X-Ray Crystallographic Studies of the Complex Between Porcine Pepsin and the Aspartic Proteinase Inhibitor PI-3 from the Nematode Ascaris suum

Abstract

Ascaris suum is an intestinal endoparasitic nematode found in pigs. In order to avoid proteolytic digestion by the host the nematode has developed a large battery of proteinase inhibitors. Among those is the aspartic proteinase inhibitor PI-3. PI-3 consists of a single polypeptide chain containing 149 residues with a molecular weight of 16.4 kD. It has three disulfide bridges. CD and secondary structure predictions indicate that the fold of PI-3 is mixed alpha/beta.¹ Until now only a very few larger aspartic proteinase inhibitors have been characterized. Aspartic proteinase inhibitors from the Filarie family of parasites have been reported (Dirofilaria, Brugia and Onchocerca). These inhibitors are more closely related to each other (~60% sequence identity) than PI-3 (less than 20%).² The inhibitor form Ascaris suum show a broad specificity towards members from the pepsin related family of aspartic proteinases having Ki’s in the range of 0.2—15 nM (porcine and human pepsin and cathepsin E). Cathepsin D however, is not inhibited by PI-3, although it shares up to 45 identity to the above mentioned aspartic proteinases.³ This has important aspects as it might be used to distinguish between cathepsin D and E. The following report describes the progress in the structure determination of the molecular complex between porcine pepsin and PI-3.