STEREOSPECIFIC DIHALOALKANE BINDING IN A PH-SENSITIVE CAVITY IN CUBICINSULIN CRYSTALS

Crystallographic analysis at 2-Angstrom resolution of the selective bi nding of dihalogenated methane, ethane, and ethylene compounds in the cavity on the cubic insulin dimer axis provides a model for anesthetic -protein interactions. At pH 6-11, 1,2-dichloroethane binds isomorphic ally in the right-handed cis-conformation, displacing four water molec ules from the invariant cavity. Lowering the pH to 5.7 in 1 M Na2SO4 w ithout dihaloalkanes induces a cooperative structural transition in wh ich the dyad cavities between B13 glutamate pairs are constricted, and SO42- ions are bound by rearranged triads of B1 NH3+ groups. In the p resence of dichloroethane at pH 5-5.5, the equilibrium is shifted to a mixture of the ligand-bound and ligand-excluding cavity structures, w ith half-occupancy of the sulfate sites, exemplifying how a volatile a nesthetic can act as an allosteric effector. Measurements at pH 9 of t he occupancies of structurally similar dihaloalkanes demonstrate a hig h degree of binding selectivity. Induced polarization of the ligand an d bound water by the charge distribution in the binding cavity apparen tly provides the selective electrostatic interactions that discriminat e between dihaloalkanes of comparable size and polarity.