The immunoglobulin fold

Each domain folds in a similar and characteristic way typical of all heavy and light chain domains as well as other polypeptides which form part of the immunoglobulin 'supergene' family. The polypeptide chain forms two layers of antiparallel (3-pleated sheet to create a cylinder, one layer (X) consists of four (3 strands, the other (Y) of three. These layers form a hydrophobic envelope containing the intrachain disulfide bond which links them. The two cysteines that form this bond are conserved in almost all molecules that form the immunoglobulin fold. The 3-strands are connected by loops which arc evidently flexible, being rich in glycine residues. All constant region domains fold according to this basic structure.

The domains of the variable region have a slightly different structure in that there are two extra strands of (3-pleated sheet associated with the three-strand (Y) face. This extension allows more scope for variety in the antigen-binding site. When the constant region domains pair, as in CL:C71, 0,2:0,2, C,3:Cy3 the four-strand faces of the constant region domains face each other and, with the exception of C72, adhere on the basis of noncovalent interactions between conserved residues. The Cy2 domains are separated by a solvent channel lined on the X face with a carbohydrate moiety attached by N-glycosidic linkage through Asn297. The spatial relationship between the Cy2 domains is maintained by the hinge disulfide bonds and the interactions of the C.,3 domains. The biological significance of this feature is not understood, but loss of the disulfide or removal of the paired C73 domains disturbs this spatial relationship and may interfere with biological effector function. Additionally, the carbohydrate may play a biological role in securing this structure. Elimination or modification of the carbohydrate often has damaging consequences for the biological activity of IgG. Carbohydrate may also occur in the variable region if the acceptor sequence Asn-X-Ser/Thr occurs, that is in approximately 15% of V regions.

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