Introduction
Protein is a complex nitrogenous complex with a high molecular mass that is a polymer of amino acids. This macromolecule is considered to be the main component of all life. It is not possible to imagine the existence of any animal without protein. All proteins are composed of carbon, hydrogen, oxygen, and nitrogen. There are seven types of proteins namely antibodies, enzyme proteins, hormonal proteins, structural proteins, storage proteins, transport proteins, and contractile proteins.
Hormonal protein is a type of protein whose molecules are formed by amino acids. These Protein hormones have an impact on the endocrine system of humans. Almost all hormones are protein in nature. The protein hormone is slow as compared to that of the nerve system in the human body. Like other proteins, this type of protein has a specific structure and some functions (1) & (2).
Hormonal protein
Some proteins help in regulating metabolism and cell function known as hormonal proteins. They are derived from amino acids. These proteins are very important for the functioning of the human body and all of its glands (3).
When a hormonal protein binds to a receptor on the surface of a cell, a second messenger is generated in the cytoplasm. This messenger regulates cell signal transport which leads to cellular responses (6).
Features of hormonal protein
1. Hormonal proteins are derived from amino acids.
2. They are large or small hydrophilic molecules.
3. These types of proteins are synthesized mainly in the rough endoplasmic reticulum.
4. Protein hormones freely travel through the bloodstream.
5. They bind to DNA to modify the transcription.
6. It is stored in vesicles.
7. They provide signals using the second messenger system.
8. Protein hormones are sometimes called peptide hormones (2).
Example of hormonal protein
- Growth hormones are produced by the pituitary gland.
- Follicle-stimulating hormones have a linked carbohydrate chain and are thus classified as glycoproteins.
- There are also a few other hormonal proteins such as insulin, glucagon, parathyroid hormone, oxytocin, etc. (3).
Structure
All hormonal proteins are made of 39 amino acids with serine at the amino or N terminal and phenylalanine at the carboxyl or C terminal. These types of proteins have a long peptide chain. They contain amino acid residues which are necessary to direct the folding of the hormone molecule into its active form. Some protein hormones interact with the intracellular receptors located in the nucleus through an endocrine process (1).
They are hydrophilic and lipophobic types. That means these types of proteins cannot pass the plasma membrane. There are various types of hormonal proteins. Their functions and structure are discussed below.
1. Growth hormone
- Growth hormone is a type of protein that consists of a polypeptide chain.
- These are somatotropin
- They are commonly known as the human growth hormone.
- It is a single polypeptide chain that is synthesized and stored by somatotropic cells.
- They are a type of mitogen. That is, it is specific only to the receptors on types of cells.
- Normal levels of human growth hormone will vary depending on age.
- It is a pituitary extracellular lobe hormone, which directly or indirectly promotes protein synthesis (3) & (4).
2. Prolactin
- Prolactin is a protein hormone that is stored by the pituitary gland also known as lactotropin.
- It is a protein best known to enable mammals, usually females, to produce milk.
- They are a protein hormone, which is encoded by the PRL gene.
- Prolactin causes the breasts to grow and make milk during pregnancy.
- Its molecular weight is about 22 kDa.
- They have a single polypeptide chain of 198 amino acids.
- Males and females have small levels of prolactin in the blood (2).
3. ACTH hormone
- It is a tropic hormone that is made of polypeptide chains.
- This type of protein hormone is produced and released by the anterior pituitary gland.
- It is also known as corticotropin or adrenocorticotropin.
- ACTH hormone is used as a diagnostic agent.
- It is made of 39 amino acids.
- Human ACTH has a molecular weight of 4540 Da.
- It controls the formation of the cortisol hormone.
- They are the primary regulator of mineralocorticoid biosynthesis (1) & (3).
4. Insulin
- Insulin is a protein hormone that consists of a double polypeptide chain.
- It is the primary hormonal protein of the pancreas.
- These are tiny simple proteins made up of 51 amino acids.
- Double polypeptide chains are attached by two disulfide bonds to form an insulin molecule.
- Its molecular mass is 5634 Da.
- They are a very old protein that was invented billions of years ago.
- This is the main anabolic hormone in the body.
- The circulation of insulin affects the synthesis of proteins in a variety of tissues.
- They convert small molecules in the blood into larger molecules inside the cells.
- It is a hetero dimer of an A-chain and B-chain. The Alpha-chain consists of 21 amino acids and the Beta-chain is made of 30 amino acid residues (3) & (5).
5. Glucagon
- Glucagon is a special type of protein hormone responsible for maintaining blood glucose levels.
- They are produced by the alpha cells of the pancreas.
- It stimulates the breakdown of glycogen.
- Glucagon mostly affects liver cells.
- This type of protein hormone is activated when glucose levels are low in the blood.
- Glucagon is a polypeptide chain with 29 amino acids.
- Its molecular mass is 3485 Da.
- They bind to a G protein-coupled receptor located in the plasma membrane of the cell (5) & (6).
6. Follicle-stimulating hormone
- FSH or follicle-stimulating hormone is a hormonal protein.
- It is a glycoprotein.
- This type of protein hormone is secreted from the gonadotropics of the anterior pituitary lobe.
- They are structurally a glycoprotein heterodimer.
- There are two polypeptide chains in FSH, alpha and beta.
- They contain about 96 amino acids in the glycoprotein alpha chain.
- The number of amino acids in the beta chain varies. Beta chains of FSH consist of about 110 amino acids.
- They are involved in the development of sex hormones.
- Its molecular mass is 30 kDa (2).
7. Oxytocin
- It is a protein hormone found in mammals.
- In the human body, it is excreted during female reproduction, especially during and after childbirth.
- This hormonal protein secreted from the pituitary gland cannot reach the brain; it causes different reactions in other parts of the body.
- Oxytocin stimulates uterine contractions and accelerates the childbirth process.
- There are only 9 amino acids present in the oxytocin hormone.
- It is first synthesized from the OXT gene as an inactive protein (3).
8. Thyroid-stimulating hormone
- The thyroid-stimulating hormone is produced by thyrotropin cells in the anterior pituitary gland.
- They bind to receptors in thyroid cells, causing these cells to produce thyroxine and triiodothyronine and release them into the bloodstream.
- This type of protein hormone stimulates the thyroid gland to secrete the hormone thyroxine, which has little effect on metabolism.
- It consists of a glycoprotein and two subunits, alphas and beta subunits.
- They have a 92-amino acid sequence in the alpha subunit and the beta chain contains 118-amino acid sequences.
- TSH concentration in children is generally higher than in adults (3).
9. Calcitonin hormone
- Calcitonin is also a hormonal protein that has a long polypeptide chain.
- It consists of 32 amino acids.
- They are secreted by parafollicular cells of the thyroid gland.
- These protein hormones are also known as thyrocalcitonin.
- It is a special type of protein hormone responsible for controlling the level of calcium in the blood.
- They indirectly increase the synthesis of the bone matrix.
- It inhibits osteoclast function and slows bone fractures (3).
10. Vasopressin
- They are protein hormones generated in the hypothalamus.
- It is also known as the antidiuretic hormone or ADH.
- Vasopressin raises blood pressure by increasing marginal vascular resistance.
- It induces the differentiation of stem cells into cardiomyocytes and stimulates the heart muscle.
- Vasopressin is a protein hormone composed of nine amino acids. These amino acids are attached by disulfide bonds (6).
Function or uses
1. Growth hormone
Increases body height, stimulates cell division and forms new cells. It increases muscle mass and decreases body fat (4).
2. Prolactin
Plays an important role in metabolism, regulation of the immune system, development of the pancreas, milk production, and ovulation.
3. ACTH hormone
Stimulates the secretion of glucocorticoid steroid hormones from adrenal cortex cells. It binds to the cell surface ACTH receptors.
4. Insulin
Increases the absorption of glucose in the cells, which helps the cells to absorb glucose. It also stores glucose in the liver and muscle cells by converting it into glycogen. The main function of insulin is to control the amount of glucose in the blood, especially when the amount of glucose in the blood increases (5).
5. Glucagon
Helps in increasing the amount of sugar or glucose in the blood. They improve liver function by repairing cells, stimulating the breakdown of amino acids, increasing blood flow to the kidneys, and normalizing calcium levels (6).
6. Follicle-stimulating hormones
In the development, growth, and formation of the reproductive system in the human body. It stimulates the egg growth of the ovaries and controls the menstrual cycle. This type of protein hormone controls sperm production in men.
7. Oxytocin
Acts as a chemical messenger in the brain. It also helps in childbirth by causing uterine contractions in the mature stages of pregnancy. Moreover, this protein has a role in mammary gland growth and milk secretion.
8. The thyroid-stimulating hormone
Helps in the normal growth of the thyroid gland, stimulates the thyroid gland, and regulates its secretion. It also helps in controlling iodine metabolism in the body.
9. Calcitonin
In reducing the concentration of calcium ions in the blood plasma. It stimulates the production and excretion of high levels of calcium in the blood. They also help to control calcium in the body as well as phosphate levels.
10. Vasopressin
Helps the water balance in the body and constricts the blood vessels. It decreases urine production and increases BP (3) & (6).
Q&A
1. What do hormonal proteins do?
Hormonal proteins provide the structural support for cells. They regulate some functions such as sleep and glucose concentration. These types of proteins are chemical messengers. They also bind to protein receptors on the cell.
2. What are hormonal proteins?
There are 7 types of proteins. Hormonal protein is one of them. A type of protein whose molecules are formed only by amino acid residues is known as hormonal protein. They are attached by disulfide bonds. These types of protein hormones have a long peptide chain.
3. What is produced in response to hormonal activation of g proteins?
The g proteins in turn activate a membrane-bound enzyme known as adenylyl cyclase.
4. What and glucagon are hormonal proteins that regulate glucose levels in the blood.
Insulin and glucagon are hormonal proteins that regulate glucose levels in the blood.
5. Which cells have lots of hormonal proteins?
Endocrine glands are a group of cells. They have lots of hormonal protein.
