The thermodynamics of the unfolding of an isolated protein subdomain The 255–316 C-terminal fragment of thermolysin

Differential scanning calorimetry has been used to study the thermal unfolding of the 255–316 C-terminal fragment of thermolysin. The concentration effect on the calorimetric transitions of the fragment in 0.1 M NaCl and 20 mM phosphate buffer, pH 7.5, shows that it behaves as a highly stable dimer in solution, whithin the concentration range 0.19–4.55 mg/ml, undergoing a reversible two-state thermal unfolding process. The thermodynamic parameters of unfolding (Δ G = 60 ± 6 kJ/(mol of dimer) at 20°C) are similar to those normally observed for small, compact, globular proteins. This and previous studies [1989, Eur. J. Biochem. 180, 513–518] show that the 255–316 fragment folds into a stable, native-like globular structure.