Reactive sites of an anticarcinogenic Bowman-Birk proteinase inhibitor are similar to other trypsin inhibitors.

The three-dimensional structure of the Bowman-Birk type proteinase inhibitor (PI-II) has been determined by x-ray crystallography and refined at 2.5-A resolution. This protein is a specific inhibitor of trypsin. Two reactive site loops, one at each end of the PI-II molecule, are structurally similar to each other and to reactive-site loops of pancreatic secretory trypsin inhibitor (Bolognesi, M., Gatti, G., Menegatti, E., Guarneri, M., Marquart, M., Papamokos, E., and Huber, R. (1982) J. Mol. Biol. 162, 839-869) and bovine pancreatic trypsin inhibitor (Deisenhofer, J., and Steigemann, W. (1975) Acta Crystallogr. B31, 238-250). PI-II is the first reported Bowman-Birk type inhibitor structure to be refined at high resolution, providing further insight into inhibitor mechanisms.