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  • Title: Intramolecular signaling upon complexation.
    Author: Guddat LW, Shan L, Fan ZC, Andersen KN, Rosauer R, Linthicum DS, Edmundson AB.
    Journal: FASEB J; 1995 Jan; 9(1):101-6. PubMed ID: 7821748.
    Abstract:
    Crystal habits can be used as indicators of conformational changes in their constituent proteins. As in the conversion of unliganded hemoglobin to the oxygenated form, the addition of a small hapten to a suspension of platy crystals of an unliganded Fab (NC6.8) results in the immediate disintegration of the plates and their replacement with prisms of the ligand-protein complex. Examination of the native and liganded forms by X-ray crystallography reveals that the space groups and protein structures are different. During complexation there are ligand-induced conformational changes both in the antigen combining site (local alterations) and in more distal portions of the molecule (allosteric changes). There is an extension of the light chain (10 A increase in length), a commensurate shortening of the heavy chain (by flexing), and a decrease in the "elbow bend" angle of 31 degrees (184 degrees to 153 degrees). Relative to the variable domains, the constant domain pair moves mainly as a unit in such a way that the carboxyl end of the heavy chain is displaced by 19 A. In an intact antibody this displacement may be relayed as a tug (by tensile forces) on the segment connecting the Fab to the Fc region, perhaps altering the orientations of the constituents responsible for such effector functions as complement activation.
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