is the simplest possible model system for allostery with two identical subunits. Time-resolved crystallographic analysis of this hemoglobin provided, for the first time, a preliminary structural description of allosteric changes in real time (Knapp et. al. 2006, PNAS 103 7649-7654). Despite the overall success of these experiments, a major drawback was the very high level of geminate rebinding in the crystal, which substantially reduced the signal during the allosteric transition. Our analysis of ligand migration, including time-resolved crystallographic experiments, solution experiments and computational analysis (Knapp et al. 2009, Structure 17, in press) strongly suggests the crystal lattice restricts ligand exit by damping transient subunit rotations that are required for exit through a distal histidine gate. These experiments also revealed a potential alternate exit route through a "back door" channel. We are producing mutants that will allow ligands to exit through this back door within the tight confines of the crystal lattice. The tetrameric hemoglobin, termed HbII, is formed from two heterodimers, each of which has a similar assembly to that of HbI. The presence of two different subunits will permit investigation of how one subunit impacts a second subunit, which is not possible in the two-fold symmetric HbI. Therefore, we propose to use time-resolved x-ray diffraction experiments to elucidate the kinetic structural pathway in the tetrameric HbII and specific mutants of HbII.