The role of antigenic conformation in antigen-antibody complex formation

Abstract

Iron binding imparts a conformational rigidity and compactness to the ovotransferrin molecule. This appears to cause the inaccessibility of certain antigenic sites to antibody in the formation of antigen-antibody complexes. Iron ovotranseferrin had a lower apparent antigenic valence and formed preferably smaller soluble antigen-antibody complexes that ovotransferrin, as measured by fluorescence polarization, saturated ammonium sulfate precipitation, and sedimentation velocity ultracentrifugation experiments. In primary Ab binding tests and in quantitative precipitin tests, however, both antigens gave similar reactivity. These results and the known identity of the primary chemical structure of ovotransferrin and iron ovotransferrin indicated that conformation differences are responsible for the observed immunochemical behavior. Molecular models, based on hydrodynamic measurements on ovotransferrin, iron ovotransferrin, and antibody molecules, also support this view. The antigenic differences between ovotransferrin and iron ovotransferrin were manifested only in the secondary phase of the antigen-antibody complex formation, emphasizing the need for appropriate secondary phase immunochemical techniques in such studies.