Antifreeze protein - from basic to application - S. Tsuda

Antifreeze protein (AFP, including AFGP) is an extraordinary biomolecule that binds to single ice crystal to inhibit its growth (i.e., thermal hysteresis activity) and to lipid bilayer to prolong the cell life time (cell-preservation activity). Prof. Arthur DeVries first discovered AFGP from Antarctic fishes in 1969 [1], then AFP I - III had successively been identified from fishes living in nothern polar sea area till 1984 [2, 3, 4]. Tsuda started AFP exploration in 2000, and has been shown that Japanese organisms also synthesize AFP that are similar but not identical to the polar AFP species {1, 2, 3, 4, 5]. It is interesting that AFP is contained in many edible fishes available in Japanese food markets [6], from which Tsuda prepared the highly purified AFP samples that contain neither cations nor buffer detergents. Insect AFPs [7] and fungal AFPs [8] were also identified from Japanese organisms. Regardless of the difference in primary and tertiary structure, AFPs exhibit the thermal hysteresis and the cell-preservation ability. The following shows the AFPs identified from Japanese organisms.

AFP I			Mw = 3.3 kDa, 40 residues, pI = 4.6, (T-X10)n, binding to 2nd prism & pyramidal planes	1, 2, 3, 4, 5
	Barfin plaice Liposetta pinnifasciata	alpha-helix	THmax = 3 degC
AFP IIa			Mw = 13 kDa, 130 residues, pI = 5.1, S-S bridges, binding to wide 2nd prism plane	1, 2, 3, 4
	Longsnout poacher Brachyopsis rostratus	C-type lectin form	THmax = 3 degC
AFP IIb			Needs 1 mol of Ca2+, Mw = 14 kDa, 128 residues, pI = 4.1, five S-S Bridges, binding to 1st prism plane	1, 2
	Japanese smelt Hypomesus nipponensis	C-type lectin form	THmax = > 0.5 ℃
AFP III			Mw = 6.5 kDa, 70 residues, pI = 4.8, Pretzel fold, binding to 1st prism & pyramidal planes	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
	Notched-fin eelpout Zoarces elongatus Kner	globular form	THmax = 2 degC
AFGP		Gal-GalNac+(AAT)n	Mw = 2.6〜33 kDa, 12〜156 residues, binding to 1st prism plane	1, 2
	Saffron cod Eleginus gracilis	PPII helix	THmax = 2 degC
Insect AFP			Mw = 10 kDa, 109 residues, 12-residue tandem repeats, 10 S-S bridges, binding to whole ice planes	1
	Stag beetle Dorcus hopei binodulosus	beta-helix	THmax = 7 degC
Fungal AFP			Mw = 22 kDa, 223 residues, pI = 9.6, binding to basal and prism planes	1, 2, 3, 4, 5, 6, 7
	Snow mold fungi Typhula ishikariensis	irregular beta-helix	THmax = 0.2 degC

The following shows photomicroscope snapshots of (A) an ordinary ice block and (B) a frozen AFP-solution. The (A) generally contains numerous bubbles, and often snapshotted like this picture. The (A) expands with time and phisically destroys the inside of the frozen material. In contrast, AFP solution becomes an assembly of numerous tiny ice crystals (B) during the freezing storage. Note that ice crystals are shaped into bipyramids in (B) as the AFP concentration is high (0.2 mg/ml). The ice crystals like in (B) are hardly sticked together by AFP-coating, and that they have less power to destroy an inside texture of cryopreserved materials. Such an AFP-induced effect was verified by simple experiments using a colored water. When it freezes the water containing red-colored ink in a home freezer, the inks are gradually excluded from the ice block. This is because ink particles cannot join the ice-crystal-growth that only accepts the waters. The ice block is formed from the portions near the vessel since it is first cooled, and that inks are concentrated into the unfrozen portion (mostly the center of the vessel), as sketched like (C). This is called "freeze-concentration phenomenon" [6]. In contrast, an entirely red-colored ice is created for an AFP solution sketched like (D). The (E) ~ (H) show the snapshots of the frozen colored ice prepared in a home freezer. Each contains 0, 0.01, 0.02, and 0.05 mg/ml of type I AFP, respectively. The efficiency of such "freeze-concentration-inhibition" might be correlated with the TH value of AFP.

Fabrication of "Feeze-tolerant Gel" is one of the examples of AFP application. The gel consists of polymer network with waters to make a desired viscoelasticity. The jelly, noodle, dough, boilded egg, cakes, and tofu are the well-known examples, which are mostly intorelant to freeze-thawing process, as the freezing induces the ice block formation in the gel to collapse its inside texture. For example, 1-hr freezing storage with a home freezer completely destroyes the structure of 0.5% (w/v) agarose gel (A, left picture), and nulifies its holding ability of waters that flow out concomitantly with thawing. In contrast, the AFP-containing gel is capable of holding its structure (A, 0.01-0.05 mg/ml). In the frozen gel, AFPs will bind onto the ice crystals to inhibit their growth and uniting, leading to preservation of the polymer network during the freeze-thawing process. It is significant only 0.02 mg/ml of AFP is enough for the preservation. The AFP-containing gel is also utilized for creation of "Porous Material", whose examples are the filters, membranes, absorbers, insulators, etc [7]. The first step is to prepare a solution containing gelatin, ceramics powder, and AFP (B, left). After creation of the gel, it is placed on a frozen plate to induce the unidirectional freezing. This procedure forms numerous needle-shaped ice crystals in the frozen gel, and elongates them from bottom to top, without chaging their size and thickness (B, middle). It is difficult to elongate ice needles without AFP. The final step is to sinter the frozen gel containing numerous ice needles at 1,000-1,500 degC. The gelatin and AFP are burning out in this step, while the ceramics are remained to form a porous material that contains extremely fine and unidirectionally aligned dendritic pores (B, right).

AFP was identified from fishes, insects, plants, bacteria, and fungi in the last 4 decades. We solved the first crystal structure of a fungal AFP, and determined which of its surfaces binds to ice [8]. Our traget was a 223-residue AFP from snow mold fungus,Typhura ishikariensis (photos a and b), which was discovered in 1930s from our place, "a plane of Ishikari" in Hokkaido, Japan. My collaborator (Hoshino, T) characterized the primary sequence of this AFP and named it TisAFP in 2003. It appeared that TisAFP forms a semipear-shaped overall structure as shown in (c), the right side of which is the putative ice-binding site (IBS). The most striking feature of TisAFP was that it can bind to both prism and basal planes of ice crystal similarly to insect AFP, while TisAFP had irregularly arrayed surface-bound waters (d) differently from the insect AFP (f). These waters were thought to share the positions of ice lattice at the moment AFP attaches onto ice. The following (f) and (g) compare the backbone architecture with illustration between insect AFP and TisAFP. The beta-loops are regularly formed from N to C-termini in insect AFP (f). In contrast, the terminal loops are side by side within an irregularly formed parallel beta-helix of TisAFP as illustrated by the spectrum colors showing blue next to red (g). Additionally, the six helical loops of TisAFP are of different lengths. This unique architecture of TisAFP will locate the surface-bound waters irregularly. The structural features of TisAFP may presumably be adopted in the other fungal AFPs.

In 1990, Prof. Rubinsky and colleagues in the University of California found that AFPs derived from polar fish serum showed a cell protection function during a hypothermic exposure (+4 degree C). Thereafter, researchers have been performing the hypothermic storage of germ cells, oocytes, and organs with fish AFPs at the temperature just before freezing. However, a scarcity of the fish AFPs that had to be purified from blood sera of polar fishes has been made it difficult to examine such techniques more closely. We found that AFPI-III and AFGP are also expressed in mid-latitude fishes captured in Japan, and their muscle can be utilized as a source material to purify a massive amount of AFP. Here we immersed an insulin production cells (pancreatic islet cell) into a storage solution (Euro-Collins) containing the fish-muscle-derived AFPI-III and AFGP, and examined how the survival rate of the cell changed with preservation time at 4 degree C. Significantly, it was shown that approximately 60% of pancreatic islet cells could alive even after 120 hrs of preservation period in the solution containing 10 mg/mL of AFP that was tentatively categorized into type I [9]. The preserved pancreatic islet cells held the insulin secretion ability. For this type I AFP an alha-helical formation was assumed (Fig. a). A preferable adsorption of the fluorescent type I AFP to the lipid bilayer of the pancreatic islet cells was suggested by a photomicroscope observation (Fig. b). This is consistent with the previous indication that fish AFP exerts the membrane-protection ability.