There are many known
nonhomologous types of AFPs.
Fish AFPs Antifreeze
glycoproteins or AFGPs are found in
Antarctic notothenioids and
northern cod. They are 2.6-3.3 kD. AFGPs evolved separately in notothenioids and northern cod. In notothenioids, the AFGP gene arose from an ancestral trypsinogen-like
serine protease gene. • Type I AFP is found in
winter flounder,
longhorn sculpin and
shorthorn sculpin. It is the best documented AFP because it was the first to have its three-dimensional structure determined. Type I AFP consists of a single, long, amphipathic alpha helix, about 3.3-4.5 kD in size. There are three faces to the 3D structure: the hydrophobic, hydrophilic, and Thr-Asx face. The ability is partially derived from its many repeats of the Type I ice-binding site. • Type II AFPs (e.g. ) are found in
sea raven,
smelt and
herring. They are cysteine-rich globular proteins containing five
disulfide bonds. Type II AFPs likely evolved from calcium dependent (c-type)
lectins. Sea ravens, smelt, and herring are quite divergent lineages of
teleost. If the AFP gene were present in the most recent common ancestor of these lineages, it is peculiar that the gene is scattered throughout those lineages, present in some orders and absent in others. It has been suggested that lateral gene transfer could be attributed to this discrepancy, such that the smelt acquired the type II AFP gene from the herring. • Type III AFPs are found in Antarctic
eelpout. They exhibit similar overall hydrophobicity at ice binding surfaces to type I AFPs. They are approximately 6kD in size. • Type IV AFPs () are found in longhorn sculpins. They are alpha helical proteins rich in glutamate and glutamine. This protein is approximately 12KDa in size and consists of a 4-helix bundle. Plant AFPs are rather different from the other AFPs in the following aspects: • They have much weaker thermal hysteresis activity when compared to other AFPs. • Their physiological function is likely in inhibiting the recrystallization of ice rather than in preventing ice formation. Other beetles (genus
Rhagium) have longer repeats without internal disulfide bonds that form a compressed beta-solenoid (beta sandwich) with four rows of threonine residus, and this AFP is structurally similar to that modelled for the non-homologous AFP from the pale beauty moth. In contrast, the AFP from the spruce budworm moth is a solenoid that superficially resembles the
Tenebrio protein, with a similar ice-binding surface, but it has a triangular cross-section, with longer repeats that lack the internal disulfide bonds. The AFP from midges is structurally similar to those from
Tenebrio and
Dendroides, but the disulfide-braced beta-solenoid is formed from shorter 10 amino-acids repeats, and instead of threonine, the ice-binding surface consists of a single row of
tyrosine residues. Springtails (
Collembola) are not insects, but like insects, they are arthropods with six legs. A species found in Canada, which is often called a "snow flea", produces hyperactive AFPs. Other insects, such as an Alaskan beetle, produce hyperactive antifreezes that are even less similar, as they are polymers of sugars (
xylomannan) rather than polymers of amino acids (proteins). Taken together, this suggests that most of the AFPs and antifreezes arose after the lineages that gave rise to these various insects diverged. The similarities they do share are the result of convergent evolution.
Sea ice organism AFPs Many microorganisms living in
sea ice possess AFPs that belong to a single family. The
diatoms
Fragilariopsis cylindrus and
F. curta play a key role in polar sea ice communities, dominating the assemblages of both platelet layer and within pack ice. AFPs are widespread in these species, and the presence of AFP
genes as a multigene family indicates the importance of this group for the genus
Fragilariopsis. AFPs identified in
F. cylindrus belong to an AFP family which is represented in different taxa and can be found in other organisms related to sea ice (
Colwellia spp.,
Navicula glaciei,
Chaetoceros neogracile and
Stephos longipes and Leucosporidium antarcticum) and Antarctic inland ice bacteria (
Flavobacteriaceae), as well as in cold-tolerant fungi (
Typhula ishikariensis,
Lentinula edodes and
Flammulina populicola). Several structures for sea ice AFPs have been solved. This family of proteins fold into a
beta helix that form a flat ice-binding surface. Unlike the other AFPs, there is not a singular sequence motif for the ice-binding site. AFP found from the metagenome of the
ciliate Euplotes focardii and psychrophilic bacteria has an efficient ice re-crystallization inhibition ability. 1 μM of
Euplotes focardii consortium ice-binding protein (
EfcIBP) is enough for the total inhibition of ice re-crystallization in −7.4 °C temperature. This ice-recrystallization inhibition ability helps bacteria to tolerate ice rather than preventing the formation of ice.
EfcIBP produces also thermal hysteresis gap, but this ability is not as efficient as the ice-recrystallization inhibition ability.
EfcIBP helps to protect both purified proteins and whole bacterial cells in freezing temperatures.
Green fluorescent protein is functional after several cycles of freezing and melting when incubated with
EfcIBP.
Escherichia coli survives longer periods in 0 °C temperature when the
efcIBP gene was inserted to
E. coli genome.
EfcIBP has a typical AFP structure consisting of multiple
beta-sheets and an
alpha-helix. Also, all the ice-binding polar residues are at the same site of the protein. == Evolution ==