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| Names | |
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| IUPAC name Adenosine 5′-(trihydrogen diphosphate) | |
| Systematic IUPAC name [(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-3,4-dihydroxyoxolan-2-yl]methyl trihydrogen diphosphate | |
| Other names Adenosine 5′-diphosphate; Adenosine 5′-pyrophosphate; Adenosine pyrophosphate | |
| Identifiers | |
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3D model (JSmol) | |
| ChEBI | |
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| DrugBank |
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| ECHA InfoCard | 100.000.356 |
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| Properties | |
| C10H15N5O10P2 | |
| Molar mass | 427.201 g/mol |
| Density | 2.49 g/mL |
| logP | −2.640 |
| Hazards | |
| Safety data sheet (SDS) | MSDS |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Adenosine diphosphate (ADP), also known asadenosine pyrophosphate (APP), is an importantorganic compound inmetabolism and is essential to the flow of energy in livingcells. ADP consists of three important structural components: asugar backbone attached toadenine and twophosphate groups bonded to the 5 carbon atom ofribose. The diphosphate group of ADP is attached to the 5’ carbon of the sugar backbone, while the adenine attaches to the 1’ carbon.[1]
ADP can be interconverted toadenosine triphosphate (ATP) andadenosine monophosphate (AMP). ATP contains one more phosphate group than ADP, while AMP contains one fewer phosphate group. Energy transfer used by all living things is a result ofdephosphorylation of ATP by enzymes known asATPases. The cleavage of a phosphate group from ATP results in the coupling of energy to metabolic reactions and a by-product of ADP.[1] ATP is continually reformed from lower-energy species ADP and AMP. The biosynthesis of ATP is achieved throughout processes such assubstrate-level phosphorylation,oxidative phosphorylation, andphotophosphorylation, all of which facilitate the addition of a phosphate group to ADP.
ADP cycling supplies theenergy needed to do work in a biological system, thethermodynamic process of transferring energy from one source to another. There are two types of energy:potential energy andkinetic energy. Potential energy can be thought of as stored energy, or usable energy that is available to do work. Kinetic energy is the energy of an object as a result of its motion. The significance of ATP is in its ability to store potential energy within the phosphate bonds. The energy stored between these bonds can then be transferred to do work. For example, the transfer of energy from ATP to the proteinmyosin causes a conformational change when connecting toactin duringmuscle contraction.[1]
It takes multiple reactions between myosin and actin to effectively produce one muscle contraction, and, therefore, the availability of large amounts of ATP is required to produce each muscle contraction. For this reason, biological processes have evolved to produce efficient ways to replenish the potential energy of ATP from ADP.[2]
Breaking one of ATP's phosphorus bonds generates approximately 30.5kilojoules permole of ATP (7.3kcal).[3] ADP can be converted, or powered back to ATP through the process of releasing the chemical energy available in food; in humans, this is constantly performed viaaerobic respiration in themitochondria.[2] Plants usephotosynthetic pathways to convert and store energy from sunlight, also conversion of ADP to ATP.[3] Animals use the energy released in the breakdown of glucose and other molecules to convert ADP to ATP, which can then be used to fuel necessary growth and cell maintenance.[2]
The ten-stepcatabolic pathway ofglycolysis is the initial phase of free-energy release in the breakdown ofglucose and can be split into two phases, the preparatory phase and payoff phase. ADP andphosphate are needed as precursors to synthesize ATP in the payoff reactions of theTCA cycle andoxidative phosphorylation mechanism.[4] During the payoff phase of glycolysis, the enzymes phosphoglycerate kinase and pyruvate kinase facilitate the addition of a phosphate group to ADP by way ofsubstrate-level phosphorylation.[5]
Glycolysis is performed by all living organisms and consists of 10 steps. The net reaction for the overall process ofglycolysis is:[6]
Steps 1 and 3 require the input of energy derived from the hydrolysis of ATP to ADP and Pi (inorganic phosphate), whereas steps 7 and 10 require the input of ADP, each yielding ATP.[7] Theenzymes necessary to break down glucose are found in thecytoplasm, the viscous fluid that fills living cells, where the glycolytic reactions take place.[1]
Thecitric acid cycle, also known as the Krebs cycle or the TCA (tricarboxylic acid) cycle is an 8-step process that takes the pyruvate generated by glycolysis and generates 4 NADH, FADH2, and GTP, which is further converted to ATP.[8] It is only in step 5, where GTP is generated, by succinyl-CoA synthetase, and then converted to ATP, that ADP is used (GTP + ADP → GDP + ATP).[9]
Oxidative phosphorylation produces 26 of the 30 equivalents of ATP generated in cellular respiration by transferring electrons from NADH or FADH2 toO2 through electron carriers.[10] The energy released when electrons are passed from higher-energy NADH or FADH2 to the lower-energy O2 is required to phosphorylate ADP and once again generate ATP.[11] It is this energy coupling and phosphorylation of ADP to ATP that gives the electron transport chain the name oxidative phosphorylation.[1]
During the initial phases ofglycolysis and theTCA cycle,cofactors such asNAD+ donate and accept electrons[12] that aid in theelectron transport chain's ability to produce a proton gradient across the inner mitochondrial membrane.[13] The ATP synthase complex exists within the mitochondrial membrane (FO portion) and protrudes into the matrix (F1 portion). The energy derived as a result of the chemical gradient is then used to synthesize ATP by coupling the reaction of inorganic phosphate to ADP in the active site of theATP synthase enzyme; the equation for this can be written as ADP + Pi → ATP.[citation needed]
Under normal conditions, small disk-shapeplatelets circulate in the blood freely and without interaction with one another. ADP is stored indense bodies insidebloodplatelets and is released upon platelet activation. ADP interacts with a family of ADP receptors found on platelets (P2Y1,P2Y12, and P2X1), which leads to platelet activation.[14]
ADP in the blood is converted toadenosine by the action ofecto-ADPases, inhibiting further platelet activation viaadenosine receptors.[citation needed]
