Adenosine 5'-triphosphate (ATP) is a multifunctional nucleotide primarily known in biochemistry as the "molecular currency" of intracellular energy transfer. In this role ATP transports chemical energy within cells. It is produced as an energy source during the processes of photosynthesis and cellular respiration. ATP is also one of four monomers required for the synthesis of ribonucleic acids. Furthermore, in signal transduction pathways, ATP is used to provide the phosphate for protein-kinase reactions.
ATP consists of adenosine and three phosphate groups (triphosphate). The phosphoryl groups, starting with that on AMP, are referred to as the alpha (α), beta (β), and gamma (γ) phosphates. ATP is extremely rich in chemical energy, in particular between the second and third phosphate groups. The net change in energy of the decomposition of ATP into ADP and an inorganic phosphate is -12 kCal / mole in vivo (inside of a living cell) and -7.3 kCal / mole in vitro (in laboratory conditions). This massive release in energy makes the decomposition of ATP extremely exergonic, and hence useful as a means for chemically storing energy.
ATP can be produced by various cellular processes: Under aerobic conditions, the majority of the synthesis occurs in mitochondria during oxidative phosphorylation and is catalyzed by ATP synthase and, to a lesser degree, under anaerobic conditions by fermentation.
The main fuels for ATP synthesis are glucose and triglycerides. The fuels that result from the breakdown of triglycerides are glycerol and fatty acids.
First, glucose and glycerol are metabolised to pyruvate in the cytosol using the glycolyitic pathway. This generates some ATP through substrate phosphorylation catalyzed by two enzymes: PGK and Pyruvate kinase. Pyruvate is then oxidised further in the mitochondrion.
In the mitochondrion, pyruvate is oxidised by pyruvate dehydrogenase to acetyl-CoA, which is fully oxidised to carbon dioxide by the Krebs cycle. Fatty acids are also broken down to acetyl CoA by beta-oxidation and metabolised by the Krebs cycle. Every turn of the Krebs cycle produces an ATP equivalent (GTP) through substrate phosphorylation catalyzed by Succinyl-CoA synthetase as well as reducing power as NADH. The electrons from NADH are used by the electron transport chain to generate a large amount of ATP by oxidative phosphorylation coupled with ATP synthase.
The whole process of oxidising glucose to carbon dioxide is known as cellular respiration and is more than 40% efficient at transferring the chemical energy in glucose to the more useful form of ATP.
ATP is also synthesized through several so-called "replenishment" reactions catalyzed by the enzyme families of NDKs (nucleoside diphosphate kinases), which use other nucleoside triphosphates as a high-energy phosphate donor, and the ATP:guanido-phosphotransferase family, which uses creatine.
ADP + GTP --> ATP + GDP
In plants, ATP is synthesized in chloroplasts during the light reactions of photosynthesis. Some of this ATP is then used to power the Calvin cycle, which synthesizes triose sugars.
If a clot causes a decrease in oxygen delivery to the cell, the amount of ATP produced in the mitochondria will decrease.
ATP energy is released when hydrolysis of the phosphate-phosphate bonds is carried out. This energy can be used by a variety of enzymes, motor proteins, and transport proteins to carry out the work of the cell. Also, the hydrolysis yields free inorganic Pi and ADP, which can be broken down further to another Pi and AMP. ATP can also be broken down to AMP directly, with the formation of PPi. This last reaction has the advantage of being an effectively irreversible process in aqueous solution.
A few examples of the use of ATP include the active transport of molecules across cell membranes, the synthesis of macromolecules (Eg. proteins), muscle contractions, endocytosis, and exocytosis.
The total quantity of ATP in the human body is about 0.1 mole. The energy used by human cells requires the hydrolysis of 200 to 300 moles of ATP daily. This means that each ATP molecule is recycled 2000 to 3000 times during a single day. ATP cannot be stored, hence its consumption must closely follow its synthesis. On a per-hour basis, 1 kilogram of ATP is created, processed and then recycled in the body.
There is talk of using ATP as a power source for nanotechnology and implants. Artificial pacemakers could become independent of batteries. ATP is also present as a neurotransmitter independent from its energy-containing function. Receptors that utilise ATP as their ligand are known as purinoceptors.
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