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Butyric acid,
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Gamma-aminobutyric acid
Butyric acid, (from
Greek βουτυρος =
butter)
IUPAC name
n-Butanoic acid, or normal butyric acid, is a
carboxylic acid with structural formula
CH3CH
2CH
2-
COOH. It is notably found in rancid
butter,
parmesan cheese, and
vomit, and has an unpleasant odor and acrid taste, with a sweetish aftertaste (similar to
ether). Butyric acid can be detected by
mammals with good scent detection abilities (e.g.,
dogs) at 10
ppb, while
humans can detect it in concentrations above 10
ppm.
Butyric acid is a
fatty acid occurring in the form of esters in animal fats and plant oils. The
glyceride of butyric acid makes up 3% to 4% of butter. When butter goes rancid, butyric acid is liberated from the glyceride by
hydrolysis leading to the unpleasant odor.
Normal butyric acid or fermentation butyric acid is also found as a hexyl
ester in the oil of
Heracleum giganteum (cow parsnip) and as an octyl ester in
parsnip (
Pastinaca sativa); it has also been noticed in the fluids of the flesh and in perspiration.
It is ordinarily prepared by the fermentation of
sugar or
starch, brought about by the addition of putrefying
cheese, with
calcium carbonate added to neutralize the acids formed in the process. The butyric fermentation of starch is aided by the direct addition of
Bacillus subtilis.
Butryic acid is used in the preparation of various
butyrate esters. Low-molecular-weight esters of butyric acid, such as
methyl butyrate, have mostly pleasant aromas or tastes. As a consequence, they find use as food and perfume additives.
The acid is an oily colorless liquid that solidifies at -8
°C; it boils at 164 °C. It is easily soluble in
water,
ethanol and
ether, and is thrown out of its aqueous solution by the addition of
calcium chloride.
Potassium dichromate and
sulfuric acid (also known as sulphuric acid) oxidize it to
carbon dioxide and
acetic acid, while alkaline
potassium permanganate oxidizes it to
carbon dioxide. The calcium salt, Ca(C
4H
7O
2)
2·H
2O, is less soluble in hot water than in cold.
There is an
isomer,
isobutyric acid, which has the same chemical formula
C4H8 O2 but a different structure. It has similar chemical properties but different physical properties.
Butyrate fermentation
Butyrate is produced as end-product of a fermentation process solely performed by obligate
anaerobic bacteria. This fermentation pathway was discovered by
Louis Pasteur in
1861. Examples of butyrate producing
species:
*
Clostridium butyricum*
Clostridium kluyveri*
Clostridium pasteurianum*
Fusobacterium nucleatum*
Butyrivibrio fibrisolvens*
Eubacterium limosumThe pathway starts with the
glycolytic cleavage of
glucose to two
molecules of
pyruvate, as happens in most organisms. Pyruvate is then
oxidized into
acetyl coenzyme A using a unique mechanism that involves an
enzyme system called pyruvate-ferredoxin oxidoreductase. Two molecules of
carbon dioxide (CO
2) and two molecules of elemental
hydrogen (H
2) are formed in the process and exit the cell. Then:
* Acetyl coenzyme A converts into acetoacetyl coenzyme A; responsible enzyme: acetyl-CoA-acetyl transferase.
* Acetoacetyl coenzyme A converts into β-hydroxybutyryl CoA; responsible enzyme: β-hydroxybutyryl-CoA dehydrogenase.
* β-hydroxybutyryl CoA converts into crotonyl CoA; responsible enzyme: crotonase.
* Crotonyl CoA converts into butyryl CoA (CH
3CH
2CH
2C=O-CoA); responsible enzyme: butyryl CoA dehydrogenase.
* A
phosphate group replaces CoA to form butyryl phosphate; responsible enzyme: phosphobutyrylase.
* The phosphate group joins
ADP to form
ATP and butyrate; responsible enzyme: butyrate
kinase.
ATP is produced, as can be seen, in the last step of the fermentation. 3 ATPs are produced for each glucose molecule, a relatively high yield. The balanced equation for this fermentation is:
C6H12O6 → C4H8O2 + 2CO2 + 2H2 Acetone and butanol fermentation
Several species form
acetone and
butanol in an alternative pathway which starts as butyrate fermentation. Some of these species are:
*
Clostridium acetobutylicum: the most prominent acetone and butanol producer, used also industrially
*
Clostridium beijerinckii*
Clostridium tetanomorphum*
Clostridium aurantibutyricumThese bacteria begin with butyrate fermentation as described above, but, when the
pH drops below 5, they switch into butanol and acetone production in order to prevent further lowering of the pH. Two molecules of butanol are formed for each molecule of acetone.
The change in the pathway occurs after acetoacetyl CoA formation. This intermidiate then takes two possible pathways:
* Acetoacetyl CoA → acetoacetate → acetone, or
* Acetoacetyl CoA → butyryl CoA →
butanal → butanol.
Butyric acid function/activity
Butyric acid has been associated with the ability to inhibit the function of histone deacetylase enzymes, thereby favouring an acetylated state of histones in the cell. Acetylated histones have a lower affinity for DNA than non-acetylated histones, due to the neutralisation of electrostatic charge interections. It is generally thought that
transcription factors will be unable to access regions where histones are tightly associated with DNA (ie non-acetylated, eg heterochromatin). Therefore, it is thought that butyric acid enhances the transcriptional activity at promoters which are typically silenced/downregulated due to histone deacetylase activity.
This article incorporates information from the 1911 encyclopedia.Category:Carboxylic acidsCategory:Fatty acidsCategory:MetabolismCategory:ButyratesCategory:Foul-smelling chemicalsda:Smørsyrede:Buttersäurefr:Acide butanoïqueit:Acido butirricola:Acidum butyricumnl:Boterzuurja:酪酸pl:Kwas masłowypt:Ácido butíricoru:Масляная кислотаsv:Smörsyra