Question

As a scientist, you must be able to conduct experiments. Pretend you are studying β2-adrenergic receptor signaling in cultured cells: Your experimental set consists of cells containing GTPγS; your control set consist of cells that do not contain GTPγS. Upon addition of epinephrine (an agonist for the  β2-adrenergic receptor), which cells will exhibit higher levels of cAMP? Explain your answer in biochemical terms.

Answer 1

Ans: The beta-2 adrenergic receptor (β₂ adrenoreceptor), also known as ADRB2, is a cell membrane-spanning beta-adrenergic receptor that binds epinephrine (adrenaline), a hormone and neurotransmitter whose signaling, via adenylate cyclase stimulation through trimeric Gs proteins, increased cAMP, and downstream L-type calcium channel interaction, mediates physiologic responses such as smooth muscle relaxation and bronchodilation.

Now as described above that epinephrine when binds to the β2-adrenergic receptor it increases cAMP production by means of trimeric Gs proteins. The activated Gs protein activates adenylate cyclase which catalyzes the production of cAMP. So this means that if the trimeric Gs protein is activated for prolonged time, this will exhibit higher levels of cAMP and trimeric Gs protein is activated when GTP is bound to it and gets inactivated by the hydrolysis of GTP into GDP.

Now GTPgammaS is a non-hydrolyzable or slowly hydrolyzable G-protein-activating analog of guanosine triphosphate (GTP). The substitution of sulfur for one of the oxygens of the γ-phosphate of GTP creates this nucleotide that either cannot be hydrolyzed or is only slowly hydrolyzed. This prevents the GTP binding proteins from being inactivated, and allows the cellular processes that they carry out when active to be more easily studied. So cultured cells containing this analog will have longer activation of trimeric Gs protein and this will result in longer activation of adenylate cyclase which will exhibit higher level of cAMP.

Hence from all the literature described above we can conclude that cells containing GTPγS will exhibit higher levels of cAMP because of the prolonged activation of trimeric Gs protein by means of GTPγS.