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Systematic Size Study of an Insect Antifreeze Protein and Its Interaction with Ice

Kai Liu ^* , Zongchao Jia , Guangju Chen ^*, Chenho Tung  and Ruozhuang Liu ^*

^* Department of Chemistry, Beijing Normal University, Beijing 100875, People's Republic of China; Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, People's Republic of China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China; and Department of Biochemistry, Queen's University, Kingston, Ontario, K7L 3N6, Canada

Correspondence: Address reprint requests to Prof. Guangju Chen, Dept. of Chemistry, Beijing Normal University, Beijing 100875, People's Republic of China. Tel.: 86-10-5880-7969; E-mail: gjchen{at}bnu.edu.cn.

Because of their remarkable ability to depress the freezing point of aqueous solutions, antifreeze proteins (AFPs) play a critical role in helping many organisms survive subzero temperatures. The ß-helical insect AFP structures solved to date, consisting of multiple repeating circular loops or coils, are perhaps the most regular protein structures discovered thus far. Taking an exceptional advantage of the unusually high structural regularity of insect AFPs, we have employed both semiempirical and quantum mechanics computational approaches to systematically investigate the relationship between the number of AFP coils and the AFP-ice interaction energy, an indicator of antifreeze activity. We generated a series of AFP models with varying numbers of 12-residue coils (sequence TCTxSxxCxxAx) and calculated their interaction energies with ice. Using several independent computational methods, we found that the AFP-ice interaction energy increased as the number of coils increased, until an upper bound was reached. The increase of interaction energy was significant for each of the first five coils, and there was a clear synergism that gradually diminished and even decreased with further increase of the number of coils. Our results are in excellent agreement with the recently reported experimental observations.