WATER-DEPENDENT DOMAIN MOTION AND FLEXIBILITY IN RIBONUCLEASE-A AND THE INVARIANT FEATURES IN ITS HYDRATION SHELL - AN X-RAY STUDY OF 2 LOW-HUMIDITY CRYSTAL FORMS OF THE ENZYME

The crystal structures of 88 and 79% relative humidity forms of ribonu clease A, resulting from water-mediated transformations, have been ref ined employing the restrained least-squares method using X-ray data co llected on an area detector to R = 0.173 for 15 326 observed reflectio ns in the 10-1.5 Angstrom resolution shell and R = 0.176 for 8534 obse rved reflections in the 10-1.8 Angstrom shell, respectively. The compa rison of these structures with those of the native, the phosphate-boun d and the sulfate-bound forms demonstrates that the mobility of the ri bonuclease A molecule involves hinge-bending movement of the two domai ns and local flexibility within them, particularly at the termini of r egular secondary structures and in loops. The comparison also leads to the identification of 31 invariant water molecules in the hydration s hell of the enzyme, many of which are involved in holding different pa rts of the molecule together and in stabilizing local structure. The c onformational changes that accompany the partial removal of the surrou nding water, particularly those observed in the 79% form, could be sim ilar to those that occur during enzyme action.