(196g) Nucleation and Crystal Growth of Insulin as a Fundamental Mechanism of Regulation in Mammalian Organisms

Rhombohedral crystals of Zn-insulin hexamers form in the islets of Langerhans in mammalian pancreases as a means to protect the insulin and to increase the degree of conversion from soluble proinsulin. To accomplish this, crystal growth should be fast and adaptable to rate fluctuations in the conversion reaction. Complete filling of the volume of the islet cells is ensured by nucleating a single insulin crystal in each of them, likely by assisted nucleation on a membrane site while maintaining extremely low homogenous nucleation rate. We present recent results on the mechanisms of growth of insulin crystals that shed light on how these three objectives may be accomplished by the insulin crystallization system. The mechanism found for insulin appears to be novel and unencountered in previously studied crystallization systems.

Zn-insulin crystals have been shown to grow by the spreading of layers so that the growth sites, the kinks are located at the edges of unfinished layers, the steps. Kinks are known to be generated either by thermal fluctuations or one-dimensional nucleation of new crystalline rows. The kink density determines the rate at which steps advance, and these two kink-generation mechanisms lead to weak near-linear responses of the growth rate to concentration variations. We demonstrate for the crystallization of Zn-insulin a novel mechanism of kink generation, whereby 2D clusters of several insulin molecules appear at the steps either by assisted 2D nucleation or association after pre-formation on the terraces between steps. This leads to several-fold faster rate of crystallization and its high sensitivity to small increases of the Zn-insulin concentration (1).

(1)Georgiou, D. K.; Vekilov, P. G. Proc. Natl. Acad. Sci. USA 2006, 103, 1681