title: The native metastable fold of C1-inhibitor is stabilized by disulfide bonds.
abstract: C1-inhibitor is a member of the serpin family of proteinase inhibitors and is an important inhibitor of complement and contact system proteinases. The native protein has the characteristic serpin feature of being in a kinetically trapped metastable state rather than in the most stable state it could adopt. A consequence of this is that it readily forms loop-sheet dimers and polymers, by a mechanism believed to be the same as observed with other serpins. An unusual feature of C1-inhibitor is that it has a unique amino-terminal domain, of unknown function, held to the serpin domain by two disulfide bonds not found in other serpins. We report here that reduction of these bonds by DTT, causes a conformational change such that the reactive center loop inserts into beta-sheet A. This form of C1-inhibitor is less stable to heat and urea than the native protein, and is more susceptible to extensive degradation by trypsin. These data show that the disulfide bonds in C1-inhibitor are required for the protein to be stabilized in the metastable state with the reactive center loop expelled from beta-sheet A.
authors.affiliation: Department of Oral Medicine and Diagnostic Sciences (MC 838), College of Dentistry, University of Illinois at Chicago, 60612, USA.
authors.forename: I
authors.initials: I
authors.lastname: Simonovic
authors.forename: P A
authors.initials: PA
authors.lastname: Patston
mesh_terms: D001665:Binding Sites; D003174:Complement C1 Inactivator Proteins / Q000737:chemistry*; D050718:Complement C1 Inhibitor Protein; D003545:Cysteine / Q000737:chemistry; D015853:Cysteine Proteinase Inhibitors / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry*; D004229:Dithiothreitol; D006358:Hot Temperature; D006801:Humans; D010084:Oxidation-Reduction; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D014508:Urea; D000515:alpha 1-Antitrypsin / Q000737:chemistry
chemical_list: D003174:Complement C1 Inactivator Proteins; D050718:Complement C1 Inhibitor Protein; D015853:Cysteine Proteinase Inhibitors; D004220:Disulfides; D000515:alpha 1-Antitrypsin; D014508:Urea; D003545:Cysteine; D004229:Dithiothreitol