Effect of Sodium Ion on the Structural Phase Transition of Hen Egg-White Lysozyme Crystals

Biological Information Research Center, National Institute of Advanced Industrial Science and Technology
â—‹Kazuaki Harata Toshihiko Akiba

Crystal-to-crystal phase transition induced by dehydration is observed when a triclinic or monoclinic lysozyme crystal is slowly desiccated. We have determined crystal structures at atomic resolution before and after the transition and reported that the phase transition is associated with the binding (triclinic crystal) or release (monoclinic crystal) of a sodium ion in the loop region Ser60-Leu75 [1, 2]. A monoclinic crystal soaked in a saturated NaCl solution was hard to transform completely in 24 hours by dehydration in a nitrogen gas stream with 263K. In this crystal, a sodium ion was fully bound to the loop region with a distorted octahedral coordination. On the other hand, a crystal soaked in a saturated KCl solution was easily transformed in an hour. The sodium ion bound to the loop region was replaced with a water molecule in the KCl soaked crystal although the structure of the loop region is still in the sodium bound form. The phase transition is accompanied with the structural transformation of the loop region from the sodium-bond form to the water-bound form. The binding of the sodium ion stabilizes the loop structure and seems to prevent the change to the water-bound form.
[1] K. Harata & T. Akiba. (2004). Acta Cryst., D60, 630.
[2] K. Harata & T. Akiba. (2006). Acta Cryst., D62, 375.