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Using a combination of X-ray crystallography and solution kinetics studies, a team of biological chemists has documented that a single antibody can take on two quite different conformations, enabling it to bind two unrelated antigens. These two conformations exist in solution, which may shed light on long-standing questions of how antibodies can bind selectively to such a vast array of potential antigens. The antibody they studied, a monoclonal immunoglobulin, ordinarily binds to the small molecule 2,4-dinitrophenol. As shown in the pink structure, this binding occurs at a small, deep hole in the antibody. But the antibody can also bind protein antigens, as shown in the lower structure, where the protein (green) binds across a broad and shallow binding site on the antibody's surface.
Researchers have known that bound antibodies sometimes have a different conformation than in solution. One explanation is that the binding itself induces structural changes. Another possibility is that structural isomers of an antibody exist in solution and can bind to different antigens. Binding then stabilizes that particular conformation. The team found that both effects occur for its antibody: It circulates in two distinct forms, each of which further changes shape as it binds
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