A predicted secondary structure of protein protease inhibitors

Basing on the Chou & Fasman method (Biochemistry, 13, 222-245; 1974) and the known amino acid sequences, the alpha-helical, beta-sheet, beta-turn and random-coil regions were predicted for protein protease inhibitors. It appears that in all the inhibitors examined there is a region conserved in the vicinity of the active site, composed of one beta-turn with an adjacent beta-sheet structure, and a second beta-turn situated in the other part of the polypeptide chain and linked with the first one by disulphide bridge. Two disulphide bridges between Cys2 and Cys25, and between Cys10 and Cys21 were proposed for the squash seed inhibitor. It is suggested that the two beta-turns play an essential role in the process of trypsin inhibition by protein protease inhibitors.