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Somatotropin and Genetic origin

Somatotropin is a polypeptide hormone produced by the somatotropic cells of the pituitary gland. Growth hormone, a protein circulating in the blood. The Human Growth Hormone has its origin on chromosome 17. There it gets encoded by the Growth hormone 1 gene along with four other related genes. Three of these genes are encoding human chorionic somatomammotropin, which is closely related to somatotropin. They are all in the same transcriptional orientation.

Functions

Somatropin plays an important role in physiological environments such as: increasing muscle mass, reducing fat mass, providing the energy necessary for tissue growth, maintaining the right level of glucose and lipids ... and the development of the individual's body ^[1]. It acts directly on a cell surface or indirectly ^[1]. In the second case, somatropin stimulates tissues such as the liver, which in turn allows the synthesis and secretion of IGF-1, thus enabling the development of cell growth, tissue, bone and thus the linear growth of the individual ^[2].

Structure

Somatotropin has two major isoforms. The predominant form is composed out of 191 amino acids and has a molecular weight of 22 kDa. The second isoform lost amino acids 32 till 46 due to alternative splicing of the pre-mRNA and therefore has a molecular weight of 20 kDa.

The primary structure, corresponding to a sequence of amino acids, of the predominant somatotropin is the following :

Somatotropin does not exist as a linear chain of amino acids - it twists and folds on itself, forming the secondary structure. The protein consists of four antiparallel aligned α-helices. The first helix starts at the 6th amino acid, which is a leucine and ends with the 37th amino acid proline. It is separated from the other three helices after the 37th position. The 38th and 39th amino acids, which are lysine and glutamic acid are spliced out of the protein and therefore disconnects the first helix from the second one. The second helix starts at position 72 till 92, the third from 106 till 128 and the fourth helix from 155 until 184.

From the secondary structure, we obtain the tertiary structure, which corresponds to the 3D structure adopted by all the alpha helixes. The structural maintenance is ensured by electrostatic, hydrophobic, hydrogen and/or covalent interactions with cysteine 53 and cysteine 165 that form a disulphide bridge as well as cysteine 182 with cysteine 189.

Structural highlights

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