Molecular structures of Fabs and Fabhapten complexes

The determination of the first three-dimensional structures of Fab complexes with the haptens phos-phorylcholine and vitamin K,OH, confirmed that the CDRs of both VH and VL participate in binding. The contacts of the ligands with the combining site included salt bridges, hydrogen bonds and van der Waals interactions. No conformational changes in the combining site or in other regions of the antibody could be seen by comparing free with liganded Fabs. Thus, an allosteric mechanism by which conformational signals would be transmitted from the combining site to the Fc region was not supported by these structure determinations. Such mechanisms were postulated as a way of activating secondary (effector) functions of antibodies. However, since haptens represent only a fraction of the antigenic determinants recognized by antibodies, the complete antigen-binding site could not be defined by that approach. Moreover, it was thought possible that conformational changes would be observed only if a complex with a macromolecular antigen, contacting a wider area at the combining site, could be studied.

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