The Three-Dimensional Structure of Ribonuclease-S

Abstract The procedures used in the collection and analysis of x-ray diffraction data from single crystals of ribonuclease-S are described. Both isomorphous and anomalous scattering were measured on heavy atom derivatives prepared with the uranyl cation, the tetracyanoplatinate(II) anion, and dichloroethylenediamine platinum(II). The amplitudes and phases of 6000 reflections were established. An electron density map with an estimated root mean square error of 0.23 electron per (A)3 and a nominal resolution of 2 A was produced. An interpretation of this map on the basis of the known chemical sequence has been made. The actual fit of the physical model to the map is shown in a series of stereophotographs of superimposed images. A preliminary list of the coordinates of all nonhydrogen atoms in this enzyme has been prepared. The level of confidence in the proposed structure varies markedly in different parts of the protein. Comments on the quality of the electron density map at individual residues and on the fit of the model in the different regions are given.