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Abstract

The Bowman‐Birk inhibitor from soybean is a small protein that contains a binary arrangement of trypsin‐reactive and chymotrypsin‐reactive subdomains. In this report, the crystal structure of this anticarcinogenic protein has been determined to 0.28‐nm resolution by molecular replacement from crystals grown at neutral pH. The crystal structure differs from a previously determined NMR structure [Werner, M. H. & Wemmer, D. E. (1992) Biochemistry 31, 999–1010] in the relative orientation of the two enzyme‐insertion loops, in some details of the main chain trace, in the presence of favourable contacts in the trypsin‐insertion loop, and in the orientation of several amino acid side chains. The proximity of Met27 and Gln48 in the X‐ray structure contradicts the solution structure, in which these two side chains point away from each other. The significant effect of a Met27?Ile replacement on the inhibitory activity of the chymotrypsin‐reactive subdomain agrees with the X‐ray structure. Exposed hydrophobic patches, the presence of charged amino acid residues, and the presence of water molecules in the protein interior are in contrast to standard proteins that comprise a hydrophobic core and exposed polar amino acids.