Co-operation between the pair of C gamma 2 domains in Clq-binding by rabbit IgG.
The single site binding constants of rabbit IgG and its plasmin-derived fragments F(acb)2, Facb and F(ab)2 for human C1q were measured by the sedimentation velocity method. The intact IgG and F(acb)2 having the paired C gamma 2 domains gave an identical association constant at 20 degrees C (Ka) of 3.02 X 10(4) M-1 in the presence of a physiological concn of salt and on the basis of six sites per C1q. The C1q-binding affinity was found to be decreased to 1.04 X 10(4) M-1 in the reduced, monomerized fragment Facb. Under the same conditions F(ab)2, which is completely unable to activate the classical complement cascade, gave an apparent C1q-affinity of 0.36 X 10(4) M-1. The results, together with previous observations, led us to the conclusion that the C1q-binding site of rabbit IgG is constituted associatively by the pair of C gamma 2 domains, each of which providing a limited, complementary part of the binding free energy between IgG and C1q.
D000915:Antibody Affinity; D001666:Binding Sites, Antibody; D003166:Complement Activating Enzymes / Q000276:immunology*; D015922:Complement C1q; D004591:Electrophoresis, Polyacrylamide Gel; D007140:Immunoglobulin Fab Fragments; D007141:Immunoglobulin Fc Fragments / Q000276:immunology; D007074:Immunoglobulin G / Q000276:immunology*; D014461:Ultracentrifugation
D007140:Immunoglobulin Fab Fragments; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; C041323:immunoglobulin Facb fragment; D015922:Complement C1q; D003166:Complement Activating Enzymes