The structure of a rhombohedral R6 insulin hexamer that binds phenol

Different hexameric forms of insulin have been crystallized from a variety of conditions. In the presence of 1% phenol, 1.0 M sodium chloride, and at a pH of 8.5, a rhombohedral form is produced with two monomers in the asymmetric unit, space group R3, a = 79.92 A and c = 40.39 A. The structure has been solved and refined, using data between 8.0 and 2.5 A resolution, to a residual of 0.157. Each of the monomers adopts an R conformation, that is residues B1-B8 are alpha-helical. As a result of the T to R transition, an elliptical cavity is created between symmetry-related monomers and is occupied by a phenol molecule. A region of density within bonding distance to one of the zinc ions has been interpreted as an additional phenol molecule.