CRYSTAL-STRUCTURE OF THE BIFUNCTIONAL SOYBEAN BOWMAN-BIRK INHIBITOR AT 0.28-NM RESOLUTION - STRUCTURAL PECULIARITIES IN A FOLDED PROTEIN CONFORMATION

The Bowman-Birk inhibitor from soybean is a small protein that contain s a binary arrangement of trypsin-reactive and chymotrypsin-reactive s ubdomains. In this report, the crystal structure of this anticarcinoge nic protein has been determined to 0.28-nm resolution by molecular rep lacement from crystals grown at neutral pH. The crystal structure diff ers from a previously determined NMR structure [Werner, hi. H. & Wemme r. D. E. (1992 Biochemistry 31, 999-1010] in the relative orientation of the two enzyme-insertion loops, in some details of the main chain t ract. in the presence of favourable contacts in the trypsin-insertion loop, and in the orientation of several amino acid side chains. The pr oximity of Met27 and Gln48 in the X-ray structure contradicts the solu tion structure, in which these two side chains point away from each ot her. The significant effect of a Met27-->Ile replacement on the inhibi tory activity of the chymotrypsin-reactive subdomain agrees with the X -ray structure. Exposed hydrophobic patches. the presence of charged a mino acid residues, and the presence of water molecules in the protein interior are in contrast to standard proteins that comprise a hydroph obic core and exposed polar amino acids.