title: Four-state equilibrium unfolding of an scFv antibody fragment.
abstract: The conformational stability of a single-chain Fv antibody fragment against a hepatitis B surface antigen (anti-HBsAg scFv) has been studied by urea and temperature denaturation followed by fluorescence and circular dichroism. At neutral pH and low protein concentration, it is a well-folded monomer, and its urea and thermal denaturations are reversible. The noncoincidence of the fluorescence and circular dichroism transitions indicates the accumulation in the urea denaturation of an intermediate (I(1)) not previously described in scFv molecules. In addition, at higher urea concentrations, a red-shift in the fluorescence emission maximum reveals an additional intermediate (I(2)), already reported in the denaturation of other scFvs. The urea equilibrium unfolding of the anti-HBsAg scFv is thus four-state. A similar four-state behavior is observed in the thermal unfolding although the intermediates involved are not identical to those found in the urea denaturation. Global analysis of the thermal unfolding data suggests that the first intermediate displays substantial secondary structure and some well-defined tertiary interactions while the second one lacks well-defined tertiary interactions but is compact and unfolds at higher temperature in a noncooperative fashion. Global analysis of the urea unfolding data (together with the modeled structure of the scFv) provides insights into the conformation of the chemical denaturation intermediates and allows calculation of the N-I(1), I(1)-I(2), and I(2)-D free energy differences. Interestingly, although the N-D free energy difference is very large, the N-I(1) one, representing the "relevant" conformational stability of the scFv, is small.
authors.affiliation: Departamento de Bioquímica y Biología Molecular y Celular, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain.
authors.forename: Idolka
authors.initials: I
authors.lastname: Pedroso
authors.forename: María P
authors.initials: MP
authors.lastname: Irún
authors.forename: Claudia
authors.initials: C
authors.lastname: Machicado
authors.forename: Javier
authors.initials: J
authors.lastname: Sancho
mesh_terms: D002942:Circular Dichroism; D006514:Hepatitis B Surface Antigens / Q000276:immunology; D006863:Hydrogen-Ion Concentration; D007128:Immunoglobulin Fragments / Q000737:chemistry* / Q000276:immunology; D008958:Models, Molecular; D011487:Protein Conformation; D011489:Protein Denaturation; D013050:Spectrometry, Fluorescence; D055672:Static Electricity
chemical_list: D006514:Hepatitis B Surface Antigens; D007128:Immunoglobulin Fragments