Squash inhibitors of serine proteinases form an uniform family of small proteins. They are built of 27-33 amino-acid residues and cross-linked with three disulfide bridges. The reactive site peptide bond (Pl-Pl') is between residue 5 (Lys, Arg or Leu) and 6 (always lie). High resolution X-ray structures are available for two squash inhibitors complexed with trypsin. NMR solution structures have also been determined for free inhibitors. The major structural motif is a distorted, triple-strai:ded antiparallel p-sheet. A similar folding motif has been recently found in a number of proteins, including: conotoxins from fish-hunting snails, carbo- xypeptidase inhibitor from potato, kalata B1 polypeptide, and in some growth factors (e.g. nerve growth factor, transforming growth factor P2, platelet-derived growth factor). Squash inhibitors are highly stable and rigid proteins. They inhibit a number of serine proteinases: trypsin, plasmin, kallikrein, blood clotting factors: Xa and XII*, cathepsin G. The inhibition spectrum can be much broadened if specific amino-acid substitutions are introduced, especially at residues which contact proteinase. Squash inhibitors inhibit proteinases via the standard mechanism. According to the mechanism, inhibitors are substrates which exibit at neutral pH a high fccWKm index for hydrolysis and resynthesis of the reactive site, and a low value of the hydrolysis constant.
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