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Cold Instability of Aponeocarzinostatin and its Stabilization by Labile Chromophore

Kandaswamy Jayachithra ^*, Thallampuranam Krishnaswamy Suresh Kumar , Ta-Jung Lu ^*, Chin Yu  and Der-Hang Chin ^*

^* Department of Chemistry, National Chung Hsing University, Taichung, Taiwan, Republic of China; and Department of Chemistry & Biochemistry, University of Arkansas, Fayetteville, Arkansas

Correspondence: Address reprint requests to Der-Hang Chin, Dept. of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road., Taichung 40227, Taiwan, R.O.C. Tel.: 886-4-22-840-411 ext. 304; Fax: 886-4-22-862-547; E-mail: chdhchin{at}dragon.nchu.edu.tw.

The conformational stability of aponeocarzinostatin, an all-ß-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.

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