Perkins S J, Nealis A S, Sim R B
Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, London, U.K.
Biochemistry. 1991 Mar 19;30(11):2847-57. doi: 10.1021/bi00225a017.
Factor H is a regulatory component of the complement system. It has a monomer Mr of 150,000. Primary structure analysis shows that the polypeptide is divided into 20 homologous regions, each 60 amino acid residues long. These are independently folding domains and are termed "short consensus repeats" (SCRs) or "complement control protein" (CCP) repeats. High-flux synchrotron X-ray and neutron scattering studies were performed in order to define its solution structure in conditions close to physiological. The Mr of factor H was determined as 250,000-320,000 to show that factor H is dimeric. This structure is maintained at concentrations between 1 and 11 mg/mL in the pH range 5-9. Zn2+ ions are an inhibitor of C3b cleavage by factor I, a reaction in which factor H acts as a cofactor. Additions of Zn2+ to factor H caused it to form oligomers containing 4-10 monomers. The radius of gyration RG of native factor H by X-rays or by neutrons in 0% or 100% 2H2O buffers is not measurable but is greater than 12.5 nm. Two cross-sectional radii of gyration RXS-1 and RXS-2 were determined as 3.0-3.1 and 1.8 nm, respectively. Analyses of the cross-sectional intensities show that factor H is composed of two distinct subunits. The RXS-1 corresponds to the cross-sectional properties of both subunits and exhibits an unusual radiation dependence on the X-ray flux. Since RXS-2 is close to the corresponding RXS of C4b binding protein (91% of which is formed from SCR/CCP domains), it is inferred that the SCR/CCP domains of factor H and C4b binding protein have similar solution structures. The use of hydrodynamic spheres to reproduce literature sedimentation coefficients of 5.5-5.6 S showed that these were compatible with a V-shaped arrangement of two rods (36 spheres each, length 87 +/- 5 nm) joined at an angle of 5 degrees. The use of a similar arrangement of 244 spheres arranged in two rods (length 77 nm) to fit the experimental X-ray and neutron scattering curves showed that the two rods are joined at an angle of 5 degrees. This model corresponds to an actual RG of 21-23 nm. The separation between each SCR/CCP in factor H is close to 4 nm. In the solution structure of factor H, the SCR/CCP domains are in a highly extended conformation.
补体因子H是补体系统的一种调节成分。它的单体分子量为150,000。一级结构分析表明，该多肽被分为20个同源区域，每个区域由60个氨基酸残基组成。这些区域是独立折叠的结构域，被称为“短共有重复序列”（SCRs）或“补体控制蛋白”（CCP）重复序列。为了确定其在接近生理条件下的溶液结构，进行了高通量同步加速器X射线和中子散射研究。补体因子H的分子量测定为250,000 - 320,000，表明补体因子H是二聚体。在pH值为5 - 9、浓度为1至11 mg/mL的范围内，这种结构得以维持。锌离子是I因子裂解C3b的抑制剂，在该反应中补体因子H作为辅因子。向补体因子H中添加锌离子会使其形成含有4 - 10个单体的寡聚体。在0%或100% 2H2O缓冲液中，通过X射线或中子测定的天然补体因子H的回转半径RG无法测量，但大于12.5 nm。两个截面回转半径RXS - 1和RXS - ２分别测定为3.0 - 3.1 nm和1.8 nm。对截面强度的分析表明，补体因子H由两个不同的亚基组成。RXS - 1对应于两个亚基的截面特性，并且对X射线通量表现出异常的辐射依赖性。由于RXS - 2接近C4b结合蛋白相应的RXS（其中91%由SCR/CCP结构域组成），因此推断补体因子H和C4b结合蛋白的SCR/CCP结构域具有相似的溶液结构。使用流体动力学球体来重现文献中5.5 - 5.6 S的沉降系数，结果表明这些系数与两根杆（每根杆由36个球体组成，长度为87 ± 5 nm）以5度角连接的V形排列相兼容。使用由两根杆（长度为77 nm）排列的244个球体的类似排列来拟合实验X射线和中子散射曲线，结果表明两根杆以5度角连接。该模型对应的实际RG为21 - 23 nm。补体因子H中每个SCR/CCP之间的间距接近4 nm。在补体因子H的溶液结构中，SCR/CCP结构域处于高度伸展的构象。
