Hormones Definition
Hormones are very important for the proper functioning of the human body and all of its glands. The endocrine system in the human body is responsible for producing hormones. There are some glands that are present in this system that help in the production of certain chemicals. These chemicals are called hormones. With the help of these glands, these hormones are released into the lymphatic system or the venous blood and through that, they get transported to the target organ. When the hormones reach out to their target organs, they are then taken up by the organ and then the functioning starts.
Almost all hormones are proteins in nature. The protein hormones are also known as peptide hormones. The hormonal response in the human body is slow as compared to that of the nervous system. The hormones act on the hormone receptors. The hormones help in regulating, activating, and inhibiting the physiological processes. Some properties of hormones are:
These hormones are released through endocrine glands and are absorbed by the organs.
The hormones are highly specific in nature.
They are not able to initiate a reaction but they can influence the rate of reaction.
The hormones are required in very small amounts.
They help in coordinating and controlling the different organs and their functions in the body.
When there are certain stimuli, these hormones are released to tackle them.
They are not responsible for producing energy.
Mechanism of Hormone Action
The hormone when entered into the bloodstream can reach almost all the organs of the body. But as we read above, hormones are very specific in nature, so they only act on specific target organs for which they were released into the bloodstream. Two types of hormones are present that are lipid-soluble and lipid-insoluble. Testosterone and estrogen are lipid-soluble hormones whereas insulin and glucagon are lipids insoluble. As the plasma membrane of the cells is made up of lipid bilayer so the lipid-soluble hormones can easily pass through it whereas lipid insoluble hormones are not able to cross the plasma membrane. There are certain receptors that are known as hormone receptors that are present on the target organs. The hormones are able to produce their effect when they bind with the hormone receptors. They are also known as membrane-bound receptors as they are present on the cell membrane of the cell. A hormone-receptor complex is formed when the hormone binds to the receptor. These receptors are specific to one hormone and that is the reason behind the specificity of hormones. The metabolism of the target tissue and all the physiological functions are regulated by hormones.
Interaction of Hormone with Membrane Receptors
Some hormones do not enter the target organ and they just interact with the membrane-bound receptors. The protein and peptide hormones usually function in such a way. They are able to produce secondary messengers. These hormones come in contact with the external domain that is present on the extracellular surface of the cell. A hormone-receptor complex is formed when this hormone binds to the receptor. This brings about some conformational changes in the cytoplasmic part of the receptor. This in turn helps to produce secondary messengers. The secondary messengers can be calcium ions, cyclic AMP, etc. These messengers help in activating the enzyme receptors and then the whole enzyme system. When this system is activated when there is an acceleration of the biochemical reactions of the cell. The hormone that binds to the receptor is known as the primary messenger and the secondary messengers are the calcium ions and cyclic AMPs that are produced afterwards. Growth hormone receptors and steroid hormone receptors also function the same way.
Insulin Receptors
Insulin hormone binds to the extracellular receptors. Insulin has a receptor that is made up of a heterotetrameric protein that consists of four subunits. Two alpha and two beta subunits are present. These subunits protrude out of the cytoplasm. When the hormone binds to the receptor there are conformational changes in the beta subunits of the membrane. Tyrosine kinase enzyme is also present in the beta-subunits. This beta-subunit helps in adding phosphate groups to the beta-subunits and also to the specific tyrosine residues that are present in the cytoplasmic domain of the receptor. G-protein is also activated by some hormones and this, in turn, activates the phosphodiesterase enzyme. These mediators then help in releasing the calcium ions that act as other messengers. They help in bringing physiological changes and effects.
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Protein Hormones
Insulin, glucagon, thyrocalcitonin, pituitary hormones, and hypothalamic hormones are examples of protein hormones. They are also known as polypeptide hormones. This is the list of peptide hormones. We will learn about them below.
Insulin
Insulin is one of the peptide hormones. The production of human insulin takes place from the beta-cells of the pancreas. The insulin hormone is peptide or protein in nature. It helps in regulating glucose homeostasis in the body. It works by acting on different cells of the body such as hepatocytes, adipocytes, and muscle cells. By its action, these cells are then able to use up the glucose that is present in the blood and thus it helps in lowering the blood sugar levels in the body. Whenever there is an increase in the level of glucose in the body then the insulin hormone is released and this helps in:
Glucose is taken by cells of the body for respiration
Amino acids are taken up by the cells and thus help in the synthesis of protein.
Synthesis of fat by adipose tissue.
Glucose is taken up by the liver and muscles.
When the insulin hormone is not released in a proper amount then there can be problems in the proper functioning of the body. Diabetes mellitus is one of the disorders that is caused by insufficient amounts of insulin. As no insulin or very little insulin is produced by the body, the glucose is not taken up by the cells and remains as it is in the bloodstream. This causes problems to almost all the organs of the body, especially the eye and brain. In these cases, insulin from outside is administered to the patient to balance the body conditions. Liver and muscle cells are the target tissues of the insulin hormone.
Glucagon
Glucagon is one of the peptide hormones. This hormone is also released from the pancreas. Alpha cells of the pancreas help in releasing the glucagon hormone. These cells are also called A-cells. They also help in maintaining glucose levels in the body. This glucagon hormone works opposite to that of insulin. On one hand, where insulin lowers the body’s glucose levels, the glucagon hormone helps in increasing the glucose levels of the blood. This hormone acts on the liver cells and then helps them to release the glucose by breaking down the stored glycogen. This hormone is responsible for the stimulation of the gluconeogenesis process. This means that more glucose formation is achieved by this process. It also helps in promoting the conversion of other nutrients like amino acids into glucose in the liver. Liver and adipose tissue are the target tissues of the glucagon hormone.
Thyrocalcitonin
This hormone is released from the thyroid gland. This hormone is known as the thyroid hormone. The functions of the thyroid hormones are:
Proteins, carbohydrates, and fat metabolism are controlled by the thyroid hormones. This in turn helps in regulating the growth and development of the body.
These hormones are helpful in supporting the process of formation of red blood cells that is erythropoiesis.
They also help in maintaining the water and electrolyte balance in the body.
The physical and mental growth of all the tissues of the body is done with the help of thyroid hormones.
The basal metabolic rate of the body is also controlled by thyroid hormones. When they increase the metabolic rate of the body then more heat energy is released from the body.
Thyroid hormone also helps in the process of tissue differentiation in the body.
The growth of tails of tadpoles is also influenced by thyroid hormones.
FAQs on Protein Hormones
1. What are protein hormones?
Protein hormones are a class of hormones composed of amino acid chains, also known as polypeptides. They are water-soluble (hydrophilic) and cannot pass through the cell membrane. Instead, they bind to specific receptors on the surface of target cells to initiate a response, often involving a second messenger system inside the cell. They are synthesised in the endocrine glands and released directly into the bloodstream.
2. What are some key examples of protein and peptide hormones in the human body?
The human body uses numerous protein and peptide hormones for regulation. Key examples include:
- Insulin and Glucagon: Produced by the pancreas to regulate blood glucose levels.
- Pituitary Hormones: Such as Growth Hormone (GH), Thyroid-Stimulating Hormone (TSH), and Adrenocorticotropic Hormone (ACTH).
- Hypothalamic Hormones: Hormones like Gonadotropin-releasing hormone (GnRH) that regulate the pituitary gland.
- Erythropoietin: Produced by the kidneys to stimulate red blood cell production.
- Atrial Natriuretic Factor (ANF): A peptide hormone from the heart that helps regulate blood pressure.
3. What is the importance of insulin and glucagon for the body?
Insulin and glucagon are crucial for maintaining blood glucose homeostasis. Insulin is a hypoglycemic hormone, meaning it lowers high blood glucose levels by promoting the uptake of glucose by cells and stimulating its conversion into glycogen in the liver and muscles. Conversely, glucagon is a hyperglycemic hormone; it raises low blood glucose levels by promoting the breakdown of glycogen (glycogenolysis) in the liver, releasing glucose into the bloodstream.
4. How do organs like the heart and kidneys function as endocrine glands?
Besides their primary functions, some organs produce hormones. The walls of the heart's atria produce a peptide hormone called Atrial Natriuretic Factor (ANF), which is released when blood pressure is high to cause vasodilation and reduce pressure. The juxtaglomerular cells of the kidney produce a protein hormone called Erythropoietin, which stimulates the bone marrow to produce more red blood cells (erythropoiesis), especially in response to low oxygen levels.
5. How do protein hormones work differently from steroid hormones?
The primary difference lies in their chemical nature and mechanism of action. Protein hormones are water-soluble and act via cell surface receptors because they cannot cross the lipid cell membrane. This binding triggers a cascade of intracellular signals using 'second messengers' like cAMP. In contrast, steroid hormones are lipid-soluble, allowing them to pass directly through the cell membrane and bind to intracellular receptors in the cytoplasm or nucleus, directly influencing gene expression and protein synthesis.
6. Are protein hormones lipid-soluble?
No, protein hormones are not lipid-soluble. They are hydrophilic (water-soluble) because they are made of chains of amino acids. This chemical property prevents them from diffusing across the lipid bilayer of the cell membrane. Consequently, they must bind to specific receptor proteins located on the outer surface of their target cells to transmit their signal.
7. Why can't protein hormones like insulin be taken orally?
Insulin cannot be taken orally because it is a protein. If ingested, the digestive enzymes in the stomach and small intestine, such as pepsin and trypsin, would break it down into its constituent amino acids. This process, called proteolysis, would inactivate the hormone before it could be absorbed into the bloodstream and perform its function. This is why insulin must be administered via injection to bypass the digestive system.
8. What is the main difference between a protein hormone and a peptide hormone?
The main difference between protein hormones and peptide hormones is their size. Both are made of amino acid chains. Peptide hormones consist of short chains, typically fewer than 50 amino acids (e.g., Oxytocin, ANF). Protein hormones are much larger, consisting of longer polypeptide chains, often with complex folding (e.g., Insulin, Growth Hormone). Despite this size difference, their mechanism of action is fundamentally the same, as both are water-soluble and bind to cell surface receptors.
