Abstract
Antifreeze proteins (AFPs) enhance the survival of organisms inhabiting cold environments by affecting the formation and/or structure of ice. We report the crystal structure of the first multi-domain AFP that has been characterized. The two ice binding domains are structurally similar. Each consists of an irregular β-helix with a triangular cross-section and a long α-helix that runs parallel on one side of the β-helix. Both domains are stabilized by hydrophobic interactions. A flat plane on the same face of each domain’s β-helix was identified as the ice binding site. Mutating any of the smaller residues on the ice binding site to bulkier ones decreased the antifreeze activity. The bulky side chain of Leu174 in domain A sterically hinders the binding of water molecules to the protein backbone, partially explaining why antifreeze activity by domain A is inferior to that of domain B. Our data provide a molecular basis for understanding differences in antifreeze activity between the two domains of this protein and general insight on how structural differences in the ice-binding sites affect the activity of AFPs.
Highlights
Ice binding proteins (IBPs) are characterized by their ability to bind to one or multiple planes of ice crystals [1]
Antifreeze proteins (AFPs) are a class of IBPs that have been documented in a number of cold-tolerant fish [2, 3], insect [4], bacterial [5, 6], fungal [7], and plant [8] species, and this phenotype permits them to prevent and/or control ice crystal formation
It is thought that thermal hysteresis (TH) is caused by the Kelvin effect because AFP binding to the ice surface generates a micro-convex structure that is thermodynamically less favorable for water molecules to bind compared with a flat ice surface [11, 12]
Summary
Ice binding proteins (IBPs) are characterized by their ability to bind to one or multiple planes of ice crystals [1]. Antifreeze proteins (AFPs) are a class of IBPs that have been documented in a number of cold-tolerant fish [2, 3], insect [4], bacterial [5, 6], fungal [7], and plant [8] species, and this phenotype permits them to prevent and/or control ice crystal formation. When bound to the ice surface, AFPs depress the freezing point without significantly altering the melting point [9]. Small ice crystals recrystallize into larger ones to minimize the surface energy (i.e., Ostwald ripening). Ice recrystallization damages cell membranes, and is one of the most lethal stresses a cell
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