Cold Instability of Aponeocarzinostatin and its Stabilization by Labile Chromophore

• Published in: Biophysical journal 2005-06, Vol.88 (6), p.4252-4261
• Main Authors: Jayachithra, Kandaswamy, Kumar, Thallampuranam Krishnaswamy Suresh, Lu, Ta-Jung, Yu, Chin, Chin, Der-Hang
• Format: Article
• Language: English
• Subjects: Antibiotics; Apoproteins - chemistry; Biophysical Phenomena; Biophysics; Circular Dichroism; Cold Temperature; Drug Stability; Hydrogen-Ion Concentration; Models, Molecular; Protein Conformation; Protein Denaturation; Protein Folding; Proteins; Stabilization; Thermodynamics; Zinostatin - chemistry
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1529/biophysj.104.051722

The conformational stability of aponeocarzinostatin, an all- β-sheet protein with 113 amino-acid residues, is investigated by thermal-induced equilibrium unfolding between pH 2.0 and 10.0 with and without urea. At room temperature, the protein is stable in a pH range of 4.0–10.0, whereas the stability of the protein drastically decreases below pH 4.0. The thermal unfolding of aponeocarzinostatin is reversible and follows a two-state mechanism. By two-dimensional unfolding studies, the enthalpy change, heat capacity change, and free energy change for unfolding of the protein are estimated. Circular dichroism profiles suggest that this protein undergoes both heat- and cold-induced unfolding. The ellipticity changes at far- and near-UV circular dichroism suggest that the tertiary structure is disrupted but the secondary structure remains folded at low temperatures. Interestingly, the labile enediyne chromophore, which is highly stabilized by the protein, is able to protect the protein against cold-induced unfolding, but not the heat-induced unfolding.