Physical-chemical characterization and stability study of α-trypsin at pH 3.0 by differential scanning calorimetry

Abstract α-Trypsin is a serine-protease with a polypeptide chain of 223 amino acid residues and six disulfide bridges. It is a globular protein with predominance of antiparallel s-sheet secondary structure and it has two domains with similar structures. In the present work, a stability study of α-trypsin in the acid pH range was performed and some physical-chemical denaturation parameters were measured by using differential scanning calorimetry (DSC). The α-trypsin has a shelf-life ( t 95% ) of about 10 months at pH 3.0 and 4 °C and its hydrolysis into the ψ-trypsin isoform is negligible during 6 months. The observed ratio Δ H cal /Δ H vH is close to unity, which suggests the occurrence of a two-state transition. At pH 3.0, α-trypsin unfolded with T m  = 325.9 K and Δ H  = 99.10 kcal mol −1 , and the change in heat capacity between the native and unfolded forms of the protein was estimated to be 1.96 ± 0.18 kcal mol −1  K −1 . The stability of α-trypsin calculated at 298 K was Δ G U  = 6.10 kcal mol −1 at pH 3.0. These values are in the range expected for a small globular protein. These results show that the thermodynamic parameters of unfolding of β-trypsin do not change substantially after its conversion to α-trypsin.