CRYSTAL-STRUCTURE OF AN ECOTIN-COLLAGENASE COMPLEX SUGGESTS A MODEL FOR RECOGNITION AND CLEAVAGE OF THE COLLAGEN TRIPLE-HELIX

The crystal structure of fiddler crab collagenase complexed with the d imeric serine protease inhibitor ecotin at 2.5 Angstrom resolution rev eals an extended cleft providing binding sites for at least 11 contigu ous substrate residues. Comparison of the positions of nine intermolec ular main chain hydrogen bonding interactions in the cleft, with the k nown sequences at the cleavage site of type I collagen, suggests that the protease binding loop of ecotin adopts a conformation mimicking th at of the cleaved strand of collagen. A well-defined groove extending across the binding surface of the enzyme readily accommodates the two other polypeptide chains of the triple-helical substrate. These observ ations permit construction of a detailed molecular model for collagen recognition and cleavage by this invertebrate serine protease. Ecotin undergoes a pronounced internal structural rearrangement which permits binding in the observed conformation. The capacity for such rearrange ment appears to be a key determinant of its ability to inhibit a wide range of serine proteases.