Sönnichsen F D, DeLuca C I, Davies P L, Sykes B D
Department of Physiology and Biophysics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106-4970, USA.
Structure. 1996 Nov 15;4(11):1325-37. doi: 10.1016/s0969-2126(96)00140-2.
Antifreeze proteins are found in certain fish inhabiting polar sea water. These proteins depress the freezing points of blood and body fluids below that of the surrounding sea water by binding to and inhibiting the growth of seed ice crystals. The proteins are believed to bind irreversibly to growing ice crystals in such a way as to change the curvature of the ice-water interface, leading to freezing point depression, but the mechanism of high-affinity ice binding is not yet fully understood.
The solution structure of the type III antifreeze protein was determined by multidimensional NMR spectroscopy. Twenty-two structures converged and display a root mean square difference from the mean of 0.26 A for backbone atoms and 0.62 A for all non-hydrogen atoms. The protein exhibits a compact fold with a relatively large hydrophobic core, several short and irregular beta sheets and one helical turn. The ice-binding site, which encompasses parts of the C-terminal sheet and a loop, is planar and relatively nonpolar. The site is further characterized by the low solvent accessibilities and the specific spatial arrangement of the polar side-chain atoms of the putative ice-binding residues Gln9, Asn14, Thr15, Thr18 and Gln44.
In agreement with the adsorption-inhibition mechanism of action, interatomic distances between active polar protein residues match the spacing of water molecules in the prism planes (¿10&1macr;0¿) of the hexagonal ice crystal. The particular side-chain conformations, however, limit the number and strength of possible proten-ice hydrogen bonds. This suggests that other entropic and enthalpic contributions, such as those arising from hydrophobic groups, could play a role in the high-affinity protein-ice adsorption.
抗冻蛋白存在于某些栖息在极地海水中的鱼类体内。这些蛋白质通过与冰晶结合并抑制其生长，将血液和体液的冰点降低至低于周围海水的冰点。据信，这些蛋白质以不可逆的方式与生长中的冰晶结合，从而改变冰水界面的曲率，导致冰点降低，但高亲和力冰结合的机制尚未完全了解。
通过多维核磁共振光谱法确定了III型抗冻蛋白的溶液结构。22个结构收敛，主链原子与平均值的均方根偏差为0.26 Å，所有非氢原子的均方根偏差为0.62 Å。该蛋白质呈现出紧密的折叠结构，具有相对较大的疏水核心、几个短而不规则的β折叠片和一个螺旋圈。冰结合位点包括C端折叠片的部分和一个环，是平面且相对非极性的。该位点的进一步特征是溶剂可及性低，以及假定的冰结合残基Gln9、Asn14、Thr15、Thr18和Gln44的极性侧链原子的特定空间排列。
与吸附抑制作用机制一致，活性极性蛋白质残基之间的原子间距离与六方冰晶棱柱面（10&1macr;0&）中水分子的间距相匹配。然而，特定的侧链构象限制了可能的蛋白质-冰氢键的数量和强度。这表明其他熵和焓的贡献，例如来自疏水基团的贡献，可能在高亲和力蛋白质-冰吸附中起作用。
