Water-mediated interaction at a protein-protein interface

The water-mediated indirect interaction between barnase and barstar was investigated by surface plasmon resonance measurement and cryogenic X-ray crystallography. Mutations of four acidic residues of barstar, D35A, D39A, E76A and E80A, decreased the binding free energies by 17.2, 25.2, 3.8 and 2.1 kJ mol(-1), respectively, in the presence of 150 mM NaCl at pH 7.4 and 25 degreesC. The changes of the hydrated structures of the complexes caused by the mutations were localized around the mutational site, suggesting that difference in the binding free energy is closely correlated with difference in the local hydrated structure. Then, the averaged binding free energy was estimated at 4.4 kJ mol(-1) per water-mediated indirect interaction on the basis of the difference in hydrogen bonding network. (C) 2004 Elsevier B.V. All rights reserved.