Ultrafast surface hydration dynamics and expression of protein functionality: alpha -Chymotrypsin.

We report studies of hydration dynamics at the surface of the enzyme protein bovine pancreatic alpha-chymotrypsin. The probe is the well known 1-anilinonaphthalene-8-sulfonate, which binds selectively in the native state of the protein, not the molten globule, as shown by x-ray crystallography. With femtosecond time resolution, we examined the hydration dynamics at two pHs, when the protein is physiologically in the inactive state (pH 3.6) or the active state (pH 6.7); the global structure and the binding site remain the same. The hydration correlation function, C(t), whose decay is governed by the rotational and translational motions of water molecules at the site, shows the behavior observed in this laboratory for other proteins, Subtilisin Carlsberg and Monellin, using the intrinsic amino acid tryptophan as a probe for surface hydration. However, the time scales and amplitudes vary drastically at the two pHs. For the inactive protein state, C(t) decays with an ultrafast component, close to bulk-type behavior, but 50% of the C(t) decays at a much slower rate, tau = 43 ps. In contrast, for the active state, the ultrafast component becomes dominant (90%) and the slow component changes to a faster decay, tau = 28 ps. These results indicate that in the active state water molecules in the hydration layer around the site have a high degree of mobility, whereas in the inactive state the water is more rigidly structured. For the substrate-enzyme complex, the function and dynamics at the probe site are correlated, and the relevance to the enzymatic action is clear.