Plasticity, hydration and accessibility in ribonuclease A. The structure of a new crystal form and its low-humidity variant

The structures of a new crystal form of ribonuclease A and its low-humidity variant, each containing two crystallographically independent molecules, h ave been determined and refined. A detailed comparison of these structures with those of the other known crystal forms of the enzyme, which have diffe rent packing arrangements and solvent composition, leads to a meaningful de lineation of the rigid and flexible regions of the protein molecule and the nature of its plasticity. Many of the water molecules which are common to all the structures are involved in bridging different regions of the protei n molecule, thus emphasizing the role of water in stabilizing the tertiary structure. The analysis of the structures shows that for a given N or O ato m, the level of hydration increases with accessible surface area, but level s off at an area of about 10 Angstrom(2). Generally, the hydration level te nds to drop when the area increases beyond about 20 Angstrom(2). This drop correlates with an increase in the displacement parameter. The analysis als o suggests that the van der Waals radii and probe radius normally used in a ccessible surface area calculations are not appropriate for dealing with al l situations.