Protein inhibitors of serine proteinases.

Serine proteinases and their natural protein inhibitors belong to the most intensively studied models of protein-protein recognition. Protein inhibitors do not form a single group but can be divided into about 20 different families. Global structures of proteins representing different inhibitor families are completely different and comprise alpha-helical proteins, beta-sheet proteins, alpha/beta-proteins and different folds of small disulfide-rich proteins. Three different types of inhibitors can be distinguished: canonical (standard mechanism) inhibitors, non-canonical inhibitors, and serpins. The canonical inhibitor binds to the enzyme through the exposed and convex binding loop, which is complementary to the active site of the enzyme. The mechanism of inhibition in this group is consistently very similar and resembles that of an ideal substrate. Non-canonical inhibitors, originating from blood sucking organisms, specifically block enzymes of the blood clotting cascade. The interaction is mediated through inhibitor N-terminus which binds to the proteinase forming a parallel beta-sheet. There are also extensive secondary interactions which provide an additional buried area and contribute significantly to the strength and specificity of recognition. Serpins are major proteinase inhibitors occurring in plasma. Similarly to canonical inhibitors, serpins interact with their target proteinases in a substrate-like manner. However, in the case of serpins, cleavage of a single peptide bond in a flexible and exposed binding loop leads to dramatic structural changes.