Differential scanning calorimetry of thermal unfolding of the methionine repressor protein (MetJ) from Escherichia coli.

The thermal stability of the methionine repressor protein from Escherichia coli (MetJ) has been examined over a wide range of pH (pH 3.5-10) and ionic strength conditions using differential scanning calorimetry. Under reducing conditions, the transitions are fully reversible, and thermograms are characteristic of the cooperative unfolding of a globular protein with a molecular weight corresponding to the MetJ dimer, indicating that no dissociation of this dimeric protein occurs before unfolding of the polypeptide chains under most conditions. In the absence of reducing agent, repeated scans in the calorimeter show only partial reversibility, though the thermodynamic parameters derived from the first scans are comparable to those obtained under fully reversible conditions. The protein is maximally stable (Tm 58.5 degrees C) at about pH 6, close to the estimated isoelectric point, and stability is enhanced by increasing ionic strength in the range I = 0.01-0.4 M. The average calorimetric transition enthalpy (delta Hm) for the dimer is 505 +/- 28 kJ mol-1 under physiological conditions (pH 7, I = 0.125, Tm = 53.2 degrees C) and shows a small temperature dependence which is consistent with an apparent denaturational heat capacity change (delta Cp) of about +8.9 kJ K-1 mol-1. The effects of both pH and ionic strength on the transition temperature and free energy of MetJ unfolding are inconsistent with any single amino acid contribution and are more likely the result of more general electrostatic interactions, possibly including significant contributions from electrostatic repulsion between the like-charged monomers which can be modeled by a Debye-Hückel screened potential.