Crystallographic structure studies of an IgG molecule and an Fc fragment

The crystal structures of a human IgG antibody molecule Kol and a human Fc fragment have been determined at 4-Å and 3.5-Å resolution respectively, by isomorphous replacement. The electron-density maps were interpreted in terms of immunoglobulin domains based on the Rei and McPC 603 models (Kol) and by model-building (Fc). The Fab parts of Kol have a different quaternary structure from that observed in isolated crystalline Fab fragments, there being no longitudinal V-C contact in Kol. The Fc part C terminal to the hinge is disordered in the Kol crystals. It is suggested that the Kol molecule is flexible in solution, whereas fragments are rigid. In the Fc fragment both C_H3 and C_H2 show the immunoglobulin fold. The C _H3 dimer aggregates as C_H1-C_L while C _H2 are widely separated from each other. The carbohydrate bound to Fc is in fixed position. From these structures a hypothetical liganded antibody molecule has been constructed, which is assumed to be rigid.