The three-dimensional structure of the carbohydrate within the Fc fragment of immunoglobulin G.

The IgG molecule, like all antibodies, is a glycoprotein, and with the discovery in 1959 that the rabbit IgG molecule could be split by the action of papain into two Fab (antigen-binding) and one Fc (crystallizable, complementand cell-binding) fragment, the major glycosylation site was found to lie within Fc (Porter, 1959). (The Fc fragment conists of the C-terminal half of each of the two ‘heavy’ polypeptide chains of IgG.) Other minor sites have since been identified in both the Fab ‘arms’, and the ‘hinge’ which connects them to the F c ‘stem’ of the Y-shaped IgG molecule (Fanger & Smyth, 1972), but all molecules of rabbit IgG contain carbohydrate N-linked to the asparagine residue 297 (human IgGl protein Eu numbering) in each of the two identical heavy chains in the Fc fragment. In this paper we report the results of an X-ray crystallographic study of rabbit F c fragment, which has revealed the conformation of these two carbohydrate chains in relation to the three-dimensional structure of the protein. The Fc fragment was prepared by papain digestion of IgG from pooled rabbit serum, without reduction of the single disulphide bridge between the two heavy chains at residue 229