Calorimetric studies on an insect antifreeze protein ApAFP752 from Anatolica polita

Many ectotherms organisms produce antifreeze proteins (AFPs), also known as thermal hysteresis proteins (THPs), which can lower the freezing temperature of body liquids without significantly affecting the melting point. In this article, thermal hysteresis activity (THA) of ApAFP752 from the desert beetle Anatolica polita was measured with differential scanning calorimetry (DSC). When the ice fraction was less than 25.3%, a delay in the onset temperature of refreezing was observed, indicating that the ApAFP752 solution has thermal hysteresis effect. When the amount of ice in the solution was less than 5.1%, THA of the ApAFP752 reached as high as 0.76 °C. THA of ApAFP752 was concentration-dependent. Hydrophilic ability of ApAFP752 was evaluated by thermal gravimetry (TG). The results of TG showed that ApAFP752 has strong hydrophilicity. The secondary structure of ApAFP752 was studied with circular dichroism (CD). The CD spectrum from 190 to 240 nm indicated a well-defined secondary structure consisting of 11.1% α-helix, 53.6% β-sheet, 8.3% turn, and 27.0% random coil.