Question

The G protein coupled receptor pathway involves the activation of many different proteins or synthesis of molecules. For the following pairs of proteins/molecules describe which of the pair is activated/synthesized by the other and how this occurs. a. protein kinase A and cAMP b. adenylyl cyclase and cAMP c. G protein coupled receptor and G protein d. protein kinase A and another protein kinase

Answer 1

Ans-a) cAMP molecule directly activates the protein kinase A .Protein Kinase A (PKA) is a cyclic AMP (cAMP) dependent protein and without cAMP, it remain in deactivated state. PKA  molecule consists of two subunits, a regulatory subunit and a calalytic subunit. These subunits are inactive when cAMP is not bound. The binding cAMP  to a regulatory subunit induces a  conformation change ,causing these subunits dissociate from the complex therby, results in activation of  catalytic subunit of PKA.

Ans-b)The activated Gs alpha subunit binds to and activates an enzyme called adenylyl cyclase, which, in turn, catalyzes the conversion of ATP into cyclic adenosine monophosphate (cAMP) by removal of pyrophosphate(ppi) from ATP and directly increasing the cAMP level in cytosol.

Ans-c) in absence of signal molecule the heteromeric G protein complex is in inactive GDP bound form. On binding of ligand or hormone on GPCR, there is a conformational change of the receptor. Binding of trimeric G protein to activated GPCR receptor leads to dissociation of GDP and binding of GTP to Gα subunit and dissociation of Gα-GTP from Gβγ. The Gα or Gβγ binds to and activate the effector.

Ans-d) protein kinase A activated by cAMP . The released activated catalytic subunit of PKA further activate the other protein kinase by removal of phosphate from ATP and addition of this on serine and threonine residues of target protein kinase. In short protein kinase A activates other protein kinase by their phosphorylation .