P50 (pressure): Difference between revisions
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{{Short description|Chemistry term for pressure}} |
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{{Other uses|P50 (disambiguation)}} |
{{Other uses|P50 (disambiguation)}} |
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In [[biochemistry]], ''' |
In [[biochemistry]], '''''p''<sub>50</sub>''' represents the [[partial pressure]] of a [[gas]] required to achieve 50% saturation of a particular protein's binding sites.<ref>[http://medical-dictionary.thefreedictionary.com/P+50 P 50 – definition of P 50 in the Medical dictionary – by the Free Online Medical Dictionary, Thesaurus and Encyclopedia]. Medical-dictionary.thefreedictionary.com. Retrieved on 2013-10-22.</ref><ref>[http://www.lexingtonpulmonary.com/education/oxyhb/oxyhb.html Oxygen/Hemoglobin] {{webarchive|url=https://web.archive.org/web/20100523084119/http://www.lexingtonpulmonary.com/education/oxyhb/oxyhb.html |date=2010-05-23 }}. Lexingtonpulmonary.com. Retrieved on 2013-10-22.</ref> Values of ''p''<sub>50</sub> are negatively correlated with substrate affinity; lower values correspond to higher affinity and ''vice versa''. The term is analogous to the [[Michaelis–Menten constant]] (''K<sub>M</sub>''), which identifies the [[concentration]] of substrate required for an enzyme to achieve 50% of its maximum [[enzyme reaction rate|reaction velocity]]. |
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The concept of ''p''<sub>50</sub> is derived from considering the fractional saturation of a protein by a gas. Imagine [[myoglobin]], a protein which is able to bind a single molecule of [[oxygen]], as per the reversible reaction below, whose [[equilibrium constant]] ''K'' (which is also a dissociation constant, since it describes a reversible association-dissociation event) is equal to the product of the concentrations (at equilibrium) of free myoglobin and free oxygen, divided by the concentration of myoglobin-oxygen complex. |
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| ⚫ | For example, |
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: <chem>Mb + O_2 <=> Mb \cdot O_2</chem> |
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: <chem>\it{K}=\rm\frac{[Mb][O_2]}{[Mb\cdot O_2]}</chem> |
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The fractional saturation ''Y''<sub>''O''2</sub> of the myoglobin is what proportion of the total myoglobin concentration is made up of oxygen-bound myoglobin, which can be rearranged as the concentration of free oxygen over the sum of that concentration and the dissociation constant ''K''. Since diatomic oxygen is a gas, its concentration in solution can be thought of as a partial pressure. |
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: <math>Y_{O_2}=\rm\frac{[Mb\cdot O_2]}{[Mb]+[Mb\cdot O_2]} |
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\Rightarrow\rm\frac{[O_2]}{\it{K}\,\rm{+\,[O_2]}} |
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\Rightarrow\rm\frac{\it{p\rm{O_2}}}{\it{K}\,\rm{+\,\it{p\rm{O_2}}}}</math> |
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From defining the ''p''<sub>50</sub> as the partial pressure at which the fractional saturation is 50%, we can deduce that it is in fact equal to the dissociation constant ''K''. |
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: <math>\frac{p_{50}}{K+p_{50}}=0.5 |
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\Rightarrow p_{50}=K |
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</math> |
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| ⚫ | For example, myoglobin's ''p''<sub>50</sub> for [[oxygen|O<sub>2</sub>]] is 130 [[pascal (unit)|pascal]]s while the ''P''<sub>50</sub> for [[hemoglobin|adult hemoglobin]] is 3.5 kPa. Thus, when O<sub>2</sub> partial pressure is low, hemoglobin-bound O<sub>2</sub> is more readily transferred to myoglobin. Myoglobin, found in high concentrations in [[muscle]] tissue, can then transfer the oxygen to muscle tissue [[muscle fiber]]s, where it will be used in the generation of energy to fuel muscle contraction.<ref>{{cite journal|pmid=2706089 | volume=11 | issue=1 | title=Modification of partial pressure of oxygen (P50) in mammalian red blood cells by incorporation of an allosteric effector of hemoglobin | year=2001| journal=Biotechnol. Appl. Biochem. | pages=31–40|last1=Bailleul|first1=C|last2=Borrelly-Villereal|first2=MC|last3=Chassaigne|first3=M|last4=Ropars|first4=C| doi=10.1111/j.1470-8744.1989.tb00049.x | s2cid=19110159 }}</ref> Another example is that of human fetal hemoglobin, which has a higher affinity (lower ''P''<sub>50</sub>) than adult hemoglobin, and therefore allows uptake of oxygen across the placental diffusion barrier.<ref>{{Cite book|last1=Widmaier|first1=Eric P.|title=Vander's Human Physiology: The Mechanisms of Body Function|last2=Raff|first2=Hershel|last3=Strang|first3=Kevin T.|publisher=[[McGraw Hill]]|year=2019|isbn=978-1-259-90388-5|edition=15th|location=New York, NY|pages=469}}</ref> |
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==References== |
==References== |
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Latest revision as of 11:31, 17 April 2023
In biochemistry, p50 represents the partial pressure of a gas required to achieve 50% saturation of a particular protein's binding sites.[1][2] Values of p50 are negatively correlated with substrate affinity; lower values correspond to higher affinity and vice versa. The term is analogous to the Michaelis–Menten constant (KM), which identifies the concentration of substrate required for an enzyme to achieve 50% of its maximum reaction velocity.
The concept of p50 is derived from considering the fractional saturation of a protein by a gas. Imagine myoglobin, a protein which is able to bind a single molecule of oxygen, as per the reversible reaction below, whose equilibrium constant K (which is also a dissociation constant, since it describes a reversible association-dissociation event) is equal to the product of the concentrations (at equilibrium) of free myoglobin and free oxygen, divided by the concentration of myoglobin-oxygen complex.
![{\displaystyle {\ce {\it {{K}={\rm {\frac {[Mb][O_{2}]}{[Mb\cdot O_{2}]}}}}}}}](https://wikimedia.org/enwiki/api/rest_v1/media/math/render/svg/0397d2f53b8b7428667d49ce59369824e43db2b9)
The fractional saturation YO2 of the myoglobin is what proportion of the total myoglobin concentration is made up of oxygen-bound myoglobin, which can be rearranged as the concentration of free oxygen over the sum of that concentration and the dissociation constant K. Since diatomic oxygen is a gas, its concentration in solution can be thought of as a partial pressure.
![{\displaystyle Y_{O_{2}}={\rm {{\frac {[Mb\cdot O_{2}]}{[Mb]+[Mb\cdot O_{2}]}}\Rightarrow {\rm {{\frac {[O_{2}]}{\it {{K}\,{\rm {+\,[O_{2}]}}}}}\Rightarrow {\rm {\frac {\it {p{\rm {O_{2}}}}}{\it {{K}\,{\rm {+\,{\it {p{\rm {O_{2}}}}}}}}}}}}}}}}](https://wikimedia.org/enwiki/api/rest_v1/media/math/render/svg/07552f610b79fff8db39ce05e9bcd554be165bbf)
From defining the p50 as the partial pressure at which the fractional saturation is 50%, we can deduce that it is in fact equal to the dissociation constant K.
For example, myoglobin's p50 for O2 is 130 pascals while the P50 for adult hemoglobin is 3.5 kPa. Thus, when O2 partial pressure is low, hemoglobin-bound O2 is more readily transferred to myoglobin. Myoglobin, found in high concentrations in muscle tissue, can then transfer the oxygen to muscle tissue muscle fibers, where it will be used in the generation of energy to fuel muscle contraction.[3] Another example is that of human fetal hemoglobin, which has a higher affinity (lower P50) than adult hemoglobin, and therefore allows uptake of oxygen across the placental diffusion barrier.[4]
References
[edit]- ^ P 50 – definition of P 50 in the Medical dictionary – by the Free Online Medical Dictionary, Thesaurus and Encyclopedia. Medical-dictionary.thefreedictionary.com. Retrieved on 2013-10-22.
- ^ Oxygen/Hemoglobin Archived 2010-05-23 at the Wayback Machine. Lexingtonpulmonary.com. Retrieved on 2013-10-22.
- ^ Bailleul, C; Borrelly-Villereal, MC; Chassaigne, M; Ropars, C (2001). "Modification of partial pressure of oxygen (P50) in mammalian red blood cells by incorporation of an allosteric effector of hemoglobin". Biotechnol. Appl. Biochem. 11 (1): 31–40. doi:10.1111/j.1470-8744.1989.tb00049.x. PMID 2706089. S2CID 19110159.
- ^ Widmaier, Eric P.; Raff, Hershel; Strang, Kevin T. (2019). Vander's Human Physiology: The Mechanisms of Body Function (15th ed.). New York, NY: McGraw Hill. p. 469. ISBN 978-1-259-90388-5.
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