G protein-coupled receptors Flashcards
What are G Protein-Coupled Receptors (GPCRs)
- Largest class of human cell surface receptors.
- Involved in smell, taste, vision, hormone detection.
- Bind a huge array of molecules.
- Characterised by 7 transmembrane domains
- The N-terminus of a GPCR is always on the outside of a cell
List some drugs that target GPCR’s
How many pharmaceuticals drugs target GPCRs?
50% of pharmaceuticals
How many of the top selling drugs target GPCRs?
25% of top selling drugs
Name some GPCR ligands
- Light (photons).
- Taste and smell molecules.
- Pheromones.
- Histamines (allergies).
- Hormones e.g. oxytocin, adrenaline.
- Any many many more!
Describe GPCR taxonomy
•Schiöth and Fredricksson (2003, 2005): Performed a phylogenetic analysis of the human genome and divided the GPCR super-family into 5 distinct families:
- Glutamate-like GPCRs
- Rhodopsin-like GPCRs
- Adhesion-like GPCRs
- Frizzled-like GPCRs
- Secretin-like GPCRs
What is the other way of naming GPCR families?
- Family A (Rhodopsin-like): Ligands ranging from light, small molecules, peptides to proteins
- Family B (Secretin-like): Peptide hormones characterised by large extracellular domain and disulfide bonds
- Family C (Glutamate-like): Bind to ions and small molecules. This family are obligate dimers that have two receptors on the outside joined by disulfide bonds
- Adhesion family: Ligands are mostly unknown but they undergo proteolytic cleavage
- Frizzled family: Bind Wnt proteins
What is the full name for G-Proteins?
•Guanine nucleotide binding proteins e.g. GTP (analogous to ATP) and GDP (analogous to ADP).
TRUE or FALSE: GPCRs signal via heterotrimeric G proteins (3 different subunits).
TRUE
- α – 16 types, β – 5 types, γ – 12 types
- α subunit binds guanine nucleotides
- β and γ form an obligate heterodimer
Describe the mode of action of G-Proteins
- G proteins act as molecular switches.
- In the inactive state they are bound to GDP.
- Activation of the G protein results in release of GDP.
- This GDP is then replaced by GTP producing an active G protein.
Explain G-Protein activation
- Ligand binds to the extracellular side.
- Changes the conformation of the receptor.
- This recruits the G protein.
- At this point the G protein is inactive (GDP-bound).
- Binding to the receptor induces GDP release; GDP is replaced by GTP.
- GTP-bound Gα separates from the GPCR and Gβγ.
- Gα goes on to activate signalling cascades.
An active GPCR can activate many G proteins (signal amplification)
What are the function of GAPs?
They are GTPase activating proteins (GAPs).
- Enhance (increase) the GTPase activity of the G protein (hydrolyses GTP to DGP).
- Cause inactivation.
- E.g. Regulator of G protein Signalling (RGS) proteins.
What are the funtion of GEFs?
•Guanine nucleotide exchange factors (GEFs).
–Accelerate the dissociation of GDP from the G protein, allowing it to be replaced by GTP.
–Enhance G-protein activation.
–E.g. Accelerator of G protein Signalling (AGS) proteins.
What is the function of Chlorea toxin?
It inactivates GTPase and stops the G-protein becoming inactive (signalling is always on)
What is the function of pertussis toxin?
It locks the G-Protein in the inactive state (signalling is always off)
G-Protein taxonomy: There are 4 families are based on sequence homology
List the 4 families
–Gs →stimulates adenylate cyclase.
–Gi/o →inhibits adenylate cyclase.
–Gq/11 →stimulates PLCβ.
–G12/13 →activates RhoGEFs.
TRUE or FALSE: Gβγ cannot signal
FALSE
Gβγ can also signal
What is Adenylate cyclase?
•Membrane bound protein (doesn’t span the membrane).
- Stimulated by Gs
- Inhibitated by Gi
- Acts as an enzyme.
- Converts ATP to cAMP (cyclic AMP).
cAMP is a second messenger
What is the function of cAMP?
•Activates protein kinase A (PKA).
–Phosphorylates a whole range of downstream targets.
- Also activates cyclic nucleotide-gated ion channels and exchange proteins activated by cAMP (EPACs).
- Broken down by cAMP phosphodiesterases.
Linked to vasodilation and also colour blindness
What is protein kinase A?
•Holoenzyme that contains 4 subunits (2 regulatory and 2 catalytic).
The regulatory domains block the active sites of the catalytic domains
How is protein kinase is affected by a low concentration of cAMP?
The 4 subunits remain together.
How is protein kinase is affected by a high concentration of cAMP?
4 AMP molecules bind to the regulatory subunits and causes a conformational change which results in the release of the catalytic subunits.
