PACKING AT THE PROTEIN-WATER INTERFACE

We have determined the packing efficiency at the protein-water interfa ce by calculating the volumes of atoms on the protein surface and near by water molecules in 22 crystal structures. We find that an atom on t he protein surface occupies, on average, a volume approximate to 7% la rger than an atom of equivalent chemical type in the protein core. In these calculations, larger volumes result from voids between atoms and thus imply a looser or less efficient packing. We further find that t he volumes of individual atoms are not related to their chemical type but rather to their structural location. More exposed atoms have large r volumes. Moreover, the packing around atoms in locally concave, groo ved regions of protein surfaces is looser than that around atoms in lo cally convex, ridge regions. This as a direct manifestation of surface curvature-dependent hydration. The net volume increase for atoms on t he protein surface is compensated by volume decreases in water molecul es near the surface. These waters occupy volumes smaller than those in the bulk solvent by up to 20%; the precise amount of this decrease is directly related to the extent of contact with the protein.