The three-dimensional structure of the insulin molecule has been determined by single crystal x-ray analysis. The two chains are compactly arranged with the A chain lying above a central helical region of the B chain. From each end of the helix the terminal residues extend as arms, and the A chain is enclosed between these. This limits the A chain to one surface of the molecule. Two surfaces, made up of B chain residues, are nonpolar and are buried by the aggregation of the molecule into a dimer and then into a hexamer. The insulin molecules of the 12,000 molecular weight dimer are related by an approximate twofold axis. Three such dimers can come together and in the presence of zinc ions form the stable 36,000 molecular weight hexamer. The three dimers have identical structures and are related by a threefold axis. The approximate twofold axis of the dimer is perpendicular to this. There is good evidence that insulin is stored in the granule as small crystals of this hexamer.
The A chain terminal residues A1 glycine, A5 glutamine, A19 tyrosine and A21 asparagine are on the surface of the molecule. They are invariant and not involved in the aggregation of the molecule. Their deletion affects both the structure and activity of insulin. Substitution at A1 glycine, especially of bulky groups, reduces activity substantially but seems to affect the molecular structure less. In proinsulin the connecting peptide appears to cover these residues. Therefore the surface A chain terminal residues apparently are important to the molecule's activity.