A detailed structural comparison between the charge relay system in chymotrypsinogen and in alpha-chymotrypsin

An improved 2.5-A electron density map of chymotrypsinogen was calculated by incorporating heavy-atom anomalous scattering effects and a new model of the molecule was constructed. Phases from x-ray structure factors (R = 0.43) computed from this model were then used in the calculation of another electron density map against which the model was further refined. The catalytic Ser-195 side chain in the new model is in the "down" or "acyl" orientation and its Ogamma atom is in position to form a normal hydrogen bond with Nepsilon2 of His-57. In contrast, the corresponding hydrogen bond in alpha-chymotrypsin (Birktoft, J.J., and Blow, D.M. (1972), J.Mol. Biol. 68, 187) is severely distorted, probably as a consequence of a 1.5-A shift in the relative positions of the two cylindrical folding domains composing most of the molecule. We suggest that this activiation induced distortion of the charge-relay, hydrogen-bonding system plays an important role in the genesis of enzymic activity, in accord with an earlier proposal by Wang concerning the role of bent hydrogen bonds in enzyme catalysis (Wang, J.J. (1970), Proc. Natl. Acad. Sci. U.S.A. 66, 874).