Evidence Convergent Evolution Of Antifreeze Glycoproteins In Fish Biology Essay


Since millions of years ago, animals, insects, plants and many other organisms have gone through series of evolutions by selective pressure to gain unique mechanisms in order to survive and adapt in harsh environments at extreme temperatures. Nearly two-third of the earth's surface is compromised of sea water and the temperatures vary according to latitude, from between -2°C at polar regions to 36°C at the Persian Gulf (http://www.windows.ucar.edu/tour/link=/earth/Water/temp.html&edu=high ). Sea water temperatures within the polar regions are constantly below the freezing point of pure water, due to the high concentration of salinity in its surroundings (http://nsidc.org/seaice/intro.html). The effect of these very low temperatures is usually extremely harmful and deadly to cells of majority of organisms.

Many researchers have investigated and obtained numbers of results suggesting the different mechanisms present in many species of fish living in the polar regions which enables them to survive in the freezing water. By analysis of these fish blood plasma and their hemoglobin molecular structure, functional features and also phylogeny, it was showed that they have a slightly higher concentration of small ions and salts in comparison to fish in temperate regions. Due to this factor, and the presence of certain distinctive series of glycoproteins and proteins, enables the fish to have a defensive effect, or the serum freezing point depression against the sub-zero temperatures.

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Antifreeze glycoproteins (AFGPs) and antifreeze proteins (AFP) have been identified in various polar fish species and they have been evolved to a total of four classes of structurally distinct types of AFPs, categorized as At least four classes of structurally independent proteins have been identified: type I, alanine-rich, a-helical 3.3 to 4.5-kDa proteins, type II, cysteine rich globular proteins that contain five disulfide bonds, type III, approximately 6 kDa globular proteins, type IV, glutamate- and glutamine-rich proteins that contain a-helices but appear to be unrelated to other proteins and a single class of AFGP (Type I `antifreeze' proteins, Structure,activity studies and mechanisms of ice growth inhibition). Figure 1 summarizes the classification and key structural differences between AFPs and AFGP. These compounds were found unusual as they caused a freezing point depression far greater than predicted to come from their colligative properties alone. They were hence concluded to exhibit thermal hysteresis, a behavior which creates difference between melting point and freezing point which causes inhibition of ice crystals growth within their body. This property is advantageous for fish to survive in subzero waters as their blood and internal fluids are prevented from crystallization and at the same time, their cell membranes are protected from any damages caused by the cold.

Unexpectedly, most fish related phyletically do not possess the same AFP types and vice versa for unrelated fish. The best example is the phylogenatically distant Antartic notothenoid and northern cods having produced near-identical AFGPs through evolution. The study of their AFGPs gives molecular evidences that strongly support convergent evolution to have had occur independently to each species during a period of time million years ago.

Antifreeze glycoprotein widely refers to a family of at least eight compositionally related glycoproteins that are found at high proportion in proteins in blood plasmid of Antartic notothenoids and Arctic cods. In Antartic notothenoids, an AFGP is composed of (Ala-Ala-Thr)n repeating units with small sequence variations and a glycoside, disaccharide β-D-galactosyl-(1→3)-α-N-acetyl-D-galactosamine joined to the hydroxyl oxygen of Thr residues, such as shown in Figure 2. The glycopeptides are divided into eight classes, ranging by their relative molecular mass from small AFGP 2.6 kDa (n = 4) to large AFGP 33.7 kDa (n = 50). Another variation of these proteins is the difference in the composition of amino acids in small AFGPs on which the first Ala is replaced by Pro in some repeats. Antartic notothenoids AFGPs basically have a straightforward primary structure, although they differ slightly in molecular size and amino acids constitution. On the other hand, Arctic cods have glycoproteins surprisingly similar to notothenoids', eventhough with the occasional replacement of an Arg residue on Thr, causing the lack of disaccharides at the positions. Further different amino acids substitutions could be tolerated, such as found in Antarctic fish species Pleuragramma antarcticum. As there are now concluded that different AFGPs have different molecular sizes, just the generic term AFGP has produced many confusion to people as it is not clear whether it has the pure glycopeptides or a mixture of different glycopeptides. Also, due to the importance nowadays for the exact amino acids compositions to be specified in order to develop further molecular understanding and research, abbreviations such as AFGP-Pro and AFGP-Arg (if there is a substitution of amino acid in the Ala-Ala-Thr tripeptide backbone) are used.

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Other than Antactic notothenioid, AFGPs were also isolated from the rock cod, Gadus odac and other northern cods from the family Gadidae. As of date, the most studied AFGPs are from Dissostichus mawsoni and Trematomas borgrevinki from Antarctic and Boreogadus saida from the northern parts of the ocean. In both of the Antarctic fish, it was found that they have a total concentration of 25 mg•mL-1 of AFGPs with 75% of them consisting of the small AFGPs. From the hard works of Chen and Chang et al., the issue concerning AFGPs' evolutionary origins was lastly resolved. There were major similarities in structures of AFGPs found in two different species of fish, the Antarctic notothenioids (Family: Nototheniidae, Artedidraconidae, Bathydraconidae and Channichthyidae) and Arctic cods (Gadidae). From the Antarctic fish Dissostichus mawsoni, it was showed that the AFGP genes were derived from a pancreatic trypsinogen encoding gene through a special mechanism which does not include recycling of the existing gene proteins.

The distinct portion of AFGP gene which encodes the function of ice-binding arises from the enrolment and the repetition of a small region within the boundary between the first intron and second exon of trypsinogen gene. Replication, amplification and repeated duplication of this portion resulted to producing 41 tandemly repeated segments which have sequences nearly identical to trypsinogen at either end. This divergence between the small sequence of nothothenioid AFGP and trypsin genes signifies that the conversion to ice-growth inhibition gene from protein gene had occurred approximately when Antarctic Ocean started to go to the freezing point. This unique conversion is a good example on how an old protein gene brought into existence a new gene with an entirely new protein function.

The notothenioid AFGP gene was compared to another study on the sequence of Arctic cod Boreogadus saida and similarities were found in their polyprotein structures which have multiple AFGP coding sequences copies which are linked by small spacers that could be cleaved. Despite this, by detailed analysis, molecular evidences strongly argue on complete independent evolution of the AFGP genes from the two fish species. The evidences include a difference in their signal peptide sequences, difference in their mechanisms of processing their polyprotein precursors due to dissimilar spacer sequences on the linkage of the AFGP molecules and polyprotein, discrete codon bias for the AFGP tripeptide on its nine nucleotide sequence, and difference on the genomic AFGP loci in nototheninoid and cod. Hence, the higly-identical AFGPs on these two completely unrelated fish is one of the few examples of protein change by convergent evolution, as similar proteins were developed through similar environmental pressure. Also, the various lengths of AFGPs produced were found out not to be caused by cleaving large ones into smaller sizes or by small AFGPs splicing, but every AFGPs are clearly encoded as individual copies within the genes.

AFGPs have the property to gather at certain sides of the interface between ice and water and adjust the growth and shape of the ice crystals. Different AFPs have different ice binding site: AFP type I, II and III have similar flat surface, while for the newly researched AFP type IV is still unknown. At recent, the best studied AFP is type I AFPs from winter flounders. Studies have shown that the distance between the repeating polar residues on the peptide and the spacing of oxygen atoms on the deduced ice planes was a match for H-bonding groups to be fitted inside. Rearrangement of the strategically placed polar residues could result to loss of antifreeze properties (Ice-interaction mutants of type III antifreeze proteins).

The typical property of both AFPs and AFGPs is thermal hysteresis, on which they interact directly with the ice surface to lower the freezing point. At the presence of AFGP, the melting point depression was resulted on the colligative properties, that is depending on the amount of solute particles present but not the natural chemical properties of the solute in a solution. But the depression of the point where temperature was low enough for ice crystals to form (ice growth point) however was much greater than expected. This can be shown at Figure 3 which illustrates the measured thermal hysteresis of different AFGPs magnitudes. The magnitudes depend on the length polymer chain, overall the proteins' molecular masses.

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Other property of AFGPs associated with the inhibition of ice growth includes accumulation and absorption at ice crystals' specific faces. At 1 atmosphere below 0°C, ice would exist as ice 1h as that is the most stable structure compared to its other polymorphic forms. Ice 1h is a hexagonal lattice unit composed of four axes: α1, α2, α3 and c, and eight faces, two faces normal to c-axis (basal faces or c-faces) while the other six to the prism faces, such as shown in Figure 4. From researches done by Raymond et. al, they discovered that AFGPs allow limited growth on the basal plane but no growth on the prism faces. Accumulation of new layers on basal planes causes the formation of bipyramidal faces and large hexagonal pits from within the basal planes. For AFPs, the modification of ice crystals is a bit different to AFGPs, on which they inhibit growth down the α-aixs (Inhibition of growth of nonbasal planes in ice by fish antifreezes).

Molecular mass of different AFGPs have different enhancement of thermal hysteresis, such as shown in Figure 3 above. It is more important if AFGPs have longer polymers, if compared to the shorter ones. Although the small AFGPs are involved in most of the antifreeze circulation, larger AFGPs have a higher activity. From a study about thermal hysteresis on purified AFGPs belonging to Gadus ogac, it was showed that they could be divided into two classes. AFGPs with molecular masses more than 13kDa provided approximately three times higher values of hysteresis than smaller (molecular mass less tahn 10kDa) AFGPs. This result though cannot be fully compared with AFPs as they have different structures than AFGPs, but the basis of molecular masses also applies to the other types of AFPs.