The mechanism by which fish antifreeze proteins cause thermal hysteresis
The mechanism by which fish antifreeze proteins cause thermal hysteresis
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DOI：
10.1016/j.cryobiol.2005.07.007
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发表时间：
2005-12-01
期刊：
影响因子：
2.7
通讯作者：
Zachariassen, KE
中科院分区：
文献类型：
--
作者：
Kristiansen, E;Zachariassen, KE
防冻蛋白的特点是能够防止冰在冷却至本体熔点以下时生长。冰的冻结温度的这种位移是有限的，并且在足够低的温度下发生快速的冰生长。融化和冻结温度的分离通常被称为热滞后，冰生长的温度被称为滞后凝固点。这种滞后现象被认为是抗冻蛋白吸附到晶体表面的结果。这导致冰在相邻的被吸附的抗冻蛋白质之间生长为凸面区域，从而降低了晶体可以明显膨胀的温度。该模型要求抗冻蛋白不可逆地吸附到冰表面的滞后间隙内。这种假设显然与这种现象的几个特征相冲突;在抗冻蛋白存在下，冰不会过热，抗冻蛋白浓度对迟滞活性的依赖性，以及不同类型的抗冻蛋白在等摩尔浓度下引起热迟滞的不同能力。此外，还有结构上的障碍，显然会阻止不可逆吸附的抗冻蛋白质的冰表面;不可逆吸附所需的键强度和缺乏一个明确界定的表面，抗冻蛋白质可以吸附。本文讨论流行理论和经验观察之间的这些明显冲突。我们首先回顾了热滞后的机制，并进行了一些修改：我们将滞后解释为冰面和滞后间隙内环境流体部分之间的蒸汽压平衡的结果，这是由于凸面生长区内的压力积聚，以及冰的生长作为滞后冰点处冰表面成核事件的结果。然后，我们继续总结的经验数据表明，抗冻蛋白的浓度的滞后的依赖性产生从冰和溶液之间的抗冻蛋白在熔点的平衡交换。当温度降低到熔点以下时，抗冻蛋白和冰之间的这种可逆缔合之后，抗冻蛋白不可逆地吸附到新形成的晶面上。晶面的形成是由于界面区域的固化，并且必要的结合强度由蛋白质“冷冻”到表面提供。实质上：抗冻蛋白质在本体熔点时从冰上“融化”下来，并在温度降低到冰点以下时“冻结”在冰上。我们解释了不同类型的抗冻蛋白在等摩尔浓度下引起的不同滞后活动，这是由于蛋白质和冰之间可逆缔合阶段的溶解度特征，即，低水溶性导致大部分蛋白质在熔点与冰结合。这导致当温度下降时，在冰面上不可逆吸附的抗冻蛋白质的密度更大，从而导致更大的滞后活性。还提到昆虫抗冻蛋白的观察，强调这种方法的普遍有效性。(C)2005年爱思唯尔公司All rights reserved.
Antifreeze proteins are characterised by their ability to prevent ice from growing upon cooling below the bulk melting point. This displacement of the freezing temperature of ice is limited and at a sufficiently low temperature a rapid ice growth takes place. The separation of the melting and freezing temperature is usually referred to as thermal hysteresis, and the temperature of ice growth is referred to as the hysteresis freezing point. The hysteresis is supposed to be the result of an adsorption of antifreeze proteins to the crystal surface. This causes the ice to grow as convex surface regions between adjacent adsorbed antifreeze proteins, thus lowering the temperature at which the crystal can visibly expand. The model requires that the antifreeze proteins are irreversibly adsorbed onto the ice surface within the hysteresis gap. This presupposition is apparently in conflict with several characteristic features of the phenomenon; the absence of superheating of ice in the presence of antifreeze proteins, the dependence of the hysteresis activity on the concentration of antifreeze proteins and the different capacities of different types of antifreeze proteins to cause thermal hysteresis at equimolar concentrations. In addition, there are structural obstacles that apparently would preclude irreversible adsorption of the antifreeze proteins to the ice surface; the bond strength necessary for irreversible adsorption and the absence of a clearly defined surface to which the antifreeze proteins may adsorb. This article deals with these apparent conflicts between the prevailing theory and the empirical observations. We first review the mechanism of thermal hysteresis with some modifications: we explain the hysteresis as a result of vapour pressure equilibrium between the ice surface and the ambient fluid fraction within the hysteresis gap due to a pressure build-up within the convex growth zones, and the ice growth as the result of an ice surface nucleation event at the hysteresis freezing point. We then go on to summarise the empirical data to show that the dependence of the hysteresis on the concentration of antifreeze proteins arises from an equilibrium exchange of antifreeze proteins between ice and solution at the melting point. This reversible association between antifreeze proteins and the ice is followed by an irreversible adsorption of the antifreeze proteins onto a newly formed crystal plane when the temperature is lowered below the melting point. The formation of the crystal plane is due to a solidification of the interfacial region, and the necessary bond strength is provided by the protein "freezing" to the surface. In essence: the antifreeze proteins are "melted off" the ice at the bulk melting point and "freeze" to the ice as the temperature is reduced to subfreezing temperatures. We explain the different hysteresis activities caused by different types of antifreeze proteins at equimolar concentrations as a consequence of their solubility features during the phase of reversible association between the proteins and the ice, i.e., at the melting point; a low water solubility results in a large fraction of the proteins being associated with the ice at the melting point. This leads to a greater density of irreversibly adsorbed antifreeze proteins at the ice surface when the temperature drops, and thus to a greater hysteresis activity. Reference is also made to observations on insect antifreeze proteins to emphasise the general validity of this approach. (C) 2005 Elsevier Inc. All rights reserved.