Refined Crystal Structure of Carboxypeptidase A at 1.54Å Resolution

The crystal structure of bovine carboxypeptidase A (Cox) has been refined at 1•54 Å resolution using the restrained least-squares algorithm of Hendrickson & Konnert (1981). The crystallographic R factor (R = Σ||F_o| − |F_c||/Σ|F_o|) for structure factors calculated from the final model is 0•190. Bond lengths and bond angles in the carboxypeptidase A model have root-mean-square deviations from ideal values of 0•025 Å and 3•6°, respectively. Four examples of a reverse turn like structure (the “Asx” turn) requiring an aspartic acid or asparagine residue are observed in this structure. The Asx turn has the same number of atoms as a reverse turn, but only one peptide bond, and the hydrogen bond that closes the turn is between the Asx side-chain CO group and a main-chain NH group. The distributions of CO-N and NH-O hydrogen bond angles in the α-helices and β-sheet structures of carboxypeptidase A are centered about 156°. A total of 192 water molecules per molecule of enzyme are included in the final model. Unlike the hydrogen bonding geometry observed in the secondary structure of the enzyme, the CO-O(wat) hydrogen bond angle is distributed about 131°, indicating the role of the lone pair electrons of the carbonyl oxygen in the hydrogen bond interaction. Twenty four solvent molecules are observed buried within the protein. Several of these waters are organized into hydrogen-bonded chains containing up to five waters. The average temperature factor for atoms in carboxypeptidase A is 8 Å^2, and varies from 5 Å^2 in the center of the protein, to over 30 Å^2 at the surface