View source for Biology:Pasteur effect

{{short description|Inhibiting effect of oxygen on the fermentation process}}
The '''Pasteur effect''' describes how available [[Chemistry:Oxygen|oxygen]] inhibits [[Chemistry:Ethanol fermentation|ethanol fermentation]], driving [[Biology:Yeast|yeast]] to switch toward [[Biology:Cellular respiration|aerobic respiration]] for increased generation of the energy carrier [[Chemistry:Adenosine triphosphate|adenosine triphosphate (ATP)]].<ref name=":0">{{Cite journal | vauthors = Lagunas R |date=1981-01-01 |title=Is Saccharomyces cerevisiae a typical facultative anaerobe?  |journal=Trends in Biochemical Sciences |language=en |volume=6 |pages=201–203 |doi=10.1016/0968-0004(81)90073-6 |issn=0968-0004}}</ref> More generally, in the medical literature, the Pasteur effect refers to how the cellular presence of oxygen causes in cells a decrease in the rate of [[Biology:Glycolysis|glycolysis]] and also a suppression of [[Chemistry:Lactic acid|lactate]] accumulation. The effect occurs in animal tissues, as well as in microorganisms belonging to the [[Biology:Fungus|fungal]] [[Biology:Kingdom|kingdom]].<ref>{{cite web|url=https://medical-dictionary.thefreedictionary.com/Pasteur+effect |title=Pasteur effect |website=The Free Dictionary by Farlex}}</ref><ref>{{cite journal|url=https://www.biomolther.org/journal/view.html?uid=897&vmd=Full | doi=10.4062/biomolther.2017.194 | title=Convergence of Cancer Metabolism and Immunity: An Overview | date=2018 | last1=Dang | first1=Chi Van | last2=Kim | first2=Jung-Whan | journal=Biomolecules & Therapeutics | volume=26 | issue=1 | pages=4–9 | pmid=29212301 | pmc=5746032 }}</ref>

==Discovery==
The effect was described by Louis Pasteur in 1857 in experiments showing that [[Physics:Aeration|aeration]] of yeasted broth causes cell growth to increase while the fermentation rate decreases, based on lowered ethanol production.<ref>{{cite journal | vauthors = Pasteur L | title = Mémoire sur la fermentation applée lactique.  | journal = Comptes rendus de l'Académie des Sciences | trans-title = Dissertation on apple lactic fermentation | language = French | date = 1857 | volume = 45 | issue = 913–916 | pages = 1032–1036 }}<br>{{cite journal|pmc=2229983 |date=1995 |last1=Pasteur |first1=L. |title=Mémoire sur la fermentation appelée lactique (Extrait par l'auteur) |journal=Molecular Medicine |volume=1 |issue=6 |pages=599–601 }}<br>{{cite journal|url=https://link.springer.com/article/10.1007/BF03401600 |doi=10.1007/BF03401600 |title=Report on the Lactic Acid Fermentation (Author's abstract) |date=1995 |last1=Brock |first1=Thomas D. |journal=Molecular Medicine |volume=1 |issue=6 |pages=601–602 |s2cid=92130442 |doi-access=free }}</ref><ref name="pmid23875175">{{cite journal | vauthors = Vadlakonda L, Dash A, Pasupuleti M, Anil Kumar K, Reddanna P | title = Did we get Pasteur, Warburg, and Crabtree on a right note? | journal = Frontiers in Oncology | volume = 3 | issue = | pages = 186 | date = 2013 | pmid = 23875175 | pmc = 3710993 | doi = 10.3389/fonc.2013.00186 | doi-access = free }}</ref>

==Explanation==
Yeast fungi, being facultative anaerobes, can either produce energy through ethanol fermentation or aerobic respiration. When the O<sub>2</sub> concentration is low, the two [[Chemistry:Pyruvic acid|pyruvate]] molecules formed through [[Biology:Glycolysis|glycolysis]] are each fermented into [[Chemistry:Ethanol|ethanol]] and [[Chemistry:Carbon dioxide|carbon dioxide]]. While only 2 ATP are produced per glucose, this method is utilized under anaerobic conditions because it oxidizes the electron shuttle NADH into NAD<sup>+</sup> for another round of glycolysis and ethanol fermentation.

If the concentration of oxygen increases, pyruvate is instead converted to acetyl CoA, used in the [[Biology:Citric acid cycle|citric acid cycle]], and undergoes [[Biology:Oxidative phosphorylation|oxidative phosphorylation]]. Per glucose, 10 NADH and 2 FADH<sub>2</sub> are produced in cellular respiration for a significant amount of proton pumping to produce a proton gradient utilized by [[Biology:ATP synthase|ATP Synthase]]. While the exact ATP output ranges based on considerations like the overall electrochemical gradient, aerobic respiration produces far more ATP than the anaerobic process of ethanol fermentation. The increased ATP and [[Chemistry:Citric acid|citrate]] from aerobic respiration [[Biology:Allosteric regulation|allosterically inhibit]] the glycolysis enzyme [[Biology:Phosphofructokinase 1|phosphofructokinase 1]] because less pyruvate is needed to produce the same amount of ATP.

Despite this energetic incentive, Rosario Lagunas has shown that yeast continue to partially ferment available glucose into ethanol for many reasons.<ref name=":0" /> First, glucose metabolism is faster through ethanol fermentation because it involves fewer enzymes and limits all reactions to the [[Biology:Cytoplasm|cytoplasm]]. Second, ethanol has bactericidal activity by causing damage to the [[Biology:Cell membrane|cell membrane]] and [[Biology:Denaturation (biochemistry)|protein denaturing]], allowing yeast fungus to outcompete environmental bacteria for resources.<ref>{{cite journal | vauthors = McDonnell G, Russell AD | title = Antiseptics and disinfectants: activity, action, and resistance | journal = Clinical Microbiology Reviews | volume = 12 | issue = 1 | pages = 147–179 | date = January 1999 | pmid = 9880479 | pmc = 88911 | doi = 10.1128/CMR.12.1.147 }}</ref> Third, partial fermentation may be a defense mechanism against environmental competitors depleting all oxygen faster than the yeast's regulatory systems could fully switch from aerobic respiration to ethanol fermentation.

==Practical implications==
The fermentation processes used in [[Chemistry:Alcohol|alcohol]] production is commonly maintained in low oxygen conditions, under a blanket of carbon dioxide, while growing yeast for biomass involves aerating the broth for maximized energy production. Despite the bactericidal effects of ethanol, acidifying effects of fermentation, and low oxygen conditions of industrial alcohol production, bacteria that undergo [[Biology:Lactic acid fermentation|lactic acid fermentation]] can contaminate such facilities because lactic acid has a low pKa of 3.86 to avoid decoupling the pH membrane gradient that supports regulated transport.<ref>{{cite journal | vauthors = Beckner M, Ivey ML, Phister TG | title = Microbial contamination of fuel ethanol fermentations | journal = Letters in Applied Microbiology | volume = 53 | issue = 4 | pages = 387–394 | date = October 2011 | pmid = 21770989 | doi = 10.1111/j.1472-765X.2011.03124.x | s2cid = 6941273 }}</ref>

== See also ==
* [[Chemistry:Ethanol fermentation|Ethanol fermentation]]
* Fermentation (biochemistry)
* [[Biology:Facultative anaerobic organism|Facultative anaerobic organism]]
* [[Biology:Allosteric regulation|Allosteric regulation]]

== References ==
{{Reflist}}

== Further reading ==
{{Refbegin}}
* {{cite journal | vauthors = Krebs HA | title = The Pasteur effect and the relations between respiration and fermentation | journal = Essays in Biochemistry | volume = 8 | issue = 8 | pages = 1–34 | year = 1972 | pmid = 4265190 | }}
{{refend}}

[[Category:Metabolism]]

{{Sourceattribution|Pasteur effect}}