Answer to Question about Hormones H-05:
(g) Ca++, Diacylglycerol and Inositol 1,4,5 triphosphate
As was discussed in the answer to H-04, some hormones that can not penetrate the plasma membrane, interact with receptors in membrane that are linked to a G-protein. As a result of the interaction hormone-receptor, the a subunit of the G-protein binds to GTP and separates from the bg subunit. The a subunit-GTP complex activates an specific effector protein, depending on the kind of a subunit. In case the a subunit is Gas, it activates Adenylate Cyclase, increasing the production of cAMP.
If the Hormone-receptor complex interacts with a Gaq/11 kind of G protein, then the activated enzyme is Phospholipase C.
Hormones that bind to receptors related to this Protein Gaq/11-Phospholipase C system include:
-Godatrophin Releasing Hormone GnRH)
-Growth Hormone Relaeasing Hormone
Phospholipase C catalyze the hydrolysis of Phosphatidyl inositol 4,5 biphosphate that is forming part of the plasma membrane.
The action of the enzyme on this substrate produces IP3 (Inositol triphosphate) and diacylglycerol.
IP3 difuses into the cytosol and binds to its receptor in the sarcoplasmic reticulum and opens a Calcium channel
Diacylglycerol remains close to the membrane and, with the participation of Ca++ released by IP3, activates Protein Kinase C, that phosphorylates other proteins, modifying its function (for the general action of Kinases, see related post)
Ca++ binds to Calmodulin, Troponin C and other Ca++ binding proteins provoking activation of some enzymes, actin myosine interaction, promotes exocytosis, synthesis of NO, and other effects.
An excellent animation here!