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{{More citations needed|date=February 2018}}

'''Phosphagens''', also known as '''macroergic''' compounds, are high energy storage compounds, also known as [[high-energy phosphate]] compounds, chiefly found in [[muscle|muscular]] tissue in [[animal]]s. They allow a high-energy phosphate pool to be maintained in a concentration range, which, if it all were [[adenosine triphosphate]] (ATP), would create problems due to the ATP-consuming reactions in these tissues. As muscle tissues can have sudden demands for much energy, these compounds can maintain a reserve of high-energy phosphates that can be used as needed, to provide the energy that could not be immediately supplied by [[glycolysis]] or [[oxidative phosphorylation]]. Phosphagens supply immediate but limited energy.

The actual [[biomolecule]] used as a phosphagen is dependent on the organism. The majority of animals use [[arginine]] as phosphagen; however, the phylum [[Chordata]] (i.e., animals with spinal cords) use [[creatine]]. [[Creatine phosphate]] (CP), or [[Creatine phosphate|phosphocreatine]] (PCr), is made from ATP by the enzyme [[creatine kinase]] in a reversible reaction:

* Creatine + ATP {{eqm}} creatine phosphate + ADP + {{H+|nolink}} (this reaction is [[Magnesium in biology#Biological chemistry|{{chem2|Mg(2+)}}]]-dependent)

However, [[annelids]] (segmented worms) use a set of unique phosphagens; for example, earthworms use the compound [[lombricine]].

Phosphagens were discovered by [[Philip Eggleton]] and his wife Grace Eggleton.<ref>Selected Topics in the History of Biochemistry, G Semenza</ref>

== Reactions ==
{{main|Bioenergetic systems#ATP–CP: the phosphagen system}}
The Phosphagen System ([[ATP-PC|ATP-PCr]]) occurs in the [[cytosol]] (a gel-like substance) of the [[sarcoplasm]] of [[skeletal muscle]], and in the [[myocyte]]'s [[Cytosol|cytosolic]] compartment of the [[cytoplasm]] of [[Cardiac muscle|cardiac]] and [[smooth muscle]].<ref>{{cite journal |vauthors=Wallimann T, Wyss M, Brdiczka D, Nicolay K, Eppenberger HM |title=Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis |journal=Biochem J |volume=281 |issue=Pt 1 |pages=21–40 |date=January 1992 |doi=10.1042/bj2810021 |pmid=1731757 |pmc=1130636}}</ref>
[[File:Creatine kinase reaction.svg|thumb|245x245px|Creatine kinase reaction]]
During muscle contraction: 

:{{H2O-nl}} + ATP → {{H+|nolink}} + ADP + {{chem2|P_{i}|}} ({{chem2|link=Magnesium in biology#Biological chemistry|Mg(2+)}} assisted, utilization of ATP for [[Muscle contraction]] by [[ATPase]])
:{{H+|nolink}} + ADP + CP → ATP + Creatine ({{chem2|Mg(2+)}} assisted, catalyzed by [[creatine kinase]], ATP is used again in the above reaction for continued muscle contraction)
:2 ADP → ATP + AMP (catalyzed by [[adenylate kinase]]/myokinase when CP is depleted, ATP is again used for muscle contraction)[[File:Purine Nucleotide Cycle.jpg|thumb|250x250px|Phosphagen System (ATP-PCr) and Purine Nucleotide Cycle (PNC)]]
Muscle at rest:
:ATP + Creatine → ADP + CP + {{H+|nolink}} ({{chem2|Mg(2+)}} assisted, catalyzed by [[creatine kinase]])
:ADP + {{chem2|P_{i}|}} → ATP (during [[anaerobic glycolysis]] and [[oxidative phosphorylation]])
When the Phosphagen System has been depleted of phosphocreatine (creatine phosphate), the resulting AMP produced from the [[adenylate kinase]] (myokinase) reaction is primarily regulated by the [[Purine nucleotide cycle|Purine Nucleotide Cycle]].<ref>{{cite book |last=Bhagavan |first=N.V. |last2=Ha |first2=Chung-Eun |chapter=19. Contractile Systems |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780124166875000191 |doi=10.1016/B978-0-12-416687-5.00019-1 |title=Essentials of Medical Biochemistry |publisher=Elsevier |edition= |date=2015 |isbn=978-0-12-416687-5 |pages=339–361 }}</ref><ref>{{Citation |last=Valberg |first=Stephanie J. |chapter=15. Skeletal Muscle Function |date=2008 |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780123704917000155 |title=Clinical Biochemistry of Domestic Animals |edition=6th  |pages=459–484 |editor-last=Kaneko |editor-first=J. Jerry |access-date=2023-10-10 |publisher=Academic Press |isbn=978-0-12-370491-7 |editor2-last=Harvey |editor2-first=John W. |editor3-last=Bruss |editor3-first=Michael L.}}</ref>
==References==
{{reflist}}

==Further reading==

*{{cite journal|last1=Ellington|first1=W Ross|title=Evolution and Physiological Roles of Phosphagen Systems|journal=Annual Review of Physiology|volume=63|issue=1|year=2001|pages=289–325 |doi=10.1146/annurev.physiol.63.1.289|pmid=11181958}}
*{{cite journal|first1=Philip | last1= Eggleton | first2=  Grace Palmer | last2 = Eggleton|title=The physiological significance of "phosphagen"|journal=Journal of Physiology |volume=63|issue=2|year=1927|pages=155–161| pmc= 1514923 | pmid=16993876 | doi=10.1113/jphysiol.1927.sp002391}}
*{{cite journal|first1=Philip | last1= Eggleton | first2=  Grace Palmer | last2 = Eggleton|title=Further observations on phosphagen|journal=Journal of Physiology |volume=65|issue=1|year=1928|pages=15–24| pmc= 1515019 | pmid=16993934 | doi=10.1113/jphysiol.1928.sp002457}}
*{{cite journal|last1=Baldwin|first1=Ernest|title=PHOSPHAGEN|journal=Biological Reviews|volume=8|issue=1|year=1933|pages=74–105|issn=1464-7931|doi=10.1111/j.1469-185X.1933.tb01088.x|s2cid=221532329}}

[[Category:Biomolecules]]
[[Category:Metabolism]]
[[Category:Organophosphates]]