Ligand binding: G-proteins Flashcards
What are the functions of G-proteins?
Act as molecular on/off switches in signalling pathways, sensory perception, vesicle transport, protein synthesis
Is a G-protein bound to GTP active or inactive?
Active
Is a G-protein bound to GDP active or inactive?
Inactive
How do you activate a G-protein?
Switch GDP with GTP
How do you inactivate a G-protein?
GTP-hydrolysis
What are the 3 types of G-proteins?
Small, translation factors, heterotrimeric
What are the 3 subunits of heterotrimeric G-proteins?
Alpha, beta and gamma
What are G-protein coupled receptors (GPCRs)?
Integral membrane receptors that will change conformation and activate the associated G-protein when a signal binds
What happens to the G protein when a signal binds to the GPCR?
GDP is switched with GTP and the protein activates. The beta and gamma subunits dissociate and the alpha stays at the membrane and is active
What happens to a G-protein when GTP hydrolysis occurs?
The beta and gamma subunits return and form the trimer again
What are the two domains in the G alpha subunit?
GTPase and helical
What are the 4 regions in the G alpha subunit?
P-loop, Switch 1, switch 2, switch 3
What is a P-loop?
A nucleotide binding motif where the GTP or GDP is bound
What is the conserved consensus sequence in the P-loop?
GXGESGKS
What does switch 1 do?
Flexible loop that creates contacts between the alpha and beta subunits
What does switch 2 do?
Interacts with the gamma phosphate of GTP and releases the beta and gamma subunits, or interacts with the beta and gamma subunits when GDP is bound
What does switch 3 do?
Flexible loop involved in receptor mediated activation
What motif is present in the G beta subunit?
Beta propellor
Which Switch loop undergoes the largest conformational change between the GTP bound and GDP bound states?
Switch 2
What two things regulate the activation and inactivation of G-proteins?
GEFs and GAPs
What do GEFs do?
Guanosine exchange factors. They switch GDP for GTP
What do GAPs do?
GTPase activator proteins. They speed up the intrinsic GTPase activity
How many human diseases are associated with defective G-proteins or GPCRs?
25%
Why are defective G proteins involved in cancer and diabetes?
A mutation in a G protein involved in a cell division pathway will make it less likely to turn off, causing uncontrolled cell division
What toxins will target G proteins? How do they affect G-proteins?
Cholera and pertussis. They covalently modify the alpha subunit and leave it permanently on
What does bee and wasp venom do?
The peptides insert themselves in the membrane and mimic GPCRs. They activate G proteins
