ABSTRACT
The crystal structure of IN was reported for the first time as the rhombohedral 2Zn form [43], named according to its zinc content [37]. In that work it was shown that three equivalent dimers associate on an approximately threefold axis to form a hexamer. The two molecules that form the dimer have nearly identical conformations and are related to each other by a twofold axis, which is perpendicular to the crystal lographic threefold axis. In this form each 2 1 -residue A chain consists of two anti parallel a helices connected by a short extended strand (A 9Ser-A 1 2 Ser). Each 30residue B chain consists of two extended strands connected by an a-helical segment, B9Ser-B19Cys (Fig. 3). Each Zn2+ ion lies on a crystallographic threefold axis in an octahedral environment formed by the three symmetry-related BlO-His residues and three water molecules.
