THE MOLECULAR-STRUCTURE OF THE COMPLEX OF ASCARIS CHYMOTRYPSIN ELASTASE INHIBITOR WITH PORCINE ELASTASE

Background: The intestinal parasitic worm, Ascaris suum, produces a va riety of protein inhibitors that defend the organism against the host' s proteinases. Eight different proteins from Ascaris suum have been id entified as inhibitors of serine proteinases, targeting chymotrypsin, elastase and trypsin, These inhibitors share 30-40% sequence identity with one another, but have virtually no sequence identity with members of any of the other families of serine proteinase inhibitors. Results : The crystal structure of the complex of porcine pancreatic elastase with a chymotrypsin/elastase inhibitor from Ascaris suum (the C/E-1 in hibitor) has been solved to 2.4 angstrom resolution by the molecular r eplacement method. The C/E-1 inhibitor exhibits a novel folding motif. There are only two small beta-sheets and two single-turn 3(10)-helice s in this inhibitor. Unlike the majority of proteins, the C/E-1 inhibi tor does not have a hydrophobic core. The presence and unique topograp hy of the five disulfide bridges suggests that they play important rol es in maintaining the tertiary structure of the inhibitor. In addition , the side chains of several charged residues form electrostatic and h ydrogen-bonding cascades, which also probably compensate for the lack of extensive secondary structures and a hydrophobic core. The reactive -site loop of this inhibitor displays a conformation that is character istic of most serine proteinase inhibitors. Conclusions: The structure of the C/E-1 inhibitor confirms that inhibitors from Ascaris suum bel ong to a novel family of proteinase inhibitors. It also provides concl usive evidence for the correct disulfide bridge connections. The C/E-1 inhibitor probably acts by a common inhibitory mechanism proposed for other substrate-like protein inhibitors of serine proteinases. The un usual molecular scaffolding presents a challenge to current folding al gorithms. Proteins like the C/E-1 inhibitor may provide a valuable mod el system to study how the primary sequence of a protein dictates its three-dimensional structure.