Characterization of β-trypsin at acid pH by differential scanning calorimetry

Trypsin is a serino-protease with a polypeptide chain of 223 amino acid residues and contains six disulfide bridges. It is a globular protein with a predominance of antiparallel beta-sheet and helix in its secondary structure and has two domains with similar structures. We assessed the stability of beta-trypsin in the acid pH range using microcalorimetric (differential scanning calorimetry) techniques. Protein concentrations varied in the range of 0.05 to 2.30 mg/ml. Buffer solutions of 50.0 mM beta-alanine and 20.0 mM CaCl2 at different pH values (from 2.0 to 4.2) and concentrations of sorbitol (1.0 and 2.0 M), urea (0.5 M) or guanidinium hydrochloride (0.5 and 1.0 M) were used. The data suggest that we are studying the same conformational transition of the protein in all experimental situations using pH, sorbitol, urea and guanidinium hydrochloride as perturbing agents. The observed van't Hoff ratios (DeltaH(cal)/DeltaH(vH)) of 1.0 to 0.5 in the pH range of 3.2 to 4.2 suggest protein aggregation. In contrast, DeltaH(cal)/DeltaH(vH) ratios equal to one in the pH range of 2.0 to 3.2 suggest that the protein unfolds as a monomer. At pH 3.00, beta-trypsin unfolded with Tm = 54degreesC and DeltaH = 101.8 kcal/mol, and the change in heat capacity between the native and unfolded forms of the protein (cCp) was estimated to be 2.50 +/- 0.07 kcal mol(-1) K-1. The stability of beta-trypsin calculated at 298 K was DeltaG(D) = 5.7 kcal/mol at pH 3.00 and DeltaG(D) = 15.2 kcal/mol at pH 7.00, values in the range expected for a small globular protein.