The refined crystal structure of ribonuclease A at 2.0 A resolution.

This paper describes the structure of bovine pancreatic ribonuclease A, refined by a restrained parameter least squares procedure at 2.0 A resolution, and rebuilt using computer graphics. The final agreement factor (formula see text) is 0.159. The positions of the 951 main chain atoms have been determined with an estimated accuracy of 0.17 A. In addition, the model includes a phosphate group in the active site and 176 waters, many of them with partial occupancy. The bond lengths in the refined structure of RNase A differ from the ideal values by an overall root mean square deviation of 0.022 A; the corresponding value for angle distances is 0.06 A. The root mean square deviation of planar atoms from ideality is 0.017 A, and root mean square deviation of the peptide torsion angles from 180 degrees is 3.4 degrees. The model is in good agreement with the final difference Fourier maps. Two active site histidines, His 12 and His 119, form hydrogen bonds to the phosphate ion. His 119 is also hydrogen bonded to the carboxyl of ASp 121 and His 12 to the carbonyl of Thr 45. The structure of the RNase A is very similar to that of RNase S, particularly in the active site region. The root mean square discrepancy of all atoms from residues 1 to 16 and 24 to 123 is 1.06 A and the root mean square discrepancy for the active site region is 0.6 A.