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Adenosine (C10H13N5O4), also known as 9-Beta-D-ribofuranosyl-9H-purin-6-amine and abbreviated Ado, is a purine (single ring structure) nucleoside. It consists of a molecule of adenine connected to a ribofuranose sugar. The following is what the actual adenosine structure looks like:
Adenosine can be quickly metabolized to inosine and adenosine monophosphate. Studies have shown that adenosine induces sleep and stifles arousal. Levels of adenosine in the body rise every hour an organism is awake. Adenosine has been known to cause flushing, headache, and undesirable atrioventricular block.
There are four adenosine receptor subtypes that adenosine cellular signaling goes through and those are A1, A2A, A2B, and A3. A1 receptors and A2A moderate myocardial oxygen usage and coronary blood flow. Adenosine's effect on the A1 receptor accounts for why the heart pauses very briefly when given adenosine during cardiac resuscitation. The A2A receptor also has bigger anti-inflammatory responses around the body. A1 receptors and A2A also moderate the release of dopamine, glutamate, and other neurotransmitters. A2B and A3 receptors are on the exterior and play a role in processes such as inflammation and immune reactions.
Upon entering the bloodstream, adenosine gets broken down by an enzyme found in red cells and the vessel wall called adenosine deaminase (ADA). An inhibitor called Dipyridamole permits adenosine to build up in the blood stream, prompting blood vessels in the heart to widen. Adenosine can resume healthy sinus rhythm in people with paroxysmal supraventricular tachycardia. Large doses of adenosine can also lower blood pressure via a decrease of peripheral resistance (resistance to blood flow through blood vessels). Adenosine is quickly removed from blood flow mainly by red blood cells and cells that line the interior surface of blood vessels within the circulatory system.
The multifunctional nucleotide adenosine-5'-triphosphate (ATP), found in the nucleoplasm and cytoplasm of all cells, is often referred to as the energy currency of life. It is composed of adenosine and three phosphate groups. Over 2 x 1026 molecules of ATP are formed in the body every day. ATP is formed by oxidative metabolism and photophosphorylation, a process that uses sunlight to produce energy. In animals, ATP is formed in the mitochondria of the cell. The actual structure of ATP is as follows:
ATP is used as a substrate in signal transduction pathways by enzymes known as phosphotransferases that add phosphate groups to proteins and lipids, as well as by adenylate cyclase, an enzyme that consumes ATP to create cAMP. ATP is also integrated into DNA and RNA during replication and transcription.
The nucleotide adenosine diphosphate (C10H15N5O10P2), abbreviated ADP, is an ester of diphosphoric acid with adenosine. ATPases produce ADP in the process known as dephosphorylation, but ADP can be changed back to ATP using ATPsynthases. ADP is kept in granules in blood platelets and is discharged when platelet activation occurs. ADP networks with a group of ADP receptors located on platelets. These receptors are P2Y1, P2Y12 (a significant regulator in blood clotting) and P2X1. This interaction leads to additional platelet activation. ADP in the blood is changed to adenosine by the operations of exterior ADPases, preventing additional platelet activation by adenosine receptors.
The following is the reaction for ADP formation: ATP ----> ADP + energy + Pi
This is what the ADP structure looks like (Naturally it is very similar to the ATP structure.):
A derivative of ADP is adenosine diphosphate ribose. In ADP ribose, the sugar ribose is attached to the last phosphate of ADP by an ester linkage. This compound is attached to chains by poly ADP ribose polymerase (PARP). PARP is an enzyme that has always been known to play a significant factor in apoptosis and DNA repair, but it is now thought to be involved in several other biological activities. ADP ribose can also regulate proteins.
Cyclic adenosine monophosphate, also known as cyclic AMP (cAMP), transmits signals from receptors on the exterior of the cell to intended molecules in the interior of the cell. The breakdown of cAMP into AMP is activated by an enzyme known as phosphodiesterase.
cAMP is used for conveying and regulating the effects of hormones such as epinephrine and glucagon because they will not pass through the membrane of the cell on their own. cAMP also helps stimulate protein kinases to activate. Additionally, cAMP plays a role in the regulation of calcium passing through ion channels.
In humans, cAMP activates protein kinase A. cAMP links to particular places on protein kinase regulatory components, which causes catalytic subunits to dissociate from them. This stimulates catalytic units and allows them to add phosphate groups to substrate proteins.
The following is an image displaying the derivation of ATP to cAMP to AMP:
Caffeine and Theophylline
Adenosine delays activity from nerve cells by binding to adenosine receptors, causing drowsiness. Nerve cells cannot accurately distinguish between caffeine, theophylline, (which are formally known as methylxanthines) and adenosine, resulting in the binding of caffeine to adenosine receptors. With the adenosine receptors being blocked by caffeine or theophylline, they can no longer bind to the adenosine, which causes the nerve cells to increase in speed instead. This causes the pituitary gland to become slightly confused and results in a release of epinephrine, causing the heart to beat faster.