STRUCTURE AND SELECTIVITY OF A MONOVALENT CATION-BINDING SITE IN CUBIC INSULIN CRYSTALS

Cubic insulin crystals contain a binding site for monovalent cations i n a cavity on the crystal dyad in which the bound cation is ligated by protein atomic dipoles and water molecules. These types of interactio n are analogous to interactions that occur in small cation-selective c arrier and channel molecules. X-ray diffraction data collected from cu bic insulin crystals containing Li+, Na+, K+, NH4+, Rb+, and Tl+ show that (i) the differences in cation size do not cause any large alterat ion in the protein structure around the cation, and (ii) the bound cat ion is co-ordinated by one or two water molecules, depending on its io nic radii. The relative binding affinities for cations at this dyad si te were obtained from an x-ray diffraction analysis of competition exp eriments in which crystals were dialyzed in mixtures of Tl+ with Li+, Na+, K+, NH4+, Rb+, or Cs+. These data show that this site provides ve ry little discrimination between Na+, K+, Rb+, and Tl+, some selectivi ty against the small Li+ and the tetrahedrally shaped NH4+, and strong er selectivity against the larger Cs+. The capacity of this site to bi nd monovalent cations of different sizes may be accounted for by the s mall number of protein ligating groups and a change from two ligating waters with Li+ and Na+ to one ligating water with the larger cations.