The X-ray structure of a new crystal form of
chymotrypsinogen A grown from
ethanol/water has been determined at 1.8 A resolution using Patterson search techniques. The crystals are of orthorhombic space group P212121 and contain two molecules in the asymmetric unit. Both independent molecules (referred to as A and B) have been crystallographically refined to a final R value of 0.173 with reflection data to 1.8 A resolution. Owing to different crystal contacts, both independent molecules show at various sites conformational differences, especially in segments 33-38, 142-153 and 215-222. If these three loops are omitted in a comparison, the root-mean-square (r.m.s.) deviation of the main-chain atoms of molecules A and B is 0.32 A. If segments 70-79, 143-152 and 215-221 are omitted, a comparison of either molecule A or molecule B with the
chymotrypsinogen model of Freer et al. (1970) reveals an r.m.s. deviation of the alpha-
carbon atoms of about 0.7 A. Compared with the active
enzyme, four spatially adjacent
peptide segments, in particular, are differently organized in the
zymogen: the amino-terminal segment 11-19 runs in a rigid but strained conformation along the molecular surface due to the covalent linkage through Cys1; also segment 184-194 is in a rigid unique conformation due to several mutually stabilizing interactions with the amino-terminal segment; segment 216-222, which also lines the specificity pocket, adapts to different crystal contacts and exists in both
chymotrypsinogen molecules in different, but defined conformations; in particular,
disulfide bridge 191-220, which covalently links both latter segments, has opposite handedness in molecules A and B; finally, the
autolysis loop 142 to 153 is organized in a variety of ways and in its terminal part is completely disordered. Thus, the allosteric activation domain (Huber & Bode, 1978) is organized in defined although different conformations in
chymotrypsinogen molecules A and B, in contrast to
trypsinogen, where all four homologous segments of the activation domain are disordered. This reflects the structural variability and deformability of the activation domain in
serine proteinase proenzymes. If the aforementioned
peptide segments are omitted, a comparison of our
chymotrypsinogen models with
gamma-chymotrypsin (Cohen et al., 1981) yields an r.m.s. deviation for alpha-
carbon atoms of about 0.5 A. The residues of the "active site triad" are arranged similarly, but the oxyanion hole is lacking in
chymotrypsinogen.(ABSTRACT TRUNCATED AT 400 WORDS)