STRUCTURAL FEATURES OF THE REACTIONS - BETWEEN ANTIBODIES AND PROTEINANTIGENS

Antibodies bind protein antigens over large sterically and electrostat ically complementary surfaces. Van der Waals forces, hydrogen bonds, a nd occasionally ion pairs provide stability to antibody-antigen comple xes. In addition, water molecules contribute hydrogen bonds linking an tigen and antibody, and increase the complementarity of antigen-antibo dy interfaces. In qualification to a strict 'lock and key' mechanism, evidence of conformational changes between free and complexed antibodi es indicate some accommodation to the antigen. Antibody-protein antige n reactions are enthalpically driven with varying degrees of entropic compensation, often dependent on the magnitude of the enthalpy of the reaction. In the case of two antibody-combining sites studied by X-ray diffraction, the relative arrangements of the variable domains of the light and heavy chains of the antibody change slightly from the free to the antigen-bound state. Furthermore, the contacting residues of bo th antibodies exhibit similar reduced mobilities when complexed to ant igen, suggesting that differences in 'solvent entropy' rather than in conformational freedom may be the source of different entropic compens ation factors. In concert, data from structural studies, reaction rate s, calorimetric measurements, molecular dynamics simulations, and site -directed mutagenesis are beginning to detail the nature of antibody-p rotein antigen interactions.