Order and disorder in water structure of crystalline proteins.

Crystals of the hydrophobic protein, crambin (MW 4700), diffract to 0.83 A resolution at 130K. At this level of detail, nearly all the solvent molecules are ordered. Thus, this protein provides an excellent opportunity to study the order and disorder of water molecules at the protein surface. Water is important in stabilizing the folded conformation of the protein and also is necessary for enzymes to be active. In crambin, there are two types of water networks: pentagonal rings associated with the hydrophobic surface chain and chains linking the polar residues. The chain-like arrays appear to be strongly influenced by the protein surface. Study of these networks may enable us to predict the solvent shell in other proteins. The precise influence of the protein atoms on the solvent structure can be deduced by superimposing identical side chain functional groups and comparing the positions of atoms hydrogen-bonded to these reference atoms. Preliminary results with crambin indicate that there is different ordering of the solvent water molecules depending on whether the hydrogen-bonding protein molecule is greater than NH or greater than C = 0. Around -OH groups, the geometry of hydrogen-bonding is even more diverse. Two disordered water oxygen networks are located in each of the four major solvent regions of the crystal. Each alternate network may represent the necessity of water to pack against an irregular surface and still maximize hydrogen bonding. In the context of nature's balance between making strong bonds and maximizing disorder, the presence of disorder in the solvent structure of crambin is not unexpected. Free energy is the sum of bond energy (enthalpy) and disorder (entropy). However, considerable insight is gained from observing where disorder is and is not found in crystals of crambin.