Crystallization and storage of insulin requires ?

Ans. is `b' i.e., Zn++ Zinc ions are essential for the formation of hexameric insulin and hormone crystallization. Following the synthesis of proinsulin, zinc promotes the formation of proinsulin hexamers and increases its solubility before conversion to insulin. Proinsulin binds 30 zinc ions, of which 2 to 4 are coordinated within the molecule. These zinc ions appear to be impoant for the solubility of proinsulin hexamers. With the removal of C-peptide from each proinsulin monomers, the resulting insulin increases its coordination of zinc to up to six ions per hexamer, which decreases its solubility and increases its crystallization within the secretory vesicle. The insulin, stored as crystalline hexamer, is resistant from proteolytic attack within the vesicle. Upon release of insulin into the bloodsteam, zinc is also released and the insulin becomes soluble in the blood.