title: The molecular structure of the complex of Ascaris chymotrypsin/elastase inhibitor with porcine elastase.
abstract: BACKGROUND
  The intestinal parasitic worm, Ascaris suum, produces a variety of protein inhibitors that defend the organism against the host's proteinases. Eight different proteins from Ascaris suum have been identified 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 inhibitor) has been solved to 2.4 A resolution by the molecular replacement method. The C/E-1 inhibitor exhibits a novel folding motif. There are only two small beta-sheets and two single-turn 3(10)-helices in this inhibitor. Unlike the majority of proteins, the C/E-1 inhibitor does not have a hydrophobic core. The presence and unique topography of the five disulfide bridges suggests that they play important roles in maintaining the tertiary structure of the inhibitor. In addition, the side chains of several charged residues from electrostatic and hydrogen-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 characteristic of most serine proteinase inhibitors.
  
  
  CONCLUSIONS
  The structure of the C/E-1 inhibitor confirms that inhibitors from Ascaris suum belong to a novel family of proteinase inhibitors. It also provides conclusive 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 unusual molecular scaffolding presents a challenge to current folding algorithms. Proteins like the C/E-1 inhibitor may provide a valuable model system to study how the primary sequence of a protein dictates its three-dimensional structure.
authors.affiliation: Department of Biochemistry, University of Alberta, Edmonton, Canada.
authors.forename: K
authors.initials: K
authors.lastname: Huang
authors.forename: N C
authors.initials: NC
authors.lastname: Strynadka
authors.forename: V D
authors.initials: VD
authors.lastname: Bernard
authors.forename: R J
authors.initials: RJ
authors.lastname: Peanasky
authors.forename: M N
authors.initials: MN
authors.lastname: James
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001200:Ascaris; D017165:Ascaris suum / Q000737:chemistry*; D001665:Binding Sites; D002918:Chymotrypsin / Q000037:antagonists & inhibitors*; D018360:Crystallography, X-Ray; D015801:Helminth Proteins; D006860:Hydrogen Bonding; D019476:Insect Proteins; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010179:Pancreas / Q000201:enzymology; D010196:Pancreatic Elastase / Q000037:antagonists & inhibitors* / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D015842:Serine Proteinase Inhibitors / Q000737:chemistry / Q000145:classification; D013552:Swine; D014361:Trypsin Inhibitors / Q000737:chemistry*
chemical_list: C089851:C-E-1 inhibitor protein, Ascaris; D015801:Helminth Proteins; D019476:Insect Proteins; D015842:Serine Proteinase Inhibitors; D014361:Trypsin Inhibitors; C089852:trypsin inhibitor, Ascaris; D002918:Chymotrypsin; D010196:Pancreatic Elastase