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Candidates who are preparing for their CSIR NET exam might in need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point of time, We at BYJU'S Exam Prep come up with short notes on the G Protein-Coupled Receptor (GPCR) which comes under the Biochemistry section of the Life Science syllabus.
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G Protein-Coupled Receptor (GPCR)
- G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors in eukaryotes.
- GPCRs consist of a single polypeptide that is folded into a globular shape and embedded in a cell's plasma membrane.
- Seven segments of this molecule span the entire width of the membrane explaining why GPCRs are sometimes called seven-transmembrane receptors and the intervening portions loop both inside and outside the cell.
- When an external signalling molecule binds to a GPCR, it causes a conformational change in the GPCR. This change then triggers the interaction between the GPCR and a nearby G protein.
- G proteins are specialized proteins with the ability to bind the nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP).
- However, the G proteins that associate with GPCRs are heterotrimeric, meaning they have three different subunits: an alpha subunit, a beta subunit, and a gamma subunit.
- A G protein alpha subunit binds either GTP or GDP depending on whether the protein is active (GTP) or inactive (GDP).
- In the absence of a signal, GDP attaches to the alpha subunit, and the entire G protein-GDP complex binds to a nearby GPCR.
- This arrangement persists until a signalling molecule joins with the GPCR.
- At this point, a change in the conformation of the GPCR activates the G protein, and GTP physically replaces the GDP bound to the alpha subunit
- As a result, the G protein subunits dissociate into two parts: the GTP-bound alpha subunit and a beta-gamma dimer.
- Both parts remain anchored to the plasma membrane, but they are no longer bound to the GPCR, so they can now diffuse laterally to interact with other membrane proteins.
- G proteins remain active as long as their alpha subunits are joined with GTP.
- However, when this GTP is hydrolyzed back to GDP, the subunits once again assume the form of an inactive heterotrimer, and the entire G protein associated with the now-inactive GPCR.
- Specific targets for activated G proteins include various enzymes that produce second messengers.
- One common target of activated G proteins is adenylyl cyclase, a membrane-associated enzyme that, when activated by the GTP-bound alpha subunit, catalyzes the synthesis of the second messenger cAMP from molecules of ATP.
- In humans, cAMP is involved in responses to sensory input, hormones, and nerve transmission, among others.
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