title: Three-dimensional structure and molecular modelling of C1- inhibitor.
abstract: New data on the structure of C1- inhibitor have become available as the result of the completion of its amino acid and nucleic acid sequences and its carbohydrate content, together with data from neutron scattering, crystal structure determinations, 1H NMR spectroscopy and Fourier transform infrared spectroscopy. C1- inhibitor is a two-domain protein. It is constructed as a heavily glycosylated N-terminal domain with 113 amino acids and 3 N-linked and 7 O-linked oligosaccharides, and a C-terminal domain with 365 amino acids and 3 N-linked oligosaccharides. Its molecular weights is calculated as 71,100, rather than the apparent value of 105,000 commonly reported from SDS-PAGE. Neutron scattering shows that C1- inhibitor possesses a distinct two domain structure in which an extended N-terminal domain with extended carbohydrate structures is attached to the C-terminal SERPIN domain. The length of C1- inhibitor is 18 nm, rather than that of 33 nm originally proposed from electron microscopy. Microcalorimetry supports the concept of two independently-folded domains in the C1- inhibitor structure. Combination of the C1- inhibitor sequence with crystal structure determinations of proteins belonging to the SERPIN superfamily of serine protease inhibitors shows that the protein structure of the C-terminal domain is well-described in terms of this structure. Comparison of the exon structure of C1- inhibitor with its presumed protein crystal structure shows that the 7 exons are distributed throughout the SERPIN fold with no obvious segregation into structural elements. Native C1- inhibitor is cleaved at its reactive centre loop to release a more stable cleaved form that is no longer active. 1H NMR spectroscopy shows that structures involved in hydrophobic interactions within the core of the SERPIN fold are preserved to within 0.05 nm before and after the transition. Fourier transform infrared spectroscopy shows the presence of alpha-helix and beta-sheet in both forms. Large changes are detectable in both the alpha-helix and beta-sheet contents of the SERPIN fold, and these changes are matched by a greater rate of 1H-2H exchange in 2H2O solvents in the native form than in the cleaved form. These data show that changes in the whole secondary structure and not just in the reactive site loop are involved in SERPIN inactivation.
authors.affiliation: Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, London, U.K.
authors.forename: S J
authors.initials: SJ
authors.lastname: Perkins
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D003174:Complement C1 Inactivator Proteins / Q000737:chemistry*; D016384:Consensus Sequence; D006801:Humans; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009502:Neutrons; D011487:Protein Conformation; D017510:Protein Folding; D012542:Scattering, Radiation; D017386:Sequence Homology, Amino Acid; D015843:Serpins / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D003174:Complement C1 Inactivator Proteins; D015843:Serpins