PDBsum entry 1h3u

Immune system

PDB id: 1h3u

Contents

Protein chain

208 a.a. *

Ligands

NAG-NAG-BMA-MAN-MAN-FUL

NAG-NAG-BMA-BMA-MAN-FUL

Waters ×145

* Residue conservation analysis

References listed in PDB file

Key reference

Title

Structural analysis of human igg-Fc glycoforms reveals a correlation between glycosylation and structural integrity.

Authors

S.Krapp, Y.Mimura, R.Jefferis, R.Huber, P.Sondermann.

Ref.

J Mol Biol, 2003, 325, 979-989. [DOI no: 10.1016/S0022-2836(02)01250-0]

PubMed id

12527303

Abstract

Antibodies may be viewed as adaptor molecules that provide a link between humoral and cellular defence mechanisms. Thus, when antigen-specific IgG antibodies form antigen/antibody immune complexes the effectively aggregated IgG can activate a wide range of effector systems. Multiple effector mechanisms result from cellular activation mediated through a family of IgG-Fc receptors differentially expressed on leucocytes. It is established that glycosylation of IgG-Fc is essential for recognition and activation of these ligands. IgG antibodies predominate in human serum and most therapeutic antibodies are of the IgG class.The IgG-Fc is a homodimer of N-linked glycopeptide chains comprised of two immunoglobulin domains (Cgamma2, Cgamma3) that dimerise via inter-heavy chain disulphide bridges at the N-terminal region and non-covalent interactions between the C-terminal Cgamma3 domains. The overall shape of the IgG-Fc is similar to that of a "horseshoe" with a majority of the internal space filled by the oligosaccharide chains, only attached through asparagine residues 297.To investigate the influence of individual sugar (monosaccharide) residues of the oligosaccharide on the structure and function of IgG-Fc we have compared the structure of "wild-type" glycosylated IgG1-Fc with that of four glycoforms bearing consecutively truncated oligosaccharides. Removal of terminal N-acetylglucosamine as well as mannose sugar residues resulted in the largest conformational changes in both the oligosaccharide and in the polypeptide loop containing the N-glycosylation site. The observed conformational changes in the Cgamma2 domain affect the interface between IgG-Fc fragments and FcgammaRs. Furthermore, we observed that the removal of sugar residues permits the mutual approach of Cgamma2 domains resulting in the generation of a "closed" conformation; in contrast to the "open" conformation which was observed for the fully galactosylated IgG-Fc, which may be optimal for FcgammaR binding. These data provide a structural rationale for the previously observed modulation of effector activities reported for this series of proteins.

Figure 1.
Figure 1. The carbohydrate sequence attached at Asn297 of human IgG1-Fc. Numbers attributes only sugars that have been observed in the electron density. The sugars in bold belong to the oligosaccharide core which is found in all naturally occurring glycoforms, the addition of the other sugar residues is variable. The biantennary mannose residues Man5 and Man8 that are glycosidically linked to Man4 form the a(1-6) and a(1-3) arms, respectively. A bisecting (GlcNAc)* occurs naturally in 5-10% of normal human IgG but not in IgG Cri used here. The arrows point to the termini of respective glycoforms that have been prepared by enzymatic truncation of native IgG-Fc fragments. The native^** oligisaccharide is not fully sialylated, but is a mixture of G0F, G1F, G2F and monosialylated one.

Figure 4.
Figure 4. The role of GlcNAc6 in stabilisation of carbohydrate-protein interactions. The structure of the oligosaccharide moiety (chain A) of Fc-glycoform (G2F)[2] in space group C222[1] (green) and P2[1]2[1]2[1] (blue) are shown. In contrast to Man5 and Gal7, GlcAc6 maintains a contact to the Cg2 domain in both structures. For GlcNAc6 of the structure in space group C222[1] the 2F[o] -F[c] electron density map is shown contoured at 0.8s.

The above figures are reprinted by permission from Elsevier: J Mol Biol (2003, 325, 979-989) copyright 2003.