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Adenosine 3’,5’-cyclic monophosphate (cyclic AMP, cAMP, or cyclic adenosine monophosphate) is a second messenger, or a molecule produced within a cell in response to a signal outside of the cell, that is formed from a reaction where the membrane enzyme adenylyl cyclase catalyzes a reaction involving adenosine triphosphate (ATP) due to stimulus from specific hormones and molecular signals. Cyclic AMP is composed of the following three parts: a double-ring purine (a nitrogenous base), five-carbon ribose (a sugar), and the phosphate-containing cyclic nucleotide. The two bonds between the 3’ and 5’ hydroxyl groups on the ribose help create the cyclic formation. Since its discovery by Dr. Earl Wilbur Sutherland, Jr. (1915–1974) in 1958, cyclic AMP has been found to be involved in several different intracellular signal responses including protein kinase A activity and cyclic nucleotide-gated ion channels.

History
Earl Wilbur Sutherland, Jr. from Vanderbilt isolated and discovered adenosine 3’,5’-cyclic monophosphate as well as its role as an intermediate regulator in numerous hormonal mechanisms in 1958 which lead to him receiving the Nobel Prize in Physiology or Medicine. Sutherland presented the now accepted theory that numerous hormones do not enter the cell but rather attach to a surface receptor which initiates the formation of cyclic adenosine monophosphate that either starts or stops the particular metabolic process.

Structure
The second messenger adenosine 3’, 5’-cyclic monophosphate is made up of the following three pieces: a nitrogenous base, a sugar, and a cyclic nucleotide. The nitrogenous base for cyclic AMP is a double-ring purine which in this case would be adenine. The sugar for cyclic AMP is explicitly the five-carbon ribose which is also known as pentose and found within ribonucleic acid more often than not. The cyclic nucleotide for cAMP must contain a phosphate. The three elements that make up adenosine 3’, 5’-cyclic monophosphate are connected in such a way that the nitrogenous base adenine is connected at the first carbon on the ribose sugar which is called 1’ carbon while the phosphate group from the cyclic nucleotide is connected at the 5’ carbon on the ribose sugar. The latter structure is universal among all nucleotides; however, the phosphate group associated with the cyclic nucleotide bonds with the five-carbon ribose at the 3’ carbon in said cyclic nucleotide. This second linkage which is separate from the bond at the 1’ carbon produces a cyclic formation.

Adenosine 3’, 5’-cyclic monophosphate is a second messenger that responds to stimulation from specific hormones as well as extracellular signals that is generated from a reaction concerning adenosine triphosphate where the intracellular membrane enzyme adenylyl cyclase functions as the catalyst for the reaction. Soluble adenylyl cyclase serves as a source of adenosine 3’, 5’-cyclic monophosphate and is controlled by bicarbonate (HCO-3) ions that are in equilibrium with pH and carbon dioxide (CO2) because of carbonic anhydrases being present within the cell which help regulate several biological processes. Adenylyl cyclase is triggered by numerous molecular signals from the galvanization of adenylate cyclase stimulatory G-protein (Gs) coupled receptors, and in turn, adenylyl cyclase is repressed by adenylate cyclase inhibitory G-protein (Gi) coupled receptors. Degradation of cyclic AMP occurs when an enzyme from the class known as phosphodiesterases acts as a catalyst for the hydrolysis reaction where the 3’ phosphodiester bond is broken and a 5’ adenosine monophosphate is created.

A cyclic nucleotide-binding domain that contains a cyclic nucleotide binding pouch produced from being between beta sheets can be found in all protein that form bonds with cyclic nucleotide monophosphates. A conformational change occurs after the cyclic adenosine monophosphate binds to the domain which greatly impacts the overall activity of the protein.

The β-adrenergic pathway
The β-adrenergic (epinephrine) receptor pathway functions by utilizing the second messenger cAMP. This receptor is a very important protein that contains seven hydrophobic, helical regions of about twenty or so amino acid residues that in turn covers the plasma membrane seven-fold. The hormone epinephrine binds to the β-adrenergic receptor on a site that is well-descended into the plasma membrane, and this complex causes guanosine triphosphate to substitute for the α-stimulatory G-protein bound guanosine diphosphate (GDP) which activates the α-stimulatory G-protein. This activated protein detaches from the β-stimulatory G-protein while gradually moving towards the adenylyl cyclase before activating it. Adenylyl cyclase acts as the catalyst in the reaction that produces adenosine 3’, 5’-cyclic monophosphate. Cyclic AMP goes on to activate the cAMP-dependent protein kinase (PKA or protein kinase A) which catalyzes the phosphorylation of certain Ser or Thr residues in certain targeted proteins. After the phosphorylation reaction on the specific proteins, there is a cellular response to the original epinephrine signal. The cyclic AMP readily breaks down through hydrolysis by cyclic nucleotide phosphodiesterase which in turn deactivates the PKA.

Second messenger in higher plants
Cyclic AMP has recently been established as a second messenger in higher plants after measurements in Vicia faba and Arabidopsis thaliana mesophyll cells displayed that there was a increase in potassium ion outflow due to concentration dependency as well as cAMP specificity. This caused for an increase in calcium ion inflow. Adenosine 3’, 5’-cyclic monophosphate also contributes to responding to water deficiency within the cell as well as salt stressors with voltage-independent channels in Arabidopis thaliana roots seeing as though they are sensitive to a small concentration of cAMP in the cytoplasm. The phosphodiesterases for higher plants have not been singled out, but the PDE activity has been recognized in several crude protein extracts from a wide variety of higher plants. In general however, plants PDEs are exuded in order to break down extracellular RNA which in turn releases inorganic phosphates that end up being transported into the cell. The accepted theory for the latter is that nucleotide pyrophosphates as well as PDEs are joined in a complex structure that allows for co-purification during the isolation process of specific pathways. This is possible due to the two parts having similar molecular weights and isoelectric points.