 Dear students, in this topic, we shall discuss in detail the cyclic nucleotide signaling systems. You know that water soluble hormones act through stimulating the formation of second messengers. The most common second messengers are the cyclic nucleotide monophosphates C or cyclic AMP and cyclic GMP. These second messengers cause subsequent intracellular effects of the hormone. We shall discuss in detail the signaling system of cyclic AMP. Many hormones use the C AMP adenylate cyclase second messenger signaling system. This system is used to stimulate the target tissues. In this system, the hormone binds to the receptor. This receptor is coupled to a G protein. Binding of the hormone to the receptor causes the activation of this G protein. The activated G protein stimulates the enzyme adenylate cyclase. This adenylate cyclase is present in the plasma membrane and its activation catalyzes the conversion of ATP into C AMP. This step occurs in the cytoplasm. This adenylate cyclase stimulates the adenylate cycle to convert ATP into a cyclic adenosine monophosphate. This cyclic AMP activates an enzyme which is called the cyclic AMP-dependent protein kinase. This protein kinase phosphorylates the specific proteins and triggers a cascade of biochemical reactions. Dear students, there are various hormones which produce cyclic AMP, but this same cyclic AMP, acting as a second messenger, causes different types of actions in different types of cells. These specific types of reactions depend on the different types of intracellular machinery and enzyme combinations which are present in different types of cells. Thus, different functions are elicited by different target cells. We can enumerate some types of actions which are produced or generated by cyclic AMP. For example, initiating the synthesis of specific chemicals, causing muscle contraction or relaxation, initiating secretions by the cells and altering cell permeability. Dear students, now we shall discuss the cyclic GMP as second messenger. Some cells also use cyclic GMP as second messenger. The pattern of cyclic GMP activities is similar to that of CAMP. However, this pathway that uses cyclic GMP uses different enzymes and different factors which stimulate these enzymes. We shall take few examples of differences between CAMP and CGMP pathways. The production of CGMP is catalyzed by the enzyme guanylate cyclase. This is the adenylate cyclase used to make CMP. Adenylate cyclase is a membrane-bound enzyme, whereas guanylate cyclase is found in two forms. It is found in the membrane-bound form as well as in the cytoplasm, in the pre-state. The guanylate cyclase enzyme is active when calcium-ion concentration is increased within the cell. Adenylate cyclase is active when calcium-ion concentration decreases in the cell. A specific GMP activates a specific enzyme which is called protein kinase G instead of protein kinase A. In this way, the functional protein or functional enzyme that has to do phosphorylation of proteins is also different and its type is G. Dear students, We shall use the CMP pathway or the CGMP pathway, but as a result of this, the two mechanisms of the cyclical nucleotide phosphates amplify the signal of the two systems. As a result of this, Cascade of chemical reactions are activated as a result of the activation of many molecules. This is a chain reaction which can generate a powerful response even if the initial response by the hormone was due to a slight amount of the hormone.