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

Abstract

An improved 2.5-.ANG. electron density map of [bovine] 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 O.gamma. atom is in position to form a normal H bond with N.epsilon.2 of His-57. The corresponding H bond in .alpha.-chymotrypsin is severely distorted, probably as a consequence of a 1.5-.ANG. shift in the relative positions of the 2 cylindrical folding domains composing most of the molecule. This activation induced distortion of the charge-relay, and H-bonding system probably plays an important role in the genesis of enzymic activity, in accord with an earlier proposal by Wang concerning the role of bent H bonds in enzyme catalysis.