STRUCTURAL BASIS OF ANTIFREEZE PROTEIN ACTION

Abstract

Antifreeze Proteins (AFPs) modify the growth rates and orientations of ice crystal facets through attainment of the best structural match. The insect-type AFPs have ice binding surfaces (IBS) with regularly spaced binding intervals in two directions that engage the primary ice surfaces and reduce their growth rates preferentially. The primary ice surfaces are kinetically stable and have fixed orientations, since they are strongly bonded in two directions. The fish-type AFPs have one-dimensional helical and irregular globular IBSs that are either linearly extended with regular ice binding intervals, or have ice binding sites lacking spacing regularity. They adjust the orientations of the secondary ice surfaces, that have indeterminate face indices since they are strongly bonded in only one direction. The fish-type AFPs stabilize secondary ice surfaces and adjust their orientations by mimicking strong bonding directions, that are not present in the ice structure. The theory agrees with experimental observation and explains hitherto unexplained phenomena. The observed broad variation in prismatic and, more importantly, pyramidal ice crystallites produced in the presence of fish-type AFPs is explained, since these faces are secondary. The observed crystals triggered by most insect-type AFPs are disk shaped, because they consist of primary ice surfaces. The allegedly exceptional ice pyramids triggered by the insect-type TmAFP are the primary ice pyramid with fixed indices, and entirely different from the pyramids of the fish-type AFPs.