| Abstrakt: |
Theory pertaining to the binding of a ligand to a series of noninteracting macromolecular acceptors is developed in detail, and methods are described whereby such systems may be usefully identified and characterized. The theory is applied directly to the binding of α-d-methylglucopyranoside to the jack bean phytohemagglutinin, concanavalin A. At pH 5.0, three dimeric types of concanavalin A are possible: homologous dimers of either 2 intact or 2 fragmented monomer units and a hybrid dimer consisting of one monomer of each form. Binding results indicate (a) that a mixture of dimeric types must exist at pH 5.0, some types having one rather than two binding sites per molecule; and (b) that such species must have similar affinity for ligand. A number of models are considered which may explain this binding behavior. It is concluded that hybrid dimers are not present in appreciable amounts. Results at pH 7.0, where the protein exists as a noninteracting mixture of dimer and tetramer species, suggest that dimer and tetramer have significantly different binding affinities. The dimer-tetramer association reaction itself affords some fractionation of intact and fragmented forms of the protein, intact units preferentially associating to form the tetrameric molecule. |
