Protease Inhibitor Ecotin; homodimeric protease inhibitor which binds two chymotrypsin-like serine proteases to form a heterotetramer. Found in bacterial periplasm. Inhibits a broad range of serine proteases including collagenase, trypsin, chymotrypsin, elastase, and factor Xa but not thrombin. Inhibition mechanism involves binding at two different protease contact sites: the primary and secondary binding sites. Primary site loops of ecotin bind to the active site of target proteases in a substrate-like manner with the P1 residue in ecotin mimicking the interactions of a canonical P1 substrate residue.