title: The native strains in the hydrophobic core and flexible reactive loop of a serine protease inhibitor: crystal structure of an uncleaved alpha1-antitrypsin at 2.7 A.
abstract: BACKGROUND
  The protein alpha1-antitrypsin is a prototype member of the serpin (serine protease inhibitor) family and is known to inhibit the activity of neutrophil elastase in the lower respiratory tract. Members of this family undergo a large structural rearrangement upon binding to a target protease, involving cleavage of the reactive-site loop. This loop is then inserted into the main body of the enzyme following the opening of a central beta sheet, leading to stabilization of the structure. Random mutageneses of alpha1-antitrypsin identified various mutations that stabilize the native structure and retard the insertion of the reactive-site loop. Structural studies of these mutations may reveal the mechanism of the conformational change.
  
  
  RESULTS
  We have determined the three-dimensional structure of an uncleaved alpha1-antitrypsin with seven such stabilizing mutations (hepta alpha1-antitrypsin) at 2.7 A resolution. From the comparison of the structure with other serpin structures, we found that hepta alpha1-antitrypsin is stabilized due to the release of various strains that exist in native wild type alpha1-antitrypsin, including unfavorable hydrophobic interactions in the central hydrophobic core. The reactive-site loop of hepta alpha1-antitrypsin is an extended strand, different from that of the previously determined structure of another uncleaved alpha1-antitrypsin, and indicates the inherent flexibility of the loop.
  
  
  CONCLUSIONS
  The present structural study suggests that the uncleaved alpha1-antitrypsin has many folding defects which can be improved by mutations. These folding defects seem to be utilized in a coordinated fashion in the regulation of the conformational switch of alpha1-antitrypsin. Some of the defects, represented by the Phe51 region and possibly the Met374 and the Thr59 regions, are part of the sheet-opening mechanism.
authors.affiliation: Protein Engineering Research Division, Korea Research Institute of Bioscience and Biotechnology, KIST, Yusong, Taejon, South Korea. ryuse@mcr1.geri.re.kr
authors.forename: S E
authors.initials: SE
authors.lastname: Ryu
authors.forename: H J
authors.initials: HJ
authors.lastname: Choi
authors.forename: K S
authors.initials: KS
authors.lastname: Kwon
authors.forename: K N
authors.initials: KN
authors.lastname: Lee
authors.forename: M H
authors.initials: MH
authors.lastname: Yu
mesh_terms: D003198:Computer Simulation; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D015842:Serine Proteinase Inhibitors / Q000737:chemistry* / Q000235:genetics; D000515:alpha 1-Antitrypsin / Q000737:chemistry* / Q000235:genetics
chemical_list: D015842:Serine Proteinase Inhibitors; D000515:alpha 1-Antitrypsin