Crystallization,crystal structure analysis and molecular model of the third domain of Japanese quail ovomucoid,a Kazal type inhibitor

The third domain of Japanese quail ovomucoid, a Kazal type inhibitor, has been crystallized and its crystal structure determined at 2.5 Å resolution using multiple isomorphous replacement techniques. The asymmetric unit contains four molecules. In the crystal the molecules are arranged in two slightly different octamers with approximate D4 symmetry. The molecules are held together mainly by interactions of the N-terminal residues, which form a novel secondary structural element, a β-channel.The molecule is globular with approximate dimensions 35 Å × 27 Å × 19 Å. The secondary structural elements are a double-stranded anti-parallel β-sheet of residues Pro22 to Gly32 and an α-helix from Asn33 to Ser44. The reactive site Lys18-Asp19 is located in an exposed loop. It is close to Asn33 at the N terminus of the helical segment. The polypeptide chain folding of ovomucoid bears some resemblance to other inhibitors in the existence of an anti-parallel double strand following the reactive site loop.