Symmetry of Insulin Dimers and Hexamers

Abstract

RECENT X-ray crystallographic studies of orthorhombic crystals of bovine insulin sulphate have provided information about the symmetry of the insulin dimer (molecular weight 2 × 5733). This dimer appears to be the stable unit in acid aqueous solution, except under special conditions¹. The dimer is the protein component of the asymmetrical unit in the wet, type A and type B sulphate and type A citrate, crystals², and also in the air-dried sulphate crystals³. Studies of the shrinkage stages of the type A insulin sulphate crystals⁴ have disclosed the existence of several new crystal forms. The space group of all but one of these is P2₁2₁2₁ (Z = 4), that of all previously examined orthorhombic insulin crystals. One shrinkage stage (type C) has the space group I2₁2₁2₁ (Z = 8). In this crystal twofold rotation axes relate pairs of insulin molecules. On the basis of crystallographic and other evidence, it was possible to establish the strong probability that the insulin dimer has basically an invariant structure, with point group 2, in all these crystal forms and in acid solution; that is, the two insulin molecules of the dimer are related to each other by a twofold rotation axis.