Question

Describe the mechanisms that limit signaling by the phosphoinositide pathway.

Answer 1

Ans) Phosphoinositide pathway -

{monophosphates (PI3P, PI4P, and PI5P), diphosphates [PI(3,4)P2, PI(3,5)P2, PI(4,5)P2], and a triphosphate [PI(3,4,5)P3] that are collectively known as phosphoinositides.}

phosphoinositide signal pathway, also sometimes called the phosphoinositide cascade uses the 7TM receptors (also called G-coupled protein receptors).

give a particular example, the angiotensin II receptor of our own body uses this pathway to regulate blood pressure in the cardiovascular system. This pathway begins with the binding of the primary messenger to the G-coupled protein receptor. The binding induces conformational changes that causes the G-protein to expel the GDP and bind a GTP. This causes the G-protein to detach from the rest of the receptor and move onto and bind to a membrane-bound enzyme called phospholipase C. Upon binding to phospholipase C, the G-protein stimulates it to begin breaking down phosphatidylinositol 4,5-bisphosphate (PIP2) into two secondary messenger molecules called diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). DAG is non-polar and remains dissolved within the membrane of the cell. It moves along the membrane and binds onto membrane-bound protein kinase C. The IP3 is water-soluble and moves across the cytoplasm to bind to a ligand-gated calcium channel on the ER membrane. Upon binding, it causes calcium ions to rush into the cytoplasm. The calcium does two things. First, it binds to protein kinase C and with the help of DAG stimulates and activates the kinase. Second, the calcium binds to calmodulin, forming the calcium-calmodulin complex that then goes on to activate effector proteins and enzymes such as protein kinases. These kinases can then phosphorylate other proteins, enzymes, channels and pumps that ultimately carry out cell processes that cause that particular physiological effect (Example increase or decrease in blood pressure).