The structure of T₆ human insulin at 1.0 Å resolution

The structure of T₆ human insulin has been determined at 120 K at a resolution of 1.0 Å and refined to a residual of 0.183. As a result of cryofreezing, the first four residues of the B chain in one of the two crystallographically independent AB monomers in the hexameric [Zn_1/3(AB)₂Zn_1/3]₃ complex undergo a conformational shift that displaces the C^α atom of PheB1 by 7.86 Å relative to the room-temperature structure. A least-squares superposition of all backbone atoms of the room-temperature and low-temperature structures yielded a mean displacement of 0.422 Å. Omitting the first four residues of the B chain reduced the mean displacement to 0.272 Å. At 120 K, nine residues were found to exhibit two discrete side-chain conformations, but only two of these residues are in common with the seven residues found to have disordered side chains in the room-temperature structure. As a result of freezing, the disorder observed at room temperature in both ArgB22 side chains is eliminated. The close contact between pairs of O^[epsilon]2 atoms in GluB13 observed at room temperature is maintained at cryotemperature and suggests that a carboxylate–carboxylic acid centered hydrogen bond exists [—­C(=O)—O···H···O—C(=O)—] such that the H atom is equally shared between the two partially charged O atoms.