Editing Nucleoside triphosphate (section)

== Other cellular roles ==

=== ATP as a source of cellular energy ===
[[File:Adenosintriphosphat protoniert.svg|left|thumb|240x240px|The energy released during hydrolysis of adenosine tripshophate (ATP), shown here, is frequently coupled with energetically unfavourable cellular reactions.]]
[[Adenosine triphosphate|ATP]] is the primary energy currency of the cell.<ref>{{cite web |title=ATP |url=https://www.nature.com/scitable/definition/atp-318/ |website=Scitable }}</ref> Despite being synthesized through the metabolic pathway described above, it is primarily synthesized during both [[cellular respiration]]<ref>{{cite web |title=Mitochondria, Cell Energy, ATP Synthase |url=https://www.nature.com/scitable/topicpage/mitochondria-14053590/ |website=Scitable }}</ref> and [[photosynthesis]]<ref>{{Cite web|url=http://plantsinaction.science.uq.edu.au/content/124-atp-synthesis|title=ATP Synthesis|website=Plants in Action|access-date=2017-11-12}}</ref> by [[ATP synthase]]. ATP synthase couples the synthesis of ATP from ADP and phosphate with an [[electrochemical gradient]] generated by the pumping of protons through either the [[inner mitochondrial membrane]] (cellular respiration) or the [[thylakoid membrane]] (photosynthesis).<ref>{{cite journal | vauthors = Jonckheere AI, Smeitink JA, Rodenburg RJ | title = Mitochondrial ATP synthase: architecture, function and pathology | journal = Journal of Inherited Metabolic Disease | volume = 35 | issue = 2 | pages = 211–25 | date = March 2012 | pmid = 21874297 | pmc = 3278611 | doi = 10.1007/s10545-011-9382-9 }}</ref> This electrochemical gradient is necessary because the formation of ATP is [[Endergonic reaction|energetically unfavourable]].

The [[ATP hydrolysis|hydrolysis of ATP]] to ADP and P<sub>i</sub> proceeds as follows:<ref>{{cite journal | vauthors = Dittrich M, Hayashi S, Schulten K | title = On the mechanism of ATP hydrolysis in F1-ATPase | journal = Biophysical Journal | volume = 85 | issue = 4 | pages = 2253–66 | date = October 2003 | pmid = 14507690 | pmc = 1303451 | doi = 10.1016/S0006-3495(03)74650-5 | bibcode = 2003BpJ....85.2253D }}</ref>

:<chem>ATP + H2O -> ADP + P_{i}</chem>

This reaction is [[Exergonic reaction|energetically favourable]] and releases 30.5 kJ/mol of energy.<ref name=Khan-ATP /> In the cell, this reaction is often coupled with unfavourable reactions to provide the energy for them to proceed.<ref>{{Cite web|url=https://courses.lumenlearning.com/boundless-biology/chapter/atp-adenosine-triphosphate/|title=ATP: Adenosine Triphosphate {{!}} Boundless Biology|website=courses.lumenlearning.com-US|access-date=2017-11-12}}</ref> [[Guanosine triphosphate|GTP]] is occasionally used for energy-coupling in a similar manner.<ref>{{cite journal | vauthors = Carvalho AT, Szeler K, Vavitsas K, Åqvist J, Kamerlin SC | title = Modeling the mechanisms of biological GTP hydrolysis | journal = Archives of Biochemistry and Biophysics | volume = 582 | issue = Supplement C | pages = 80–90 | date = September 2015 | pmid = 25731854 | doi = 10.1016/j.abb.2015.02.027 | series = Special issue in computational modeling on biological systems | url = http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-264885 | doi-access = free }}</ref> [[File:Activatoin-Adenylate cyclase-outlined.svg|thumb|236x236px|Binding of a ligand to a G protein-coupled receptor allows GTP to bind the G protein. This causes the alpha subunit to leave and act as a downstream effector.]]

=== GTP signal transduction ===
GTP is essential for [[signal transduction]], especially with [[G protein]]s. G proteins are coupled with a cell membrane bound receptor.<ref name=gpcr14047471/> This whole complex is called a [[G protein–coupled receptor|G protein-coupled receptor]] (GPCR). G proteins can bind either GDP or GTP. When bound to GDP, G proteins are inactive. When a [[ligand]] binds a GPCR, an [[Allosteric regulation|allosteric]] change in the G protein is triggered, causing GDP to leave and be replaced by GTP.<ref name=":3">{{Cite encyclopedia|url=https://www.britannica.com/science/G-protein-coupled-receptor|title=G protein-coupled receptor (GPCR) {{!}} biochemistry|encyclopedia=Encyclopedia Britannica|access-date=2017-11-12}}</ref> GTP activates the alpha subunit of the G protein, causing it to dissociate from the G protein and act as a downstream effector.<ref name=":3" />