Strange bedfellows: interactions between acidic side-chains in proteins.

The oxygen atoms of two acidic side-chains are frequently found within hydrogen-bonding distance of each other in proteins. Two distinct types of cases are common. In metal-binding sites, the oxygen atoms are brought near (average closest approach 3.0 A) by their common role as metal ligands. In a different location, either buried or on the protein surface, the two acidic groups can share a proton. The corresponding O-O distances in the latter case are shorter (usually 2.7 or less), and the geometry is typical of hydrogen-bonding interactions. The glucose/galactose-binding protein of Salmonella typhimurium provides an example of a well-ordered Asp-Glu pair on the surface of a protein with a very short O-O distance, at a pH of 7.0. Other instances have been found at pH values as high as 8.0, suggesting substantial alteration of the pKa involved. These observations have implications for the study of enzymes that use pairs of acidic residues in binding and catalysts.