Phosphorylcholine, antibody binding

Haptens, a special class of antigen, are small molecules that induce specific antibody production when they are attached to a protein that acts as a carrier. Phosphorylcholine is one such hapten that has been widely used in the investigation of immune responses. The specific binding of this hapten... 

Another example that I now recall of poor fit between shape of a binding site and that of ligand is the immunoglobulin, or Bence-Jones protein, that binds the hapten phosphorylcholine. The X-ray structure of the antibody has been worked out and it is evident that the binding area is much larger than that of the hapten, and hardly complementary in shape. 

F. S. Lee, Z.-T. Chu, M. B. Bolger, and A. Warshel, Protein Eng., 5, 215 (1992). Calculations of Antibody—Antigen Interactions Microscopic and Semimicroscopic Evaluation of the Free Energies of Binding of Phosphorylcholine Analogs to McP603. 

The binding of phosphorylcholine does not significantly change the structure of the antibody, yet induced fit plays a role in the formation of many antibody-antigen complexes. A malleable binding site can accommodate many more kinds of ligands than can a rigid one. Thus, induced fit increases the repertoire of antibody specificities. 

Figure 33.11. Binding of a Small Antigen. The structure of a complex between an Fj j, fragment of an antibody and its target in this case, phosphoryl-choline. Residues from the antibody interact with phosphorylcholine through...

Lee, P.S., Chu, Z-T., Bolger, M.B., and Warshel, A. (1992) Calculations of antibody-antigen interactions microscopic and semi-microscopic evaluation of the free energies of binding of phosphorylcholine analogs to McPC603, Prot. Eng. 5, 215-228. 

Figure 8.11c. Binding of phosphorylcholine to an antibody-combining site. Adapted from E.A. Padlan et al.PO ].

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