Structure and selectivity of a monovalent cation binding site in cubic insulin crystals

Cubic insulin crystals contain a binding site for monovalent cations in a cavity of the crystal dyad in which the bound cation is ligated by protein atomic dipoles and water molecules. These types of interaction are analogous to interactions that occur in small cation-selective carrier and channel molecules. X-ray diffraction data collected from cubic insulin crystals containing Li+, Na+, K+, NH4+, Rb+, and Tl+ show that (i) the differences in cation size do not cause any large alteration in the protein structure around the cation, and (ii) the bound cation is co-ordinated by one or two water molecules, depending on its ionic radii. The relative binding affinities for cations at this dyad site were obtained from an x-ray diffraction analysis of competition experiments in which crystals were dialyzed in mixtures of Tl+ with Li+, Na+, NH4+, Rb+, or Cs+. These data show that this site provides very little discrimination between Na+, K+, Rb+, and Tl+, some selectivity against the small Li+ and the tetrahedrally shaped NH4+, and stronger selectivity against the larger Cs+. The capacity of this site to bind monovalent cations of different sizes may be accounted for by the small number of protein ligating groups and a change from two ligating waters with Li+ and Na+ to one ligating water with the larger cations.