Classification and genetic features
Peptide hormones are characterized by an enormous variety of structures. Based on molecular size, they are traditionally divided into two large groups:
- Short peptides. This group includes molecules with a small number of amino acid residues. A classic example is thyrotropin-releasing hormone, which is produced in the hypothalamus and is merely a tripeptide.
- Glycoproteins. These are large, complex protein molecules containing a carbohydrate component. Most hormones secreted by the anterior pituitary belong to this class.
An interesting genetic feature of this group of substances is that hormones with different functions can be encoded by a single common gene. All the information encoded in DNA is transcribed, and then during translation, a single polypeptide chain is assembled, which is subsequently cleaved into separate functional blocks.
Stages of processing: from preprohormone to active forms
The synthesis of peptide hormones never leads to the instantaneous appearance of an active substance. First, ribosomes produce a large molecule—a preprohormone. For it to turn into a full-fledged regulator, partial proteolysis is required, which takes place in several stages.
- Cleavage of the signal peptide. A special signal sequence (usually consisting of 26 amino acids) is located at the N-terminus of the molecule. Its main task is to direct the transport of the protein. Once this function is performed, the peptide is removed by enzymes.
- Primary cleavage. The remaining polypeptide chain is cleaved into two large fragments. For example, adrenocorticotropic hormone (ACTH), consisting of 39 amino acids, and $\beta$-lipotropin ($\beta$-LPH), comprising 42 to 134 amino acids, are formed from a common precursor.
- Further tissue-specific proteolysis. This stage proceeds differently depending on the tissue in which the hormone is located. For example, in the anterior and intermediate lobes of the pituitary gland, the fate of the molecules diverges:
- ACTH gives rise to $\alpha$-melanocyte-stimulating hormone ($\alpha$-MSH) and corticotropin-like intermediate lobe peptide (CLIP).
- $\beta$-LPH first forms $\gamma$-LPH (the precursor of $\beta$-MSH), as well as $\beta$-endorphin, which is then sequentially cleaved into $\gamma$-endorphin and $\alpha$-endorphin.
Insulin biosynthesis as a classic example
Let us examine the maturation process using insulin as an example, which demonstrates a clear spatial organization of processing.
Synthesis in the endoplasmic reticulum (ER) The process begins with the signal peptide directing the growing amino acid chain directly into the lumen of the ER. There, the assembly of preproinsulin is completed. Immediately afterwards, the signal sequence is cleaved off, and the molecule turns into proinsulin.
Processing in the Golgi apparatus The proinsulin molecule, consisting of 86 amino acids, is transported to the Golgi apparatus. Here, specific proteases come into play. They excise a fragment from the central part of the molecule, resulting in the formation of two separate substances: insulin itself (51 amino acids) and the C-peptide (31 amino acids).
Storage and secretion The finished molecules of insulin and C-peptide are packaged into secretory granules. It is important to note that they are present there in a strict equimolar ratio (1:1). For compact storage within mature granules, insulin associates into dimers and hexamers. Hormone release occurs via the fusion of granules with the plasma membrane—exocytosis into the extracellular fluid. Upon entering the bloodstream, the oligomeric insulin complexes rapidly dissociate into active monomers.
Briefly: answers to topical questions
What is the name of the common precursor protein from which ACTH and beta-lipotropin are formed?
The common precursor protein from which these hormones are formed is pro-opiomelanocortin (POMC). This is a polypeptide chain consisting of 265 amino acid residues. The synthesis of this protein is localized in the anterior and intermediate lobes of the pituitary gland, as well as in the intestine and placenta. During partial proteolysis, a number of active peptides are formed from POMC, including adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormone, and other derivatives.
Which trace element is necessary for the stabilization of insulin hexamers in secretory granules?
Zinc (Zn). Sources indicate that Zn stimulates insulin synthesis and is part of its structures in the secretory granules of $\beta$-cells. The granules of B-cells contain crystalloids—a complex of insulin with zinc; within the granules, insulin forms dimers and hexamers. Zn deficiency reduces pancreatic insulin incretion and its biological effect on hepatocytes.
Which specific anterior pituitary hormones belong to the class of glycoproteins?
The following anterior pituitary hormones belong to the class of glycoproteins:
- Thyroid-stimulating hormone (TSH) — also referred to as thyrotropin.
- Follicle-stimulating hormone (FSH) — also referred to as follitropin.
- Luteinizing hormone (LH).
Most anterior pituitary hormones are classified precisely as glycoproteins based on their size and chemical structure.
Which peptide hormones are produced in the hypothalamus?
The following groups of peptide hormones are produced in the hypothalamus:
- Effector hormones — vasopressin (antidiuretic hormone) and oxytocin. These are nonapeptides that are synthesized in the hypothalamus but stored and released into the blood from the posterior pituitary.
- Releasing hormones — include liberins (stimulate the synthesis of pituitary hormones) and statins (inhibit synthesis). Well-known representatives include:
- Corticotropin-releasing hormone
- Growth hormone-releasing hormone
- Thyrotropin-releasing hormone (thyrotropin-releasing factor)
- Gonadotropin-releasing hormone
To easily remember tissue-specific proteolysis in the pituitary gland, use the association of a "tree": the trunk is the common gene, the two large branches are ACTH and $\beta$-LPH, and the small leaves at the ends of the branches are endorphins and melanocyte-stimulating hormones (MSH).
Class and exam questions
What happens to insulin inside secretory granules?
The preprohormone contains an N-terminal signal peptide (usually 26 amino acids), which is necessary for directing the molecule into the lumen of the ER. After its cleavage, a prohormone is formed, which then undergoes further cleavage by specific proteases.
In what ratio are insulin and C-peptide released?
They are incorporated into secretory granules and released into the blood via exocytosis in equimolar amounts, i.e., in a strict 1:1 ratio.
What happens to the insulin molecule inside secretory granules?
Inside mature secretory granules, insulin molecules associate to form dimers and hexamers. They are stored in this form, and after secretion into the blood, these oligomers dissociate.
What else to study on this topic
- The mechanism of targeted transport of the polypeptide chain into the lumen of the endoplasmic reticulum.
- Specific proteases of the Golgi apparatus involved in proinsulin processing.
- The tissue specificity of enzyme systems in the anterior and intermediate lobes of the pituitary gland.
- The molecular structure of insulin dimers and hexamers in secretory granules.
- The role of the C-peptide and its clinical significance in assessing insulin secretion.
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