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Adenosine triphosphate?(ATP)

is a complex?organic chemical?that participates in many processes. Found in all forms of life, ATP is often referred to as the ?molecular unit of?currency? of intracellular?energy transfer.[1]?When consumed in metabolic processes, it converts to either the di- or monophosphates, respectively ADP and AMP. Other processes regenerate ATP such that the human body recycles its own body weight equivalent in ATP each day.[2]?It is also a precursor to DNA and RNA.

From the perspective of?biochemistry, ATP is classified as?nucleoside triphosphate, which indicates that it consists of three components, a nitrogenous base (adenine), the sugar ribose, and the triphosphate. It is used in?cellsas a?coenzyme.

In terms of its structure, ATP consists of an?adenine?attached by the 9-nitrogen atom to the 1? carbon atom of a sugar (ribose), which in turn is attached at the 5? carbon atom of the sugar to a triphosphate group. In its many reactions related to metabolism, the adenine and sugar groups remain unchanged, but the triphosphate is converted to di- and monophosphate, giving respectively the derivatives?ADP?and?AMP. The three phosphoryl groups are referred to as the alpha (?), beta (?), and, for the terminal phosphate, gamma (?).

In neutral solution, ionized ATP exists mostly as ATP4?, with a small proportion of ATP3?.[3]

?

Being polyanionic and featuring a potentially?chelatable?polyphosphate group, ATP binds metal cations with high affinity. The?binding constant?for?Mg2+ is (9554).[4]?The binding of a?divalent?cation, almost always?magnesium, strongly affects the interaction of ATP with various proteins Due to the strength of the ATP-Mg2+?interaction, ATP exists in the cell mostly as a complex with?Mg2+ bonded to the phosphate oxygen centers.[3][5]

A second magnesium ion is critical for ATP binding in the kinase domain.[6]?The presence of Mg2+?regulates kinase activity.[7]

Salts of ATP can be isolated as colorless solids.[8]

ATP is stable in aqueous solutions between pH?6.8 and 7.4, but it is rapidly?hydrolysed?at more extreme pH?s. ATP?hydrolyses?to?ADP?and phosphate. Living cells maintain the ratio of ATP to ADP at a point ten orders of magnitude from equilibrium, with ATP concentrations fivefold higher than the concentration of ADP.[9]

[10]?In the context of biochemical reactions, these anhydride bonds are frequently referred to as?high-energy bonds.[11]

The hydrolysis of ATP into ADP and inorganic phosphate releases 30.5?kJ/mol?of enthalpy, with a change in?free energy?of 3.4?kJ/mol.[12]?The energy released by cleaving either a phosphate (Pi) or pyrophosphate (PPi) unit from ATP at?standard state?of 1?M are:[13]

ATP +?H2O?? ADP + Pi????G? = ?30.5?kJ/mol (?7.3?kcal/mol)

ATP +?H2O?? AMP + PPi????G? = ?45.6?kJ/mol (?10.9?kcal/mol)

These abbreviated equations can be written more explicitly (R =?adenosyl):

[RO-P(O)2-O-P(O)2-O-PO3]4-?+?H
2O
?? [RO-P(O)2-O-PO3]3-?+ [PO4]3-?+ 2 H+
[RO-P(O)2-O-P(O)2-O-PO3]4-?+?H
2O
?? [RO-PO3]2-?+ [O3P-O-PO4]4-?+ 2 H+

Production, aerobic conditions[edit]

With a typical intracellular?concentration?of 1?10?mM, ATP is abundant.[14]?The dephosphorylation of ATP and rephosphorylation of ADP and AMP occur repeatedly in the course of aerobic metabolism.

ATP can be produced by a number of distinct cellular processes; the three main pathways in?eukaryotes?are (1)?glycolysis, (2) the?citric acid cycle/oxidative phosphorylation, and (3)?beta-oxidation. The overall process of oxidizing glucose to?carbon dioxide, the combination of pathways 1 and 2, is known as?cellular respiration, produces about 30 equivalents of ATP from each molecule of glucose.[15]

ATP production by a non-photosynthetic?aerobic eukaryote occurs mainly in the?mitochondria, which comprise nearly 25% of the volume of a typical cell.[16]

Glycolysis

In glycolysis, glucose and glycerol are metabolized to?pyruvate. Glycolysis generates two equivalents of ATP through?substrate phosphorylation?catalyzed by two enzymes,?PGK?and?pyruvate kinase. Two equivalents of?NADH?are also produced, which can be oxidized via the?electron transport chain?and result in the generation of additional ATP by?ATP synthase. The pyruvate generated as an end-product of glycolysis is a substrate for the?Krebs Cycle.[17]

Regulation

In glycolysis,?hexokinase?is directly inhibited by its product, glucose-6-phosphate, and?pyruvate kinase?is inhibited by ATP itself. The main control point for the glycolytic pathway is?phosphofructokinase?(PFK), which is allosterically inhibited by high concentrations of ATP and activated by high concentrations of AMP. The inhibition of PFK by ATP is unusual, since ATP is also a substrate in the reaction catalyzed by PFK; the active form of the enzyme is a?tetramer?that exists in two conformations, only one of which binds the second substrate fructose-6-phosphate (F6P). The protein has two?binding sites?for ATP?? the?active site?is accessible in either protein conformation, but ATP binding to the inhibitor site stabilizes the conformation that binds F6P poorly.[17]?A number of other small molecules can compensate for the ATP-induced shift in equilibrium conformation and reactivate PFK, including?cyclic AMP,?ammonium?ions, inorganic phosphate, and fructose-1,6- and -2,6-biphosphate.[17]

Citric acid cycle

In the?mitochondrion, pyruvate is oxidized by the?pyruvate dehydrogenase complex?to the?acetyl?group, which is fully oxidized to?carbon dioxide?by the?citric acid cycle?(also known as the Krebs cycle). Every ?turn? of the citric acid cycle produces two molecules of carbon dioxide, one equivalent of ATP?guanosine triphosphate?(GTP) through?substrate-level phosphorylation?catalyzed by?succinyl-CoA synthetase, three molecules of?NADH, and one equivalent of?FADH2. NADH and FADH2?are recycled to (NAD+?and?FAD, respectively), generating additional ATP by?oxidative phosphorylation. The oxidation of NADH results in the synthesis of 2?3 equivalents of ATP, and the oxidation of one FADH2?yields between 1?2 equivalents of ATP.[15]?The majority of cellular ATP is generated by this process. Although the citric acid cycle itself does not involve molecular?oxygen, it is an obligately?aerobic?process because O2?is used to recycle the NADH and FADH2. In the absence of oxygen, the citric acid cycle ceases.[16]

The generation of ATP by the mitochondrion from cytosolic NADH relies on the?malate-aspartate shuttle?(and to a lesser extent, the?glycerol-phosphate shuttle) because the inner mitochondrial membrane is impermeable to NADH and NAD+. Instead of transferring the generated NADH, a?malate dehydrogenase?enzyme converts?oxaloacetate?to?malate, which is translocated to the mitochondrial matrix. Another malate dehydrogenase-catalyzed reaction occurs in the opposite direction, producing oxaloacetate and NADH from the newly transported malate and the mitochondrion?s interior store of NAD+. A?transaminase?converts the oxaloacetate to?aspartate?for transport back across the membrane and into the intermembrane space.[16]

In oxidative phosphorylation, the passage of electrons from NADH and FADH2?through the electron transport chain pumps?protons?out of the mitochondrial matrix and into the intermembrane space. This pumping generates a?proton motive force?that is the net effect of a?pH?gradient and an?electric potential?gradient across the inner mitochondrial membrane. Flow of protons down this potential gradient?? that is, from the intermembrane space to the matrix?? yields ATP by?ATP synthase.[18]

Most of the ATP synthesized in the mitochondria will be used for cellular processes in the cytosol; thus it must be exported from its site of synthesis in the mitochondrial matrix. The inner membrane contains an?antiporter, the?ADP/ATP translocase, which is an?integral membrane protein?used to exchange newly synthesized ATP in the matrix for?ADP?in the intermembrane space.[19]?This translocase is driven by the membrane potential, as it results in the movement of about 4 negative charges out of the mitochondrial membrane in exchange for 3 negative charges moved inside. However, it is also necessary to transport phosphate into the mitochondrion; the phosphate carrier moves a proton in with each phosphate, partially dissipating the proton gradient.

Regulation

The citric acid cycle is regulated mainly by the availability of key substrates, particularly the ratio of NAD+?to NADH and the concentrations of?calcium, inorganic phosphate, ATP,?ADP, and AMP.?Citrate?? the molecule that gives its name to the cycle?? is a feedback inhibitor of?citrate synthase?and also inhibits PFK, providing a direct link between the regulation of the citric acid cycle and glycolysis.[17]

Beta oxidation

In the presence of air and various cofactors and enzymes, fatty acids are degraded to?acetyl-CoA. The pathway is called?beta-oxidation. Each cycle of beta-oxidation shortens the fatty acid chain by two carbon atoms and produces one equivalent each of NADH and one FADH2. The NADH and FADH2?are used to generate ATP by oxidative phosphorylation. Dozens of ATP equivalents are generated by the beta-oxidation of a single long acyl chain.[20]?The acetyl-CoA produced by beta-oxidation can be subsequently metabolized by the citric acid cycle, yielding further equivalents of ATP.

Regulation

In oxidative phosphorylation, the key control point is the reaction catalyzed by?cytochrome c oxidase, which is regulated by the availability of its substrate?? the reduced form of?cytochrome c. The amount of reduced cytochrome c available is directly related to the amounts of other substrates:

?1?2?NADH + cyt?cox?+ ADP + Pi????1?2?NAD+?+ cyt?cred?+ ATP

which directly implies this equation:

{displaystyle {frac {[mathrm {cyt~c_{red}} ]}{[mathrm {cyt~c_{ox}} ]}}=left({frac {[mathrm {NADH} ]}{[mathrm {NAD} ]^{+}}}right)^{frac {1}{2}}left({frac {[mathrm {ADP} ][mathrm {P_{i}} ]}{[mathrm {ATP} ]}}right)K_{mathrm {eq} }}{displaystyle {frac {[mathrm {cyt~c_{red}} ]}{[mathrm {cyt~c_{ox}} ]}}=left({frac {[mathrm {NADH} ]}{[mathrm {NAD} ]^{+}}}right)^{frac {1}{2}}left({frac {[mathrm {ADP} ][mathrm {P_{i}} ]}{[mathrm {ATP} ]}}right)K_{mathrm {eq} }}

Thus, a high ratio of [NADH] to [NAD+] or a high ratio of [ADP][Pi] to [ATP] imply a high amount of reduced cytochrome c and a high level of cytochrome c oxidase activity.[17]?An additional level of regulation is introduced by the transport rates of ATP and NADH between the mitochondrial matrix and the cytoplasm.[19]

Production, anaerobic conditions

Fermentation?is the metabolism of organic compounds in the absence of air. It involves?substrate-level phosphorylation?in the absence of a respiratory?electron transport chain. The equation for the oxidation of glucose to?lactic acid?is:

C
6H
12O
6
?? 2?CH
3CH(OH)COOH
?+ 2?ATP

Anaerobic respiration?is respiration in the absence of?O
2
. Prokaryotes can utilize a variety of electron acceptors. These include?nitrate,?sulfate, and carbon dioxide.

ATP replenishment by nucleoside diphosphate kinases

ATP can also be synthesized through several so-called ?replenishment? reactions catalyzed by the enzyme families of?nucleoside diphosphate kinases?(NDKs), which use other nucleoside triphosphates as a high-energy phosphate donor, and the?ATP:guanido-phosphotransferase?family.

ATP production during photosynthesis

In plants, ATP is synthesized in the?thylakoid membrane?of the?chloroplast. The process is called photophosphorylation. The ?machinery? is similar to that in mitochondria except that light energy is used to pump protons across a membrane to produce a proton-motive force. ATP synthase then ensues exactly as in oxidative phosphorylation.[21]?Some of the ATP produced in the chloroplasts is consumed in the?Calvin cycle, which produces?triose?sugars.

ATP recycling

The total quantity of ATP in the human body is about 0.2?moles. The majority of ATP is recycled from?ADP?by the aforementioned processes. Thus, at any given time, the total amount of ATP +?ADP?remains fairly constant.

The energy used by human cells requires the?hydrolysis?of 100 to 150?moles of ATP daily, which is around 50 to 75?kg. A human will typically use up his or her body weight of ATP over the course of the day. Each equivalent of ATP is recycled 500-750 times during a single day (100 / 0.2 = 500).

Intracellular signaling

ATP is involved?signal transduction?by serving as substrate for kinases, enzymes that transfer phosphate groups. Kinases are the most common ATP-binding proteins. They share a small number of common folds.[22]Phosphorylation?of a protein by a kinase can activate a cascade such as the?mitogen-activated protein kinase?cascade.[23]

ATP is also a substrate of?adenylate cyclase, most commonly in?G protein-coupled receptor?signal transduction pathways and is transformed to?second messenger, cyclic AMP, which is involved in triggering calcium signals by the release of calcium from intracellular stores.[24]?This form of signal transduction is particularly important in brain function, although it is involved in the regulation of a multitude of other cellular processes.[25]

DNA and RNA synthesis

ATP is one of four ?monomers? required in the synthesis of?RNA. The process is promoted by?RNA polymerases.[26]?A similar process occurs in the formation of DNA, except that ATP is first converted to the?deoxyribonucleotide?dATP. Like many condensation reactions in nature,?DNA replication?and?DNA transcription?also consumes ATP.

Amino acid activation in protein synthesis

Aminoacyl-tRNA synthetase?enzymes consume ATP in the attachment tRNA to amino acids, forming aminoacyl-tRNA complexes. Aminoacyl transferase binds AMP-amino acid to tRNA. The coupling reaction proceeds in two steps:

  1. aa + ATP ?> aa-AMP +?PPi
  2. aa-AMP + tRNA ?> aa-tRNA + AMP

The amino acid is coupled to the penultimate nucleotide at the 3?-end of the tRNA (the A in the sequence CCA) via an ester bond (roll over in illustration).

ATP binding cassette transporter

Transporting chemicals out of a cell against a gradient is often associated with ATP hydrolysis. Transport is mediated by?ATP binding cassette transporters. The human genome encodes 48 ABC transporters, that are used for exporting drugs, lipids, and other compounds.[27]

Biochemistry laboratories often use?in vitro?studies to explore ATP-dependent molecular processes.?Enzyme inhibitors?of ATP-dependent enzymes such as?kinases?are needed to examine the?binding sites?and?transition states?involved in ATP-dependent reactions. ATP analogs are also used in?X-ray crystallography?to determine a?protein structure?in complex with ATP, often together with other substrates. Most useful ATP analogs cannot be hydrolyzed as ATP would be; instead they trap the enzyme in a structure closely related to the ATP-bound state. Adenosine 5?-(?-thiotriphosphate) is an extremely common ATP analog in which one of the gamma-phosphate oxygens is replaced by a?sulfur?atom; this anion is hydrolyzed at a dramatically slower rate than ATP itself and functions as an inhibitor of ATP-dependent processes. In crystallographic studies, hydrolysis transition states are modeled by the bound?vanadate?ion. However, caution is warranted in interpreting the results of experiments using ATP analogs, since some enzymes can hydrolyze them at appreciable rates at high concentration.[28]

ATP was discovered in 1929 by Karl Lohmann,[29]?and independently by Cyrus Fiske and?Yellapragada Subbarow?of?Harvard Medical School,[30]?but its correct structure was not determined until some years later.[citation needed]?It was proposed to be the intermediary between energy-yielding and energy-requiring reactions in cells by?Fritz Albert Lipmann?in 1941.[31]?It was first synthesized in the laboratory by?Alexander Todd?in 1948.[32]

Adenosine-5?-triphosphate (ATP) is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the ?molecular unit of currency? of intracellular energy transfer. ATP transports chemical energy within cells for metabolism. It is one of the end products of photophosphorylation and cellular respiration and used by enzymes and structural proteins in many cellular processes, including biosynthetic reactions, motility, and cell division. One molecule of ATP contains three phosphate groups, and it is produced by ATP synthase from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP).?

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