DeLuca C I, Chao H, Sönnichsen F D, Sykes B D, Davies P L
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
Biophys J. 1996 Nov;71(5):2346-55. doi: 10.1016/S0006-3495(96)79476-6.
Mutation of residues at the ice-binding site of type III antifreeze protein (AFP) not only reduced antifreeze activity as indicated by the failure to halt ice crystal growth, but also altered ice crystal morphology to produce elongated hexagonal bipyramids. In general, the c axis to a axis ratio of the ice crystal increased from approximately 2 to over 10 with the severity of the mutation. It also increased during ice crystal growth upon serial dilution of the wild-type AFP. This is in marked contrast to the behavior of the alpha-helical type I AFPs, where neither dilution nor mutation of ice-binding residues increases the c:a axial ratio of the ice crystal above the standard 3.3. We suggest that the ice crystal morphology produced by type III AFP and its mutants can be accounted for by the protein binding to the prism faces of ice and operating by step growth inhibition. In this model a decrease in the affinity of the AFP for ice leads to filling in of individual steps at the prism surfaces, causing the ice crystals to grow with a longer c:a axial ratio.
III型抗冻蛋白（AFP）冰结合位点残基的突变不仅如无法阻止冰晶生长所示那样降低了抗冻活性，还改变了冰晶形态，产生了细长的六方双锥体。一般来说，随着突变严重程度的增加，冰晶的c轴与a轴之比从约2增加到超过10。在野生型AFP连续稀释的冰晶生长过程中，该比值也会增加。这与α-螺旋I型AFP的行为形成显著对比，在I型AFP中，冰结合残基的稀释或突变都不会使冰晶的c:a轴比增加到标准的3.3以上。我们认为，III型AFP及其突变体产生的冰晶形态可以通过蛋白质与冰的棱柱面结合并通过台阶生长抑制来解释。在这个模型中，AFP对冰的亲和力降低导致棱柱表面的各个台阶被填满，使冰晶以更长的c:a轴比生长。
