RECEPTOR AND ITS TYPES:

A receptor is defined as a protein molecule with in a cell or on the cell surface to which a substance like hormone, a drug, or an antigen can bind to it , and cause a change in the cell activity.(1)

Types of receptors:- (2)

• Receptors acting as enzymes: These type of receptors generally span the cell membrane once, and in response to the binding of ligand, usually there is an increase in the phosphorylation ofthe intracellular proteins, for instance the tyrosine residues on vital regulatory components (sometimes, serine or threonine residues are also phosphorylated). (2)
• Receptors that activate the transmembrane ion channels. These channels are called 'ligand-gated ion channels'receptors.
• Receptors that use as their transducer as 'G-protein'. these receptors often can couple with many different G-proteins, which in turn are not particularly faithful to single receptor. The results at the end are effected by the G protein stimulating one of the variety of pathways, for instance adenyl cyclases and phospholipases, or even ion channels that permit only some of the ions to enter (or) leave the cell. (2)
• the most interesting of the lot - receptors that are located within the cell called as (transcription factors) that once bound to the ligand,they either increase or decrease the process of DNA transcription, either by binding DNA or by modulating the effects of histone proteins. (2)

Functions of a typical receptor:

* Receives the signal and

* Transduces a signal to

* an effector mechanism

* Receptors are proteins that interact with extracellular physiological signals and

* convert them into intracellular effects . (2)

Ligand-gated ion channels (LGICs) are oneof the type of ionotropic receptor or channel-linked receptor. They are group of transmembrane ion channels which are opened or closed in response to a binding of a chemical messenger (may be , a ligand), or a neurotransmitter(3)

The binding site ofan endogenous ligand on Ligand Gated Ion Channels protein complexes are normally located on a different portion of protein (an allosteric binding site) compared to the ion where the conduction pore is situated. (3)

The ion channel is regulated by ligand and is usually very selective to1 or more ions like Na+, K+, Ca2+, or Cl-ions. Such receptors are located at synapses to convert the chemical signal of presynaptically released neurotransmitter and very fastly into a postsynaptic electrical signal. (3

) Many Ligand Gated Ion Channels are additionally modulated by the allosteric ligands, by channel blockers, ions, or by membrane potential. (3)

Example: nicotinic acetylcholine receptor (6)

G protein-coupled receptors: G protein-coupled receptors (GPCRs),are also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors (GPLR), comprises a large protein family of transmembrane receptors that sense molecules outside the cell .

they also activate inside signal transduction pathways and, ultimately, cellular responses too. G protein-coupled receptors are found only in the eukaryotes, that includes yeast, choano flagellates and animals.

The ligands that bind and activate these receptors include light-sensitive compounds,like odours, pheromones, hormones, and neurotransmitters, and these vary in size from small molecules to peptides to large protein structures.

G protein-coupled receptors are mainly involved in many of the diseases, and are also the target nearly 30% of all modern medicinal drug. .(4)

There are two main signal transduction pathways that involves G protein-coupled receptors.they are: .(4)

? cAMP signal pathway and

? Phosphatidylinositol signal pathway.

When a ligand binds to GPCR it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor(GNEF) .(4)

The GPCR then activates an associated G-protein by exchanging its bound GuanineDiPhosphate for a GuanineTriPhosphate. The G-protein's a subunit, together with bound GTP, can then dissociate from the v and g subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the a subunit type. .(4)

G-ProteinCoupledReceptorss are involved in wide variety of the physiological processes.

Some of the examples of their physiological roles includes the following:

? the visual sense: the opsins use photoisomerization reaction to translate the electromagnetic radiation into cellular signals. For example:Rhodopsin acts in this case.

? the sense of smell: receptors of the olfactory epithelium bindto the odorants (olfactory receptors) and pheromones too .

? mood and behavioural regulation: receptors in the mammalian brain bind to several different neurotransmitters, including some like serotonin, dopamine, GABA, and glutamate

? regulation of the immune system activity and inflammation: chemokine receptors bind to the ligands that mediate the intercellular communication between the cells of immune system;

? receptors such as histamine receptors bind inflammatory mediators and engage in the target cell types in the inflammatory responses.

? autonomic nervous system transmission:here in this type of transmission the sympathetic as well as parasympathetic nervous systems are regulated by G-ProteinCoupledReceptor pathwaysthat is , responsible for control of many of the automatic functions of the body such as blood pressure, heart rate, and digestive processes.

? cell density sensing: A novel GPCR role in regulating cell density sensing is used these days.

Nuclear receptors:

* nuclear receptors are a class of proteins that are found within the cells that are responsible for sensing steroid and thyroid hormones and other molecules. (5)

* As a response, these receptors work with other proteins to regulate the expression of specific genes, thereby controlling the activities like development, homeostasis, and metabolism of an organism. (5)

* Nuclear receptors have the capability to directly bind to the DNA and regulate the expression of genes, hence these receptors are called as transcription factors.The regulation of gene expression by nuclear receptors generally occurs when a ligand -- a molecule that affects the receptor's behavior is present. (5)

* A unique and special property of the nuclear receptors is that it differentiates them from other classes of receptors in their ability to directly interact with and it can control the expression of genomic DNA. (5)

* As a result, nuclear receptors play key roles in both embryonic development and as well as adult homeostasis. (5)

THIS BLOG DOESNOT CONTAIN ANY PLAGIARISED MATERIAL.

REFERENCES:

1.http://celiacdisease.about.com/od/celiacdiseaseglossary/g/Receptor.htm (accessed on 27/3/11,8:12 a.m) 2.http://hsc.uwe.ac.uk/synapses_neuro/receptorlocations.htm (accessed on 27/3/11,8:12 a.m)

3.http://en.wikipedia.org/wiki/Ligand-gated_ion_channel(accessed on 27/3/11,8:12 a.m)

4. http://en.wikipedia.org/wiki/G_protein-coupled_receptor( accessed on 27/3/11,8:12 a.m) 5.http://en.wikipedia.org/wiki/Nuclear_receptor(accessed on 27/3/11,8:12 a.m)

6. http://en.wikipedia.org/wiki/File:LGIC.png(accessed on 27/3/11,8:12 a.m)