Study of the Structure and Function of Bowman-Birk Inhibitor of Serine Proteases

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The structure analyses of a family of the Bowman-Birk type protease inhibitors and their complexes with proteases have successfully been carried out. The Bowman-Birk inhibitors which inhibit serine proteases are small proteins with 60-80 amino acid residues. An inhibitor molecule has seven disulfide bridges, and contains very few hydrophobic residues. It consists of two domains, of which the amino acid sequences are very similar to each other.1. 2.3 A structure analysis of the trypsin-AB-I (azuki bean inhibitor) complex: Azuki bean inhibitor AB-I inhibits trypsin and chymotrypsin. The complex between AB-I and trypsin (1:1) gave fine crystals suitable for the X-ray analysis. Of the inhibitor only the trypsin-binding domain could be determined. Including 140 water molecules, R = 0.20. The binding site of the inhibitor including Lys26 is tightly bound in the trypsin active center by several hydrogen bonds and van der Waals contacts. The structure of the binding site of the inhibitor seems to be very stable, and any deviation from the stable structure which is necessary to induce a proteolytic reaction seems hardly to occur.2. 3.3 A analysis of peanut inhibitor A-II: The inhibitor molecule has the dimension of 45x15x15 A, and consists of two distinct domains of which the structures are very similar with each other. Their structures are also essentially the same as that of the AB-I trypsin-binding domain. The two domains are related by an intramolecular pseudo twofold axis, and linked by two rather flexible peptide chains, and each domain has one binding site for proteases at the edges of the molecule.