The Evidence of Convergent Evolution of Antifreeze Glycoproteins in Fish
Since 1000000s of old ages ago, animate beings, insects, workss and many other beings have gone through series of developments by selective force per unit area to derive alone mechanisms in order to last and accommodate in rough environments at utmost temperatures. About two-third of the Earth ‘s surface is compromised of sea H2O and the temperatures vary harmonizing to latitude, from between -2?C at polar parts to 36?C at the Persian Gulf ( hypertext transfer protocol: //www.windows.ucar.edu/tour/link=/earth/Water/temp.html & A ; edu=high ) . Sea H2O temperatures within the polar parts are invariably below the stop deading point of pure H2O, due to the high concentration of salt in its milieus ( hypertext transfer protocol: //nsidc.org/seaice/intro.html ) . The consequence of these really low temperatures is normally highly harmful and lifelessly to cells of bulk of beings.
Many research workers have investigated and obtained Numberss of consequences proposing the different mechanisms present in many species of fish life in the polar parts which enables them to last in the freezing H2O. By analysis of these fish blood plasma and their hemoglobin molecular construction, functional characteristics and besides evolution, it was showed that they have a somewhat higher concentration of little ions and salts in comparing to angle in temperate parts. Due to this factor, and the presence of certain typical series of glycoproteins and proteins, enables the fish to hold a defensive consequence, or the serum freeze point depression against the sub-zero temperatures.
Antifreeze glycoproteins ( AFGPs ) and antifreeze proteins ( AFP ) have been identified in assorted polar fish species and they have been evolved to a sum of four categories of structurally distinguishable types of AFPs, categorized as type I, type II, type III and type IV and a individual category of AFGP. Figure 1 summarizes the categorization and cardinal 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 belongingss entirely. They were therefore concluded to exhibit thermic hysteresis, a behaviour which creates difference between runing point and freeze point which causes suppression of ice crystals growing within their organic structure. This belongings is advantageous for fish to last in subzero Waterss as their blood and internal fluids are prevented from crystallisation and at the same clip, their cell membranes are protected from any amendss caused by the cold.
Unexpectedly, most fish related phyletically do non possess the same AFP types and frailty versa for unrelated fish. The best illustration is the phylogenatically distant Antartic notothenoid and northern pods holding produced near-identical AFGPs through development. The survey of their AFGPs gives molecular groundss that strongly back up convergent development to hold had occur independently to each species during a period of clip million old ages ago.
Antifreeze glycoprotein widely refers to a household of at least eight compositionally related glycoproteins that are found at high proportion in proteins in blood plasmid of Antartic notothenoids and Arctic pods. In Antartic notothenoids, an AFGP is composed of ( Ala-Ala-Thr ) n reiterating units with little sequence fluctuations and a glycoside, disaccharide ?-D-galactosyl- ( 1>3 ) -?-N-acetyl-D-galactosamine joined to the hydroxyl O of Thr residues, such as shown in Figure 2. The glycopeptides are divided into eight categories, runing by their comparative molecular mass from little AFGP 2.6 kDa ( n = 4 ) to big AFGP 33.7 kDa ( n = 50 ) . Another fluctuation of these proteins is the difference in the composing of aminic acids in little AFGPs on which the first Ala is replaced by Pro in some repetitions. Antartic notothenoids AFGPs fundamentally have a straightforward primary construction, although they differ somewhat in molecular size and amino acids fundamental law. On the other manus, Arctic pods have glycoproteins surprisingly similar to notothenoids ‘ , eventhough with the occasional replacing of an Arg residue on Thr, doing the deficiency of disaccharides at the places. Further different amino acids permutations could be tolerated, such as found in South-polar fish species Pleuragramma antarcticum. As there are now concluded that different AFGPs have different molecular sizes, merely the generic term AFGP has produced many confusion to people as it is non clear whether it has the pure glycopeptides or a mixture of different glycopeptides. Besides, due to the importance presents for the exact amino acids composings to be specified in order to develop farther molecular apprehension and research, abbreviations such as AFGP-Pro and AFGP-Arg ( if there is a permutation of amino acid in the Ala-Ala-Thr tripeptide anchor ) are used.
Other than Antactic notothenioid, AFGPs were besides isolated from the stone pod, Gadus odac and other northern pods from the household Gadidae. As of day of the month, the most studied AFGPs are from Dissostichus mawsoni and Trematomas borgrevinki from Antarctic and Boreogadus Sayda from the northern parts of the ocean. In both of the Antarctic fish, it was found that they have a entire concentration of 25 mg•mL-1 of AFGPs with 75 % of them dwelling of the little AFGPs. From the difficult plant of Chen and Chang et al. , the issue refering AFGPs ‘ evolutionary beginnings was in conclusion resolved. There were major similarities in constructions of AFGPs found in two different species of fish, the Antarctic notothenioids ( Family: Nototheniidae, Artedidraconidae, Bathydraconidae and Channichthyidae ) and Arctic pods ( Gadidae ) . From the Antarctic fish Dissostichus mawsoni, it was showed that the AFGP cistrons were derived from a pancreatic trypsinogen encoding cistron through a particular mechanism which does non include recycling of the bing cistron proteins.
The distinguishable part of AFGP cistron which encodes the map of ice-binding arises from the registration and the repeat of a little part within the boundary between the first noncoding DNA and 2nd coding DNA of trypsinogen cistron. Replication, elaboration and perennial duplicate of this part resulted to bring forthing 41 tandemly repeated sections which have sequences about indistinguishable to trypsinogen at either terminal. This divergency between the little sequence of nothothenioid AFGP and trypsin cistrons signifies that the transition to ice-growth suppression cistron from protein cistron had occurred about when South-polar Ocean started to travel to the freeze point. This alone transition is a good illustration on how an old protein cistron brought into being a new cistron with an wholly new protein map.
The notothenioid AFGP cistron was compared to another survey on the sequence of Arctic pod Boreogadus Sayda and similarities were found in their polyprotein constructions which have multiple AFGP cryptography sequences transcripts which are linked by little spacers that could be cleaved. Despite this, by elaborate analysis, molecular groundss strongly argue on complete independent development of the AFGP cistrons from the two fish species. The groundss include a difference in their signal peptide sequences, difference in their mechanisms of treating their polyprotein precursors due to dissimilar spacer sequences on the linkage of the AFGP molecules and polyprotein, distinct codon prejudice for the AFGP tripeptide on its nine nucleotide sequence, and difference on the genomic AFGP venue in nototheninoid and pod. Hence, the higly-identical AFGPs on these two wholly unrelated fish is one of the few illustrations of protein alteration by convergent development, as similar proteins were developed through similar environmental force per unit area. Besides, the assorted lengths of AFGPs produced were found out non to be caused by spliting big 1s into smaller sizes or by little AFGPs splice, but every AFGPs are clearly encoded as single transcripts within the cistrons.