Hormones have no direct effect on target cells. It first binds to a receptor on the target cells, forming a hormone-receptor complex. This hormone-receptor complex causes a variety of alterations or responses in target cells. Here, let’s learn more about the different types of hormone receptors.
Table of Contents
- What are Hormone Receptors?
- Types of Hormone Receptors
- Regulation of Hormone Receptors
- Mechanism of Hormonal Action
- Frequently Asked Questions
What are Hormone Receptors?
Hormone receptors are big protein molecules found in target cells. There are thousands of receptors on each cell. An important aspect of receptors is that each receptor recognises only one hormone. Thus, a hormone may only function on a target cell only if the receptor for that hormone is present on the target cell.
Examples of hormone receptors:
- Glucagon receptors
- Androgen receptors
- Insulin receptors
- Progesterone receptors
- Thyroid hormone receptors
These hormone receptors enable the transmission of signals and also aid in the regulation of gene expression. Moreover, the receptor’s location indicates where the hormone must be physiologically active.
Types of Hormone Receptors
Lipophilic hormones (lipid-soluble hormones) like thyroid and steroids can pass through the nuclear and cell membrane. These hormones target the specific DNA sequences by diffusing into the cell. The lipophilic hormone receptors, upon binding with these hormones, undergo conformational changes and influence the transcription process.
Hydrophilic hormones (water-soluble hormones) are lipophobic in nature. Thus, they cannot diffuse through the cell membranes. Examples include catecholamines, glycoproteins, insulin, etc. The receptors for these hormones are localised on the plasma membrane
Thus, hormone receptors are found in the target cells’ cell membrane, cytoplasm, or nucleus, as follows:
- Cell membrane – The cell membrane contains receptors for protein hormones and adrenal medullary hormones (catecholamines)
- Cytoplasm – Steroid hormone receptors are found in the cytoplasm of target cells.
- Nucleus – Thyroid hormone receptors are found in the nucleus of the cell.
Cell surface receptors
These receptors are embedded in the cell membrane and bind to the extracellular molecules to initiate cell signalling. Cell surface receptors are specialised IMPs (integral membrane proteins) which allow communication between the extracellular space and the cell. The 2 main classes of cell surface receptors are
G Protein-coupled Receptors
They are a large group of proteins that can detect molecules present outside the cell and activate further cellular responses. They possess 7 transmembrane helices that can activate a G-protein upon binding. Example – Thyrotropin receptor.
Enzyme-linked Receptors
They possess both receptor and catalytic functions. Here, an extracellular ligand binds to cause an enzymatic action on the intracellular side. Example – Receptor tyrosine kinase.
Intracellular receptors
These receptors are found inside the cell. The two classes of intracellular receptors are as follows:
Nuclear Receptors
Receptors located within the cytoplasm are also termed nuclear receptors. In particular, these receptors are proteins present within the cells that are responsible for recognising thyroid and steroid hormones. They work with other proteins to regulate the homeostasis, gene expression and metabolism of the organism. They also have the ability to directly bind to the DNA. Example – Thyroid hormone receptor
InsP3 Receptor
Inositol triphosphate receptor acts as a calcium channel by activating the InsP3. These receptors have a broad tissue distribution and are abundantly found in the cerebellum. They are mostly found integrated into the endoplasmic reticulum. InsP3R are vital for the control of several physiological and cellular processes.
See more:Types of Receptors
Regulation of Hormone Receptors
Receptor proteins are not static cell components. Their number grows or falls under different conditions.
When a hormone is released in excess, the number of receptors for that hormone decreases owing to hormone binding to receptors. This is known as downregulation. The number of receptors rises during hormone deficit and this is termed as upregulation.
Hormone enters the target cell as a hormone-receptor complex via endocytosis and performs the activities. This whole process is termed internalisation. Some receptors are recycled after internalisation, whereas many are destroyed and new receptors are produced. It takes a long time for new receptors to form. Thus, the number of receptors diminishes, as the hormone levels rise.
Mechanism of Hormonal Action
Hormones have no direct effect on the target cell. It joins with the receptor to create the hormone-receptor complex. This complex performs hormonal activity by one of the following mechanisms:
1. By modifying the permeability of the cell membrane – Neurotransmitters function by modifying the permeability of the postsynaptic membrane in the neuromuscular junction or synapse.
2. By inducing intracellular enzyme activation – Protein hormones and catecholamines produce their effects via activating intracellular enzymes.
3. By influencing genes – Steroid hormones and thyroid hormones execute their function by acting on genes in the target cells
Keep exploring BYJU’S Biology to learn more such exciting topics.
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