The association and dissociation of small ligands regulate the functions of proteins through structural changes in the protein. Such structural changes propagate long distances, and this allostery plays a key role in molecular functions. However, the mechanism by which structural changes are transmitted is poorly understood. Here we show that nonbonded atomic contacts play an essential role in driving the displacement of a helix in picosecond time scale primary structural changes following the dissociation of carbon monoxide from the heme group in myoglobin. The present time-resolved ultraviolet resonance Raman study revealed that the amplitude of this helix displacement was reduced upon substitution of Val68, which contacts the heme in wild-type myoglobin, with a less bulky side chain (Ala). Our findings provided the first direct evidence that structural changes are transmitted not only by covalent bonds, salt bridges and hydrogen bonds but also by nonbonded atomic contacts in the primary protein response upon ligand dissociation. Furthermore, the present results indicate the importance of dense atomic packing in a protein structure for responding to the association and dissociation of small molecules. The high compactness of protein structures makes possible the propagation of structural changes, providing useful clues to the design of molecular machines.
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