Why Don’t Fish in the Arctic Ocean Freeze?
Scientists have discovered why fish don’t freeze in the Arctic Ocean. Together with cooperation partners from the U.S., the researchers surrounding Prof. Dr. Martina Havenith (Physical Chemistry II of the RUB) describe their discovery in a so-termed Rapid Communication in the prestigious American chemistry journal, the Journal of the American Chemical Society (JACS).
Temperatures of minus 1.8 degree C should really be enough to freeze any fish: the freezing point of fish blood is about minus 0.9 degree C. How Antarctic fish are able to keep moving at these temperatures has interested researchers for a long time. As long as 50 years ago, special frost protection proteins were found in the blood of these fish. These so-called antifreeze proteins work better than any household antifreeze. How they work, however, was still unclear.
The Bochum researchers used a special technique, terahertz spectroscopy, to unravel the underlying mechanism. With the aid of terahertz radiation, the collective motion of water molecules and proteins can be recorded. Thus, the working group has already been able to show that water molecules, which usually perform a permanent dance in liquid water, and constantly enter new bonds, dance a more ordered dance in the presence of proteins – “the disco dance becomes a minuet” says Prof. Havenith.
The subject of the current investigations was the antifreeze glycoproteins of the Antarctic toothfish Dissostichus mawsoni, which one of the American partners, Arthur L. Devries, had fished himself on an Antarctic expedition. “We could see that the protein has an especially long-range effect on the water molecules around it. We speak of an extended dynamical hydration shell”, says co-author Konrad Meister. “This effect, which prevents ice crystallization, is even more pronounced at low temperatures than at room temperature”, adds Prof. Havenith.
Antifreeze proteins (AFPs) or ice structuring proteins (ISPs) refer to a class of polypeptides produced by certain vertebrates, plants, fungi and bacteria that permit their survival in subzero environments. AFPs bind to small ice crystals to inhibit growth and recrystallization of ice that would otherwise be fatal. There is also increasing evidence that AFPs interact with mammalian cell membranes to protect them from cold damage. This work suggests the involvement of AFPs in cold acclimatization.
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.
Type I-hyp AFP (where hyp stands for hyperactive) are found in several righteye flounders. It is approximately 32 kD (two 17 kD dimeric molecules). The protein was isolated from the blood plasma of winter flounder. It is considerably better at depressing freezing temperature than most fish AFPs.
Type II AFPs 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’s 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 III AFPs likely evolved from a sialic acid synthase gene present in Antarctic eelpout. Through a gene duplication event, this gene—which has been shown to exhibit some ice-binding activity of its own—evolved into an effective AFP gene?
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. Its only posttranslational modification is a pyroglutamate residue, a cyclized glutamine residue at its N-terminus. Scientists at the University of Guelph in Canada are currently examining the role of this pyroglutame residue in the antifreeze activity of type IV AFP from the longhorn sculpin.