Circular dichroism study on the conformational stability of the dimerization domain of transcription factor LFB1

LFB1, a dimeric DNA binding protein, is a major determinant of hepatocyte-specific transcription. The thermal and chemical equilibrium unfolding of a 32-residue alpha-helical peptide comprising its dimerization domain (B1-Dim) was monitored by circular dichroism spectroscopy. The conformational stability of this peptide is shown to be concentration dependent, and the unfolding reaction is described as a two-state transition between folded dimers and unfolded monomers. The thermodynamic parameters associated with the unfolding reaction were determined under the two-state assumption by the van't Hoff procedure. The enthalpy of unfolding increases linearly with temperature, and the corresponding value of delta Cp, the difference in heat capacity between the unfolded and the folded forms of the peptide, is estimated to be ca. 0.7 kcal mol-1 K-1. The dimeric folded structure of the peptide is stabilized, at 25 degrees C, by a delta G of about 11.5 kcal mol-1, which is equivalent to a dimerization constant greater than 10(8) mol-1. These results indicate that the dimerization domain of LFB1 can fold and dimerize independently of the rest of the protein, with a thermodynamic stability comparable to that of a small globular protein.