title: The binding and activation of the Clr-Cls subunit of the first component of human complement.
abstract: The value of the functional affinity constant between 125I-labelled Clq and the Clr-Cls tetramer (when free in solution) in the formation of Cl was found to be 3.6 X 10(7) M-1. When Clq was bound to activating immune complexes, the value of K was about 10-fold higher before initiation of activation and there was a further two to three-fold rise as activation proceeded. The addition of an excess of unlabelled Clq increased the rate of activation of 125I-labelled Cl, suggesting an interaction between Clr-Cls and two neighbouring Clq molecules. It is suggested that the tetramer Clr-Cls may bind bivalently to Clq when free in solution, but on binding to activating complexes, one of the Clr-Cls binding sites is detached from Clq and becomes bound to a site on the complex. The resultant spatial rearrangement of the Clr molecules within the tetramer may be optimal for autocatalytic activation of Clr.
mesh_terms: D000936:Antigen-Antibody Complex; D003166:Complement Activating Enzymes / Q000276:immunology; D003167:Complement Activation; D003172:Complement C1 / Q000276:immunology*; D015922:Complement C1q; D015923:Complement C1r; D003173:Complement C1s; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D008970:Molecular Weight; D010047:Ovalbumin / Q000276:immunology; D011485:Protein Binding
chemical_list: D000936:Antigen-Antibody Complex; D003172:Complement C1; D015922:Complement C1q; D010047:Ovalbumin; D003166:Complement Activating Enzymes; D015923:Complement C1r; D003173:Complement C1s