Three-dimensional structure of a Kunitz-type inhibitor in complex with an elastase-like enzyme [Enzymology]

April 15th, 2015 by Garcia-Fernandez, R., Perbandt, M., Rehders, D., Ziegelmueller, P., Piganeau, N., Hahn, U., Betzel, C., de los Angeles Chavez, M., Redecke, L.

Elastase-like enzymes are involved in important diseases, such as acute pancreatitis, chronic inflammatory lung diseases, and cancer. Structural insights into the interaction with specific inhibitors will contribute to develop novel anti-elastase compounds that resist rapid oxidation and proteolysis. Proteinaceous Kunitz-type inhibitors homologous to the bovine pancreatic trypsin inhibitor (BPTI) provide a suitable scaffold, but structural aspects of their interaction with elastase-like enzymes have not been elucidated. Here, we increased the selectivity of ShPI-1, a versatile serine protease inhibitor from the sea anemone Stichodactyla helianthus with high biomedical and biotechnological potential, towards elastase-like enzymes by substitution of the P1 residue (Lys13) with leucine. The variant (rShPI-1/K13L) exhibits a novel anti-porcine pancreatic elastase (PPE) activity, together with a significantly improved inhibition of human neuthrophil elastase (HNE) and chymotrypsin. The crystal structure of the PPE-rShPI-1/K13L complex determined at 2.0 Å resolution provided first details of the canonical interaction between a BPTI-Kunitz-type domain and elastase-like enzymes. In addition to the essential impact of the variant P1 residue for complex stability, the interface is improved by increased contributions of the primary and secondary binding loop as compared to similar trypsin and chymotrypsin complexes. Comparison of the interaction network with elastase complexes of canonical inhibitors from the chelonianin family supports a key role of the P3 site in ShPI-1 in directing its selectivity against pancreatic and neutrophil elastases. Our results provide the structural basis for site-specific mutagenesis to further improve the binding affinity and/or to direct the selectivity of BPTI-Kunitz-type inhibitors towards elastase-like enzymes.