engleski

Zn(II) coordination in human and bovine bromo- and iodo-insulin hexamers

The insulin hexamer is an allosteric protein that exists in three conformational states: T6, T3R3 f and R6 [1]. Transitions between conformational states are mediated by the binding of phenolic compounds or by the coordination of anions to the bound metal ions [2-5]. It was found that T3R3f hexamer in the chloro-derivative can accommodate different number of Zn2+ ions per hexamer with different coordination of the zinc ion [4, 5]. In the present study four new insulin derivatives (two human and two bovine) crystallized in the presence of bromide or iodide ions and were structurally characterized. Single crystal diffraction data at 100 K were collected to high resolution at synchrotrons ELETTRA and ESRF. In the bromo-derivatives four different coordinations of the Zn2+ ion were found: i) tetrahedral with two histidines and two bromide ions, ii) disordered site – tetrahedral with three histidines and one bromide ion and – octahedral with three histidines and three water molecules and ; iii) tetrahedral tetraaquazinc(II) ion which was not found in the published chloro-derivatives and is rarely found in small molecule crystal structures. All bromo-derivatives are of the T3R3f type. In the iodo-derivative only the ii) type of the disordered octahedral-tetrahedral site is found. The human bromo-derivative is a superstructure with a doubled c-axis. This derivative is also T3R3 f but is of the 2Zn type with Zn2+ ions only on the three-fold axis. [1] Kaarsholm, N. C. et al. (1989). Biochemistry 28, 4427. [2] Whittingham J. L. et al. (1995) Biochemistry 34, 15553-15563. [3] Smith G. D. et al. (1996). Protein Science 5, 1502-1511. [4] Smith G. D. et al. (1984). Proc. Natl. Acad. Sci. 81, 7093-7097. [5] Ciszak, E. & Smith G .D. (1994).Biochemistry, 33, 1512- 1517.

insulin; bromine; iodine

doi: 10.1107/S018767312095165