Question

How do we get from a ligand binding to a heterotrimeric GTP-binding protein to the activation of PKA?

Answer 1

GPCR (G-Protein Coupled Binding receptors) are a large family of transmembrane proteins. They consist of seven transmembrane alpha-helices with three extracellular and three intracellular loops. On the extracellular surface of this receptor binds the ligand, while on the intracellular surface is present the binding site for the GTP-activated protein (G-protein).

The structure of the G-protein consists of three subunits (alpha, beta and gamma). This G-protein binds GDP at the alpha subunit in the inactive state. However, when the ligand binds to the ligand-binding site at the receptor extracellular surface, activation of the receptor leads to the replacement of GDP by GTP at the alpha subunit. This alpha subunit-GTP complex then displaces itself from the beta and gamma subunits and brings about response effectuation by binding to the effector (ion channels/proteins/enzymes).

There are different types of G-proteins, which are divided based on their response to the effector molecule. Generally, these types differ only in their alpha-subunit.

G_s- activation of adenylyl cyclase, the opening of calcium channels

G_i- inhibition of adenylyl cyclase, the opening of potassium channels

G_q - the activation of phospholipase C (PL_C)

G_o- inhibition of calcium channels

There are different pathways through which the GPCR can perform its activity:

a. Adenylyl-cyclase: cAMP pathway

b. Phospholipase C: IP₃-DAG Pathway

c. Channel regulation

The activation of PK_A (Protein Kinase A) occurs in the first pathway, i.e., the Adenylyl-cyclase: cAMP pathway. This pathway uses G_s subunit of the G-protein. The activation/inhibition of the effector does not take place directly but involves certain second messengers which relay the information from the receptor to the effector molecule. The second messenger, in this case, is the PK_A molecule. After the ligand binds to the receptor, the receptor is activated which in turn activates the G-protein. The GDP bound to the alpha subunit is replaced by GTP, after which the alpha-GTP dissociates from the beta and gamma subunits. This activated Gs-alpha subunit binds to and activates adenylyl cyclase (AC). Adenylyl cyclase is responsible for catalyzing the conversion of ATP to cAMP (cyclic AMP). Intracellular accumulation of cAMP activates the enzyme PK_A. Thus, the activation of cAMP-dependent PK_A results in the phosphorylation of many proteins, ion channels, enzymes and transporters, thus altering their function. Thus, the GPCR brings about a response (activation/inhibition) by altering the activity of the effector molecule.