published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

Thesis (M. Med. Sc.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 73-90).

Thesis (M.Med.Sc.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 73-90). Also available in print.

This thesis is concerned with structural aspects of the recognition and effector functions of antibody molecules. The recognition process is investigated in the dinitrophenyl (DNP) binding mouse IgA produced by the myeloma MOPC 315. The studies on effector functions utilize a DNP binding mouse hybridoma IgG2a to examine the role of N-glycosylation in IgG. The combining site of protein 315. The involvement of tyrosyl residues in the combining site of protein 315 was examined by preparing specifically nitrated NO₂-Tyr-33_H and NO₂-Tyr-34_L derivatives of the Fv fragment of this protein. The ionizations of tnese derivatives were studied in the presence and absence of various DNP-ligands. Perturbations to the nitrotyrosine ionizations were found to be caused by the side chains of certain of these ligands, allowing an indication of the distance of these tyrosines from the bound hapten. On examination of the compatibility of these data with the model of the combining site of protein 315 proposed by Dower <en>et al. (1977) (Biochem. J. 165, 207-225) it was found that while the location of Tyr-33_H is consistent with this model, the position of Tyr-34_L is not. A remodelled combining site using the modified ring-current treatment of Perkins and Dwek (1980) (Biochemistry 19, 245-258) is presented. This allows a better rationalization of the nitration data and of previous experimental observations on protein 315. The role of the conserved C 2 domain oligosaccharide of IgG. This was examined by a functional comparison of native IgG with an aglycosylated IgG preparation. Aglycosylation was acheived by cell culture of the hybridoma cells in the presence of the glycosylation inhibitor tunicamycin. The conditions for preparation and purification of this aglycosyl IgG are described. Aglycosylated IgG is found to be correctly assembled as an H₂L₂ unit. It retains the antigen binding and Staphylococcal protein A binding abilities of the native glycosylated molecule. Using an assay system designed specifically to overcome certain problems in comparing Clq binding to different preparations of IgG it was found that the aglycosylated preparation showed only slightly reduced affinity for Clq. In addition the aglycosylated IgG is able to activate bound Cl. The above results are consistent with the structure of the Fc region being only minimally altered in the absence of oligosaccharide. The structural integrity of the aglycosylated molecule may be compromised however, as its ability to bind to monocyte Fc receptor is significantly reduced. In addition the aglycosylated molecule becomes much more susceptible to proteolytic digestion. A computational model-building analysis of the quaternary structure of Fc allows an explanation of at least some of the effects of aglycosylation in terms of reduced conformational stability of the C_H2 domains.