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Mechanism of hydrogen-deuterium exchange in trp repressor studied by 1H-15N NMR.

Amide proton exchange rates have been measured for fast-exchanging amides in trp aporepressor, and compared with the rates measured in the holorepressor. The results indicate that the presence of the ligand stabilizes all of the amide protons in the molecule against exchange, not just those whose access to solvent it directly hinders. This global hindering of the exchange process by tryptophan implies that there is a non-random element in the transmission mechanism, so that damping of the exchange in one part of the molecule also damps exchange in another region. This damping at a distance is not associated with any measurable changes in the intervening average secondary structure. This suggests the existence of a concerted dynamic process in the protein backbone that is modulated by ligand binding and in turn affects the observed backbone proton exchange.