| Summary: | Two new protease inhibitors (PIs), CmPI-II and AdKI were purified and characterized from mollusks Cenchritis muricatus and Aplysia dactylomela, respectively. They showed different specificities, CmPI-II for human neutrophil elastase (HNE) and AdKI for human plasma kallikrein (HPK). Purification procedures were established, rendering good yields and high purification degree. CmPI-II (UNIPROT: P84755) is a 5480 Da polypeptide of three disulphide bridges belong to the "non-classical"Kazal-type inhibitors. A new group was proposed according to the location of the CysI-CysV disulfide bridge. The presence of a basic residue at the inhibitor active site changed the pre-established requirement of a hydrophobic residue for elastase inhibition. The three-dimensional CmPI-II/HNE complex model contributes to explain the CmPI-II specificity for the enzyme. This is the first PI molecule isolated from C. muricatus. On the other hand, AdKI (2.9 kDa polypeptide) is an exception among invertebrate inhibitors in terms of inhibitory strength and selectivity against HPK. A new serine protease, AdSP, was also purified and characterized from the same extract, which could be the target for AdKI. CmPI-II and AdKI are the first inhibitors isolated from the phyllum Mollusca against ENH and HPK, respectively.
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