Ribonuclease-A: least-squares refinement of the structure at 1.45 Å resolution

The crystal structure of bovine pancreatic ribonuclease-A has been refined by restrained least-squares analysis employing X-ray diffractometer data to 1.45/k resolution. The current R factor for 19 238 reflections is 0.26 and 0.24 for 17 427 reflections with I(hkl) > 0. The r.m.s, deviation from ideality of bond lengths is 0.01A. Corrections, mostly of minor character, to previous models of the secondary structure have been made and a quantitative analysis of intramolecular hydrogen bonds is given. A total of 79 solvent molecules have been clearly identified in the first coordination sphere around the enzyme molecule and included in the least-squares analysis. A sulphate anion occurs in the active site and has also been refined as part of the structure. Further new features of the structure to emerge are: alternative sites for the His-119 side group with occupancies, refined in the analysis, of 0.80 and 0.20 respectively; a solvent molecule hydrogen bonded to the N-terminal amino group; and extensive disorder of the side chains in the region of residues 35-39. The r.m.s, deviation in atomic position between the current model and the starting model is 1-1 ./~ including some shifts of 7-8/k where major rebuilding of side groups was necessary.