Oxytocin is an extremely important peptide hormone that is involved in the reproduction of virtually all vertebrates. Oxytocin is a nonapeptide, meaning it has a total of nine amino acid residues which include Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly. [2] There is a disulfide bond between the cysteine residues at positions 1 and 6. [1]As a result, oxytocin is made up of a cyclic 6 amino acid part, and an alpha mediated three residue tail with a COOH terminal. [1] Oxytocin is always in association with its much larger inactive binding protein, neurophysin I when in neurosecretory granules which have an acidic pH of approximately 5.6. Thus when released into the blood which has a higher pH of approximately 7.4, the complex often dissociates. [1]
Figure 1: A three dimensional representation of oxtytocin (space filling molecule to the left of the figure) and neurophyisin (ribbon molecule to the right of the figure).
The Oxytocin receptor is a member of the rhodopsin type (class I) GPCR family. A receptor becomes active when there is an orientation change in the transmembrane domains 3 and 6 which reveals G protein binding sites for oxytocin. [1] Residues in both the transmemrane domains and extracellular domains are involved in ligand binding. The typical G protein receptor has seven trans membrane alpha helicies, three extracellular loops, and three intracellular loops. Oxytocin binds its cyclic portion of the molecule to one extracellular loop, and its 3 amino acid linear chain part to another extracellular loop. [2]
The gene encoding both oxytocin and neurophysin I is in humans is found on chromosome 20p13. The first exon of the protein contains the nine amino acid oxytocin peptide, Part of the first, second, and third exons of the gene encode neurophysin I. [1] When translated, this protien is called the pre-pro hormone and is stored in secretory granules of the paraventricual neurons of the hypothalamus. The function of neurophsyin involves the proper targeting, packaging, and storage of Oxytoxin within its nuerosecretory granules before it is released in the bloodstream. [1]
Figure 2: The coding region of oxytocin gene which codes for the nonapeptide oxytocin and its corresponding binding protein neurophysin. (Gimpl and Fahrenholz, 2001)After searches of Oxytocin-neurophysin 1 hormone for three different species were performed, online software including BLAST (Basic Local Alignment Search Tool) was used to compare the oxytocin-neurophysin I hormone protein sequences of the three species. The results are as follows:
Click the above image to enlarge!
Key:
* Identical Residues
: Conserved Substitutions
. Semi-Conserved Substitutions
The above image investigates the similiarity of oxytocin and neurophysin between each of three species, Homo sapiens (human), Rattus norvegicus (rat), and Bos taurus(pig). The score in the clustalw score table represents the percent identity between oxytocin in each species. Thus, as shown from the image, evolution of oxytocin in humans differs equally when compared to the rat and pig. The high score of 88 shows that humans are closely related to both pig and rat, which is not surprising considering they are both mammals. However, their oxytocin-neurophysin proteins do differ in structure. Rat and pig are more closely related to each other then they are to humans which can be illustrated through their score of 90, which means a 90% similarity. This can also be shown by the below diagram which was obtained from BLAST, and is a phylogram illustration the phylogentic relationship between the three species, showing that rat and pig are more closely related than humans. (Click on the image to enlarge)
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Nonapeptide-1 can prevent melanin synthesis and unwanted pigmentation by preventing activation of the tyrosinase, thus allowing for a better control over skin tone. Nonapeptide-1
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