Demonstration of high and low affinity complexes between protein A and rabbit immunoglobulin C antibodies depending on hapten density

Abstract

Protein A of Staphylococcus , aureus forms high (K D = 3×10 −11M) and low (K D = 1×10 −9M) affinity complexes with rabbit immunoglobulin G antibodies that are involved in immune binding to the immobilized monovalent hapten methotrexate. Low affinity complexes are formed regardless of hapten density whereas high affinity complexes are formed only when there is a relatively high density of ligand. The high affinity complexes constitute 7–12% of the total and involve two antibody molecules separated by 40–45Å and cross-linked by a single divalent protein A molecule. If more than one protein A can bind to each antibody molecule in the low affinity complexes, then binding at one site does not affect binding at the other.