By: Richard D. Cummings
Antifreeze glycoproteins (AFGPs) are named due to their unusual ability to inhibit ice formation (1). Antifreeze glycoproteins and proteins are also called ice structuring proteins or ISPs (2). AFGPs were originally discovered by DeVries and Wohlschlag in the blood of Antarctic fish in 1969 (3) [also see references in (4)]. Such glycoproteins are most often studied in fish from the polar seas, but related AFGPs are also in insects, amphibians and even plants (5). In animals their concentrations in blood and tissue may be quite high, in the range of 4-50 mg/mL. [Note: There are also antifreeze proteins (AFPs) that lack glycans, in which the normally glycosylated Thr is replaced by Arg (6).] The temperature of water in the polar oceans is ~1.8°C, thus it is important for animals to have mechanisms to prevent their blood from freezing. The AFGPs can depress the freezing point beyond the typical colligative properties of small molecules (7-9).
A typical glycan found on AFGPs from animals is D-Galβ1-3D-GalNAcα1-Ser/Thr, the common core 1 O-glycan disaccharide (T antigen) found in virtually all vertebrates (10). The glycosylation often occurs at Thr residues within the repeating units of the tripeptide Ala-Ala-Thr. There is evidence that AFGP genes in fish may have arisen from a portion of the gene for trypsinogen, in which there was a small region that was iteratively duplicated to produce many dozens of tandemly repeated segments (11), and also in a gene termed sculpin, derived from the lunapark gene, which is an ER protein (12). Thus, in some ways the AFGPs resemble mucins found in all animals. Some of the AFGPs range in size from 2.65-34 kDa. Different derivatives of the disaccharide have been synthesized that illustrate the specific roles of each sugar residue in its antifreeze activity (13).
While the mechanisms are unclear, it appears the AFGPs may bind to the surface of ice crystals, blocking further ice formation (13). So-called antifreeze glycoproteins have also been identified in plants, e.g., winter wheat, in which the glycoprotein is a chitinase that contains unusual glycans not well defined, but not typical N- or O-glycans which are removable by conventional enzymes (14). It is not completely clear as to the role of glycans in all forms of AFGPs, as for example, plant AFGP in which glycans do not seem to be required for its antifreeze activity. Synthetic antifreeze glycoproteins have been produced with various modifications by carbohydrate and while many forms are highly active, the antifreeze properties did not depend much on the specific type of sugar present or its linkage to protein (15). Nevertheless, in general the carbohydrate residues in different antifreeze glycoproteins appear to be structurally important in affecting the structure of water at different interfaces around ice, though the peptide domain may be more important in binding to the ice surface (8). Binding of AFGPs to ice is very high affinity and essentially irreversible (16). There is great interest in AFGPs as they have many potential applications, as food additives, and in cryopreservation as for tissues and organs (17).
Trehalose is an unusual glucose disaccharide that is non-reducing (α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside) (18). It is synthesized by insects, but also by some bacteria, fungi, plants and invertebrates. It has sweetener activity (19), and neuroprotective roles (20). Interestingly, trehalose also has anti-freeze properties, and is highly synthesized in freeze-tolerant insects, where it acts as a cryoprotectant and as a supercooling agent (21).
References:
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