3-FOLD STRUCTURAL PATTERN IN THE SOYBEAN TRYPSIN-INHIBITOR (KUNITZ)

The molecule contains three very similar irregular Y-shaped lobes of antiparallel twisted β-sheet, which are grouped symmetrically round a central axis and linked by hydrogen bonds to form a six-stranded barrel. Each lobe can be superposed on either neighbour by a rotation of approximately 120 °. Of the 160 residues seen in the X-ray electron density map, 101 may be superposed onto other residues within a root-mean-square distance of 2.1 Å. The bond which reacts with trypsin lies on a loop between the first two lobes. It is suggested that the protein evolved from a primitive symmetrical trimer of identical subunits by tandem gene triplication. © 1979.