Types of Receptors: Definition, Location and Flow Chart

Receptors are proteins that attach to ligands and cause immune system responses. The cells release chemical messengers and another cell receives the messages with receptors. The binding of the messenger with the receptor triggers the downstream events that ultimately lead to the response by the cell. 

Keyterms: Receptors, Immune system, Cell, Protein, Biochemistry, Pharmacology, Molecule, Hormone, Hydrophobic

Read More: Difference between cerebellum and cerebrum

---

What are Receptors?

[Click Here for Sample Questions] 

In biochemistry and pharmacology, receptors are chemical structures that are composed of protein. They receive and transduce signals that are integrated into biological systems.

Receptors have two domains: 

(a) A ligand-binding domain that recognizes a specific ligand molecule. 

(b) An effector domain that undergoes conformational changes that produce some downstream effect.

---

Location of Receptors

[Click Here for Previous Year Questions]

Receptors are found inside the cell and on the cell. The membrane receptors are for hydrophilic messengers for e.g. peptide hormones. Such molecules are too big and also being hydrophilic, they cannot cross the hydrophobic lipid bilayer. So, receptors for such messengers are expressed on the cell surface.

Intracellular receptors are for hydrophobic messengers. For example Steroid hormones. Such molecules are smaller and hydrophobic. They can pass through the lipid bilayer and bind with receptors inside the cell.

Read More:

---

Types of Receptors

[Click Here for Sample Questions]

There are three types of receptors:

1. Intracellular Receptors
2. Cell Surface Receptors
3. Receptors in Immune System

Read More: Medulla Oblongata

---

Flow Chart: Types of Receptors

[Click Here for Previous Year Questions]

Intracellular Receptors

Intracellular receptors are located either in the cytoplasm or in the nucleus of the cell. The things that bind with these receptors are going to have to be either hydrophobic molecules like hormones or thyroid hormones because they can diffuse through the plasma membrane since they are hydrophobic. 

Also, molecules that are small enough to be able to fit through the membrane can do the same thing. 

Intracellular Receptor

Either hydrophilic ligands or molecules that are small enough to fit through the plasma membrane will go through and bind to the intracellular receptor.

Now, Let’s take an example of a molecule that diffuses through the plasma membrane and binds to the intracellular receptor to form a hormone-receptor complex. It will get translated into nuclei and bind to particular genes and act as a transcription factor(a regulatory protein that binds to DNA and affects transcription of specific genes).

It will help to regulate the transcription of particular genes and that will change our protein expression, which will change the behavior of the cell and what our cell is able to do. Intracellular receptors can alter gene expression directly without requiring the signal to be passed on to other receptors or messengers.

Intracellular is subdivided into the nuclear receptors.

Nuclear Receptor

• Nuclear receptors are located inside the cell, so it is known as an intracellular receptor.
• The ligands of nuclear receptors are lipid-soluble because they can cross the membrane and bind the receptor located inside the cell.
• These receptors are transcription factors. They regulate gene expression and protein synthesis.
• Nuclear receptors have two subfamilies of these receptors

Steroid Hormones 

Example: Glucocorticoids, mineralocorticoids, estrogen etc.

Vitamin D, Thyroid Hormone and Retinoic Acid

• Glucocorticoids and mineralocorticoids are located in the cytoplasm under resting conditions.
• Estrogen and progesterone receptors are found in the nucleus.
• And receptors for thyroid hormone and retinoic acid are attached to DNA even under resting conditions.
• Nuclear receptors have three main domains:

Ligand Binding Domain: Binds to Ligand.

• DNA binding domain: Binds to DNA
• Transactivation domain: Brings change in DNA conformation to initiate transcription.

Cell–surface Receptors

These receptors are located in the cell membrane. Cell surface receptors are integral (pass through the whole membrane) membrane proteins. These receptors are located on the membrane. Cell surface receptors bind to hydrophilic molecules. 

Example: Protein hormone, insulin, and glucagon. This receptor crosses the plasma membrane and converts extracellular signals to intracellular signals.

Because they are particular to individual cell types, they are also known as cell-specific proteins or markers. There are three primary domains in cell surface receptors.

1. Intracellular domain situated in the cell (transfers signal to the interior of cell).
2. External ligand-binding domain (Interact with ligand from outside of the cells).
3. A hydrophobic membrane is also called a crossing region (present between lipid bilayer that is why it is hydrophobic).

Cell-Surface receptors have three subtypes:

Ion-channel linked receptors:

• They are also known as transmitter gated ion channels or ionotropic receptors.
• Specific ions can pass via membrane as Ion- channel receptors connect to the ligand and open a channel.
• This type of cell-surface receptor contains a large membrane-spanning region to generate a channel.
• A conformational shift in the protein structure occurs when a ligand attaches to the extracellular area of the channel, allowing ions such as sodium, calcium, magnesium, and hydrogen to pass through. (refer to figure 3)

G Protein Receptors

• Ligands bind to G-protein-linked receptors, which activate a membrane protein called G-protein.
• A membrane ion channel or enzyme interacts with the activated G-protein. Despite the fact that all G-protein-linked receptors have seven transmembrane domains, each one has its own extracellular domain and G-protein-binding site.
• G-protein-linked receptor-mediated cell signaling is a cyclical process. The inactive G-protein can bind to a newly found binding site on the receptor before the ligand can bind.
• The shape change that occurs when the G-protein attaches to the receptor activates the G-protein, causing GDP to be released and GTP to be gathered up.
• The subunit and the subunit were then created from the G-protein subunits. As a result, it's possible that one or both of these G-protein fragments can activate additional proteins.
• The active G-protein subunit's GTP is hydrolyzed to GDP later, and the subunit is deactivated. The cycle begins again when the subunits recombine to create the inactive G-protein.

Enzyme Linked Receptors

• Cell surface receptors with intracellular domains that are coupled to an enzyme are known as enzyme linked receptors
• In some circumstances, the receptor's intracellular domain functions as an enzyme.
• Other enzyme-linked receptors contain a tiny intracellular region that interacts with an enzyme directly.
• A signal is sent through the membrane when a ligand attaches to the extracellular domain, activating the enzyme.
• The enzyme's activation initiates a series of actions within the cell that finally result in a response.

Receptor in Immune System

An immune receptor (or immunologic receptor) is a cell membrane receptor that attaches to a substance (for example, a cytokine) and triggers an immune response. Immune receptors are further subdivided into two types:

T-Cells Receptors

• Lymphocytes that mature in the thymus are called T-lymphocytes or T cells receptors.
• The T-cell receptor is a molecule located on T cells' surfaces that is responsible for recognizing antigens bound to major histocompatibility complex (MHC) molecules.
• In humans, alpha and beta chains (encoded by TRA and TRB, respectively) are found in 95% of T cells, while gamma chains are found in 5% of T cells. and delta () chains (encoded by TRA and TRB, respectively) (encoded by TRG and TRD, respectively).
• During development and in sick conditions, this ratio changes (such as leukemia).
• In a range of species, orthologues of the four loci have been discovered.

B-cell Receptors

• A transmembrane protein present on the surface of B cells is known as the B cell receptor.
• B cell receptors are type 1 transmembrane receptor proteins made up of immunoglobulin molecules that are commonly located on the surface of lymphocyte cells.

• B cell activation is regulated by the BCR through biochemical signaling and physical antigen acquisition from immunological synapses.

• Biochemical modules for receptor clustering, cell spreading, the creation of pulling forces, and receptor trafficking enable B cells to gather and seize antigens, leading to endocytosis and antigen presentation.
• B cells' mechanical activity is governed by a pattern of negative and positive feedback that governs the amount of antigen eliminated by altering the dynamic of BCR–antigen connections.
• The antigen-BCR connection is enhanced by grouping and spreading, resulting in sensitivity and amplification.
• Pulling forces, on the other hand, delink the antigen from the BCR, allowing the antigen-binding quality to be tested.
• The antigen-specific BCR is a critical sensor for B cell activation, survival, and development.
• When a B cell comes into contact with an antigen that binds to its receptor (its "cognate antigen"), it divides and proliferates, producing antibody-secreting plasma B cells and memory B cells.

• When the B cell receptor (BCR) binds with an antigen, it accomplishes two important tasks.

• Changes in receptor oligomerization are involved in signal transduction.

---

Cytokine & Granulocyte Receptor

[Click Here for Sample Questions]

Cytokines are specific extracellular ligands that stimulate biological responses in cell types by binding to, and activating, a family of structurally conserved cytokine receptors.

Cytokines of the hematopoietic system consist of interleukins (ILs), colony-stimulating factors (CSFs), interferons, erythropoietin (EPO) and thrombopoietin (TPO). 

Granulocyte is an immune cell that contains granules with enzymes released in infections, allergic reactions and asthma. Neutrophils, eosinophils, and basophils are the three forms of granulocytes.

Also Read:

---

Things to Remember

• Ion-channel, G-protein, and enzyme-linked protein receptors are the three types of cell-surface receptors.
• Intracellular receptors are found in the cell's cytoplasm and are activated by hydrophobic ligand molecules that can cross the plasma membrane.
• Ion channel-linked receptors bind to a ligand and open a membrane channel via which certain ions can pass.
• Lymphocytes refer to B-cells and T-cells. The complex development of lymphocytes involves primary and secondary organs, but B- and T-lymphocytes are created in the bone marrow and the thymus in the majority of cases.

---

Previous Year Questions

1. Bowman's glands are located in the...[NEET 2007]
2. A sagittal section of human brain is shown here. Identify at least two labels from A-D....[NEET 2013]
3. Which one of the following is the correct difference between rod cells and cone cells of our retina ?...[NEET 2008]
4. Afferent nerve fibre carries impulses from...[NEET 1992]
5. Alzheimer disease in humans is associated with the deficiency of...[NEET 2009]
6. Destruction of the anterior horn cells of the spinal cord would result in loss of...[NEET 2015]
7. Function of iris is to...[NEET 1993]
8. Good vision depends on adequate intake of carotene rich food : Select the best option from the following statements...[NEET 2017]
9. Injury localized to the hypothalamus would most likely disrupt...[NEET 2014]
10. Iris is part of...[NEET 1993]
11. Myelin sheath is produced by….[NEET 2017]
12. Receptor sites for neurotransmitters are present on :…..[NEET 2017]
13. A diagram showing axon terminal and synapse is given. Identify correctly at least two of A-D...[NEET 2013]
14. Injury to vagus nerve in human is not likely to affect...[NEET 2004]
15. A gymnast is able to balance his body upside down even in the total darkness because of..[NEET 2015]