17. A heterotrimeric G-protein that is associated with a G-protein coupled receptor is fully activated when...

17. A heterotrimeric G-protein that is associated with a G-protein coupled receptor is fully activated when

A. the GDP that is associated with G-protein's alpha subunit is phosphorylated to GTP.

B. the GDP that is associated with the G-protein's alpha subunit is exchanged for a GTP.

C. the alpha subunit associates with the beta and gamma subunits.

A and C

B and C

Expert Solution

The G-protein is associated with the membrane, which implies whenever any ligand binds to the receptor on the membrane, the G-protein undergoes certain changes. It is heterotrimeric which means, it is made of three different subunits: alpha, beta and gamma.
In the inactive state, the GDP (guanosine diphosphate) is bound to the alpha subunit of the G-protein.
Whenever some ligand binds to the receptor, the GDP bound to the alpha subunit is detached from it and inturn gets exchanged for the GTP (guanosine triphosphate) present in the cytosol of the cell and thus gets activated.. So, option B is correct.
As, there is no phosphorylation of GDP to GTP here, option A is incorrect.
The alpha subunit along with GTP dissociates from the two other subunits (beta and gamma) and binds to the effector after the activation. So, the activation is accompanied by dissociation of alpha subunit from beta and gamma subunits and not association.Therefore, option C is incorrect.

gladiator answered 1 hour ago

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