Mariuzza R A, Phillips S E, Poljak R J
Annu Rev Biophys Biophys Chem. 1987;16:139-59. doi: 10.1146/annurev.bb.16.060187.001035.
We have reviewed here the three-dimensional structure of an antigen-Fab complex as determined by X-ray crystallographic studies. The antigen is hen egg-white lysozyme (HEL), a protein whose three-dimensional structure and antigenic properties are well known. The Fab was prepared from a murine monoclonal anti-HEL antibody, IgGl,kappa, obtained by cell-hybridization techniques. The equilibrium association constant for the complex is 4.5 X 10(7) mol-1. The complex was crystallized and its three-dimensional structure was determined at 6-A and 2.8-A resolution. A three-dimensional model of the structure was built based on electron-density maps and the amino-acid sequence [determined from the nucleotide sequence of cDNA clones (M. Verhoeyen, C. Berek, J.M. Jarvis, G. Winter, in preparation)]. The three-dimensional structure of the complex shows that 17 antibody residues make close contacts (less than or equal to 4 A) with 16 antigen residues. Fifteen of the contacting antibody residues belong to the six complementarity-determining regions of the light chain (6 residues) and of the heavy chain (9 residues). The remaining two are located in regions of constant or nearly constant sequence ["framework" regions]. The 16 contacting lysozyme residues form a discontinuous, topographical determinant, since they are widely separated in the linear amino-acid sequence but are brought to relative spatial proximity by the three-dimensional folding of the polypeptide chain. The contacting surfaces are relatively flat, with protruding side chains of antigen and antibody penetrating each other over an area with maximum dimensions of 30 X 20 A. As in several other systems of protein-protein interactions, the contacts are chemically characterized as van der Waals interactions and hydrogen bonds. Detailed analysis of the interactions reveals that the antibody's recognition of the antigen is finely specific and is affected by antigenic variation (as observed in lysozymes from other avian species). The quaternary structure of the complexed Fab is elongated, with the axes of the variable (VH + VL) and constant (CH1 + CL) domains making an angle close to 180 degrees. Comparison of the three-dimensional structure of the complexed lysozyme with that of native lysozyme showed no significant conformational change at the current resolution (2.8 A). Comparison of the Fab moiety of the complex with other Fabs of known three-dimensional structure suggested that upon complexing no conformational change takes place in the tertiary structure of Fab either.(ABSTRACT TRUNCATED AT 400 WORDS)
我们在此回顾了通过X射线晶体学研究确定的抗原 - Fab复合物的三维结构。抗原是鸡蛋清溶菌酶（HEL），一种三维结构和抗原特性已为人熟知的蛋白质。Fab是由通过细胞杂交技术获得的鼠单克隆抗HEL抗体IgG1κ制备的。该复合物的平衡缔合常数为4.5×10⁷ mol⁻¹。该复合物被结晶，并在6埃和2.8埃分辨率下确定了其三维结构。基于电子密度图和氨基酸序列（根据cDNA克隆的核苷酸序列确定，M. Verhoeyen、C. Berek、J.M. Jarvis、G. Winter，正在准备中）构建了该结构的三维模型。复合物的三维结构表明，17个抗体残基与16个抗原残基紧密接触（小于或等于4埃）。其中15个接触的抗体残基属于轻链（6个残基）和重链（9个残基）的六个互补决定区。其余两个位于恒定或近乎恒定序列的区域（“框架”区）。16个接触的溶菌酶残基形成一个不连续的、拓扑学决定簇，因为它们在氨基酸线性序列中相距甚远，但通过多肽链的三维折叠在空间上相对靠近。接触表面相对平坦，抗原和抗体突出的侧链在最大尺寸为30×20埃的区域相互穿插。如同其他一些蛋白质 - 蛋白质相互作用系统一样，这些接触在化学上的特征是范德华相互作用和氢键。对相互作用的详细分析表明，抗体对抗原的识别具有高度特异性，并受抗原变异的影响（如在其他鸟类来源的溶菌酶中观察到的）。复合Fab的四级结构是拉长的，可变区（VH + VL）和恒定区（CH1 + CL）的轴形成接近180度的夹角。将复合溶菌酶的三维结构与天然溶菌酶的三维结构进行比较，发现在当前分辨率（2.8埃）下没有明显的构象变化。将复合物的Fab部分与其他已知三维结构的Fab进行比较表明，复合后Fab的三级结构也没有发生构象变化。（摘要截断于400字）
