Phenol stabilizes more helix in a new symmetrical zinc insulin hexamer

SINCE insulin was first shown by Scott to crystallize in the presence of zinc ions in 1934, a variety of Zn-containing insulin crystals have been grown. The structures of insulin in the related rhombohedral crystals of 2Zn-insulin and 4Zn-insulin have been solved and reveal that the molecule is a hexamer, organized as three dimers, each containing a 2-fold symmetry axis and held together by Zn ions. In 2Zn-insulin the hexamer is nearly symmetrical with the two axial Zn ions and the two molecules of the dimer related closely by a local 2-fold axis. But in 4Zn-insulin the two molecules in the dimer differ remarkably, creating an asymmetric 4Zn-hexamer in which one trimer is essentially equivalent to that in 2Zn-insulin and the other is different by virtue of an additional stretch of N-terminal helix between residues B1 and B8 (refs 6, 7). We report here the structure of a new symmetrical hexamer, in which all six molecules have the B1-B8 helix seen in 4Zn-insulin. Phenol molecules, found bonding specifically to each molecule, evidently stabilize this new helical conformation.