Structure of ribonuclease A: results of joint neutron and X-ray refinement at 2.0-A resolution.

The structure of ribonuclease A has been refined jointly with the neutron and X-ray data extending to 2.0 A. The results of an earlier X-ray refinement provided the starting model [Wlodawer, A., Bott, R., & Sjölin, L. (1982) J. Biol. Chem. 257, 1325-1332]. The final R factors were 0.159 (X-ray) and 0.183 (neutron) for a model containing all of the atoms expected in the protein, 128 waters, and a phosphate molecule in the active site. The joint refinement necessitated modifications in the orientation of a number of side chains, including the catalytically active lysine-41, which is now thought to form a salt link to the phosphate. Major modifications of the previous model of the bound solvent were necessary. The refinement of all atom occupancies with the neutron data only provided the information about the amide hydrogen exchange. A fourth of all amide hydrogens were found to be at least partially protected from exchange after a year of exchange with D2O.