What are the similarities and differences between the reactions that lead to the activation of trimeric G proteins (G protein-coupled receptor signaling pathway) and those that lead to the activation of Ras (enzyme-coupled receptor signaling pathway)?

Explanation:

Their similarities between the reactions  include:

 1.  Both proteins are trans membrane in nature.

 2.  GPCR and Ras both have an active GTP bound form and an inactive GDP bound form.

  3. When the ligan binds in both pathways, a cytosolic Guanine nucleotide exchange factor (GEF) is responsible for exchanging GDP with GTP and the proteins become active.

   4. GTPase activating protein (GAP) is responsible for termination of the response in both GPCR and Ras.

Differences :- The differncees between the reactions  include

1.   Ras is always monomeric while GPCR is a heterotrimeric receptor .

 2.  Ras protein is not physically associated with the receptor whereas G protein is.

 3. On binding of ligand (In GPCR) to the receptor,  vibrational changes are produced in the receptor which are responsible for converting Trimeric G protein into monomeric G protein. A cytosolic protein GEF is responsible for exchanging GDP with GTP and G-Protein becomes active, while in Ras pathway which is actually a Receptor Tyrosine kinase pathway, where ligand bind to the RTK, causing auto-phosphorylation of the receptor which activates the downstream protein, Grb-2. Grb-2 interacts with a protein named SOS which is actually a GEF.   The Ras-protein is being activated by SOS by replacing the GDP bound to it with GTP.

 4.  A secondary messenger cAMP is involved in GPCR pathway which is not the case in Ras pathway.

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