Stereospecific dihaloalkane binding in a pH-sensitive cavity in cubic insulin crystals

Crystallographic analysis at 2-A resolution of the selective binding 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 isomorphically in the right-handed cis-conformation, displacing four water molecules from the invariant cavity. Lowering the pH to 5.7 in 1 M Na2SO4 without dihaloalkanes induces a cooperative structural transition in which the dyad cavities between B13 glutamate pairs are constricted, and SO4(2-) ions are bound by rearranged triads of B1 NH+3 groups. In the presence of dichloroethane at pH 5-5.5, the equilibrium is shifted to a mixture of the ligand-bound and ligand-excluding cavity structures, with half-occupancy of the sulfate sites, exemplifying how a volatile anesthetic can act as an allosteric effector. Measurements at pH 9 of the occupancies of structurally similar dihaloalkanes demonstrate a high degree of binding selectivity. Induced polarization of the ligand and bound water by the charge distribution in the binding cavity apparently provides the selective electrostatic interactions that discriminate between dihaloalkanes of comparable size and polarity.