Name the Largest Family of Cell Surface Receptor?
Image: "Solid cell nests" by
Ken Berean. License: CC BY-SA 2.0
Receptors
Image: "External Reactions and the Internal Reactions of Receptors" by Laozhengzz. License: Public Domain
A receptor is a molecule that receives signals (chemical or hormonal) from outside the cell and is usually located on the jail cell surface. Receptors are proteins that undergo a conformational change upon attachment of their corresponding signaling molecule, which in turn induces a concatenation reaction (as well known as indicate transduction) within the cell leading to various cellular responses, including cell death.
The signaling molecule that binds to the receptor (as well known as a ligand) can exist a peptide, a hormone, neurotransmitter, drug, toxin, etc. Each receptor possesses ii functional domains: the recognition domain which binds ligands and the coupling domain which is involved in bespeak transduction.
Cell Surface Receptors
Cell surface receptors are transmembrane proteins embedded into the plasma membrane which play an essential role in maintaining communication between the internal processes inside the cell and various types of extracellular signals.
Such extracellular signals include hormones, cytokines, growth factors, neurotransmitters, lipophilic signaling molecules such as prostaglandins, and jail cell recognition molecules. When any of these ligands bind to their corresponding receptor, a conformational change is triggered which initiates an intracellular signaling pathway. Note that each ligand has its own specific cell surface receptor.
Image: "A schematic of a transmembrane receptor; E = extracellular space P = plasma membrane I = intracellular infinite" by Mouagip. License: CC Past-SA iii.0
Additionally, cell surface receptors are specific to individual jail cell types and thus are also known as cell-specific proteins. These receptors regulate a multitude of biological pathways required for prison cell growth, survival, differentiation, proliferation, equally well equally many other cellular processes. Jail cell surface receptors are responsible for most of the signaling in multicellular organisms.
Cell surface receptors have the following components/domains:
- The extracellular domain which binds ligands and is exposed to the outer surface of the cell; also known equally the recognition domain
- The membrane-spanning region made upwardly of hydrophobic protein molecules
- The intracellular domain which is in contact with the cytoplasm; too known as the coupling domain
Several factors govern the properties of these domains, including the size and extent of the domains, which may vary co-ordinate to the type of cell surface receptor.
Jail cell Surface Receptors: Types
Jail cell surface receptors are mostly classified into the following categories:
- Ligand-gated ion channel-linked receptors
- Enzyme-linked receptors
- 1000-protein-linked receptors
Ion channel-linked receptors
Ion channel-linked receptors besides referred to every bit ionotropic receptors, are responsible for regulating the transduction of chemical signals across the cell membrane in response to the chemic messenger (east.one thousand., neurotransmitter) binding. This class of receptor regulates the opening or endmost of ion channels that allow ions like Na + , K + , Ca 2+ , or Cl − , etc. to move across the plasma membrane.
Ion channels are pore-forming proteins also referred to as jail cell-membrane bound receptors. Ions pass downward their electrochemical gradient through ion channels without requiring ATP or metabolic energy. Ion channels are mostly institute on synaptic structures which are primarily (but not exclusively) involved in neuronal activities. Ion channels are, therefore, an important component of the nervous organisation because they mediate conduction across nerve synapses when activated by neurotransmitters.
Ion channels also play a vital role in exerting cellular response to toxins and venoms. Various biological processes involving fast changes in cells such as the contraction of cardiac , skeletal , and smooth muscles , activation of T-cells, and the release of hormones are likewise mediated through ion channels.
Downstream mechanism
When a ligand binds to ion-channel linked receptors, the extracellular domain of the receptor undergoes changes in its conformation, opening a channel across the plasma membrane. This allows specific ions (such equally Na + , Ca 2+ , H + , and Mg two+ , etc.) or other key molecules to pass through the open channel. The membrane-spanning region of these receptors helps to form a channel through which ions can laissez passer.
Ligands that bind to ion aqueduct-linked receptors include neurotransmitters and peptide hormones and the passing molecules are ions such as sodium (Na + ) and potassium (K + ).
The amino acids that occupy the membrane-spanning region of ion-channel receptors are hydrophobic in nature, making it easier for the membrane phospholipid fat acid chains to interact with them. On the other hand, amino acids lining the inside of these channels are hydrophilic, allowing easy passage of ions and water. The ion channels (or pores) remain open up simply for a limited time, afterward which, the ligand dissociates from the receptor making it bachelor to bind with a new ligand.
Non-chemical stimuli tin can also cause the ion channel receptors to human action in the same way. Such non-chemical stimuli include changes in electrical charge or mechanical disturbances of the membrane.
Enzyme-linked receptors
Image: "Enzyme-linked receptor construction (structure of IGF-1R) " by Laozhengzz. License: Public Domain
Enzyme-linked receptors are typically single-laissez passer transmembrane proteins that act as enzymes or are associated with enzymes. Enzyme-linked receptors accept both an extracellular binding site for chemical signaling and an intracellular domain whose catalytic action is controlled by the binding of an extracellular ligand and are thus likewise called catalytic receptors. There are 6 types of enzyme-linked receptors:
- Receptor tyrosine kinases which phosphorylate specific tyrosine residues on specific intracellular signaling proteins (EGFR); they bind to polypeptide growth factors which are responsible for controlling jail cell proliferation and differentiation
- Tyrosine-kinase-associated receptors which are enzymes that associate with intracellular proteins that have tyrosine kinase activity (Cytokines)
- Receptor-like tyrosine phosphatases which remove phosphate groups from tyrosines of their target intracellular proteins
- Receptor serine/threonine kinases which phosphorylate specific serines or threonines on associated gene regulatory proteins
- Receptor guanylyl cyclases which straight catalyze the production of cyclic GMP in the cytosol (natriuretic peptide receptor)
- Histidine-kinase-associated receptors which actuate a ii-component signaling pathway where the kinase phosphorylates itself on histidine residues (autophosphorylation) so immediately transfers the phosphate to a second intracellular protein; not present in animal cells
Downstream mechanism
When the enzyme-linked receptor or an enzyme associated with this blazon of receptor is activated, a multitude of intracellular pathways are finer regulated. The intracellular domains of enzyme-linked receptors are associated with an enzyme, straight collaborate with an enzyme, or itself is the enzyme.
Irrespective of large intracellular and extracellular domains of enzyme-linked receptors, a unmarried alpha-helical region of the peptide concatenation is responsible for forming the membrane-spanning region of enzyme-linked receptors. Activation of the enzyme takes identify as a result of the point transmitted through the aqueduct after a ligand binds to the extracellular domain of the receptor.
In almost cases, activation of the enzyme takes place because the receptor is dimerized upon binding of a ligand. The activated enzyme leads to an intracellular cascade of events executing the response.
G-protein linked receptors
Grand-protein-coupled receptors (GPCRs) are the largest cell surface receptors, composed of 7 transmembrane proteins in the plasma membrane. GPCRs are responsible for activating the trimeric membrane-bound G-proteins (GTP bounden proteins) which subsequently actuate either an ion channel (effector) or an enzyme in the cell membrane.
G-proteins proteins function as an intermediate transducer molecule that plays a vital role in regulating intracellular functions through a secondary machinery which is in turn activated past G-protein coupled receptors. Many unlike types of G-poly peptide-coupled receptors are known, such as the acetylcholine (Ach) receptor, β-adrenergic receptor, metabotropic glutamate receptors, sure olfactory receptors, receptors for peptide hormones, and rhodopsin (a photosensitive receptor).
Many neurotransmitters, neuropeptides, peptide hormones, as well as a number of others tin actuate G-protein coupled receptors. GPCRs are responsible for targeting diverse signaling pathways, including sensory perception such every bit sight, sense of taste, scent , and pain sensations. GPCRs are amongst the most important cell surface receptors, with almost half of the drugs we use exerting their activity by modifying these receptors.
Downstream mechanism
When the alpha subunit is reversibly bound to GDP, GPCR is inactivated. However, the Gdp can be exchanged for GTP with the help of a guanine-nucleotide exchange factor (GEF), allowing for receptor activation.
Afterwards a ligand binds to GPCR, the 1000-protein is activated. The activated G-protein, in turn, activates either an ion channel (effector) or an enzyme in the cell membrane. A cyclic series of events takes identify as a result of cell signaling with Yard-protein-linked receptors.
Source: https://www.lecturio.com/magazine/cell-surface-receptors-types-downstream-mechanisms/
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