Proton assignments and secondary structure determination of the soybean trypsin/chymotrypsin Bowman-Birk inhibitor

The 1H resonance assignments and secondary structure of the trypsin/chymotrypsin Bowman-Birk inhibitor from soybeans were determined by nuclear magnetic resonance spectroscopy (NMR) at 600 MHz in an 18% acetonitrile-d3/aqueous cosolvent. Resonances from 69 of 71 amino acids were assigned sequence specifically. Residues Q11-T15 form an antiparallel beta-sheet with residues Q21-S25 in the tryptic inhibitory domain and an analogous region of antiparallel sheet forms between residues S38-A42 and Q48-V52 in the chymotryptic inhibitory domain. The inhibitory sites of each fragment (K16-S17 for trypsin, L43-S44 for chymotrypsin) are each part of a type VI like turn at one end of their respective region of the antiparallel beta-sheet. These structural elements are compared to those found in other Bowman-Birk inhibitors.