The three‐dimensional structure of the bifunctional proteinase k/α‐amylase inhibitor from wheat (PKI3) at 2.5 Å resolution

Abstract
Wheat germ contains an inhibitor for proicinase K, called PK13 (M 1 ∼ 19600) which simultaneously inhibits α‐amylase. PK13 was crystallized, space group P21, α = 43.02 (5) Å, n = 65.18 (7) Å, c = 32.33 (4) Å, β = 112.79 (9), X‐ray data were collected to 2.5 Å resolution, the structure solved by molecular replacement on the basis of the atomic coordinates of the homologous Erythrina caffra DE‐3 inhibitor, and refined with simulated annealing techniques with a current R‐factor of 21%. The three‐dimensional structure of PK13 is stabilized by two disulfide bridges and has a central β‐barrel with distorted β‐structure. In analogy to related inhibitors, the binding site for proteinase K is assumed to be located on the surface of the protein (amino acids residues 66–67), although the 75–76 peptide bond is cleaved upon binding.