The answer is (a)
The mechanism of the “second messenger” is used by hormones that can not cross the plasma membrane, like peptides hormones and hormones derived from amino acids (T3 is an exception, since it has a hydrophobic lateral chain). Since these hormones can not cross the plasma membrane, they interact with a receptor located in the membrane, like this transmembrane protein:
The interaction between the receptor (a membrane protein that frequently has seven intramembrane domains) and the hormone provokes the activation of one of the G-Proteins, a family of amphipatic proteins associated to the inner surface of the plasma membrane.
In this graphic, the receptor is represented as a transmembrane protein in blue, and the G-Protein is represented in pink color:
The interaction between the hormone and the receptor provokes changes in conformation of the G-Protein associated to the receptor, facilitating the release of GDP and the binding of GTP in the alpha subunit of the G-Protein. The alpha subunit-GTP complex dissociates from the Beta-Gamma subunits, and it can produce activation of membrane associated enzymes like Adenyl Cyclase, Phospholipase C or other enzymes, depending on the specific Hormone-Receptor-G Protein system.
If the GTP-alpha subunit complex released in the Hormone-Receptor-G Protein System activates Adenyl Cyclase, then this enzyme catalyses the transformation of ATP to cAMP; if the system activates Phospholipase C, then it produces the hydrolysis of phosphatidyl inositol diphosphate, releasing Inositol triphosphate (IP3) and diacylglycerol, that act as second messengers.
Molecules as different as cAMP, IP3, diacylglicerol, Ca++ and Nitric Oxide can function as second messengers.
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