Citrate

Citrate

Citrate
Names
IUPAC name
2-hydroxypropane-1,2,3-tricarboxylate
Identifiers
ChemSpider  Y
Jmol-3D images Image
PubChem
Properties
C
6
H
5
O3−
7
Molar mass 189 g mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

A citrate is a derivative of citric acid; that is, the salts, esters, and the polyatomic anion found in solution. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate. When part of a salt, the formula of the citrate ion is written as C6H5O73− or C3H5O(COO)33−.

Contents

  • Other citric acid ions 1
  • Acidity 2
  • Buffering 3
  • Chelating 4
  • Metabolism 5
    • Interactive pathway map 5.1
  • Fatty acid synthesis 6
    • Role in glycolysis 6.1
  • References 7

Other citric acid ions

Since citric acid is a multifunctional acid, intermediate ions exist, hydrogen citrate ion, HC6H5O72− and dihydrogen citrate ion, H2C6H5O7. These may form salts as well, called acid salts.

Acidity

Hydrogen citrate is weakly acidic, while salts of the citrate ion itself (such as sodium citrate) are weakly basic.

Buffering

As a weak acid, citrate can be used as a component in buffer solutions, including the commonly used SSC 20X hybridization buffer.[1] This buffer uses sodium citrate and sodium chloride to maintain a neutral 7.0 pH. Other buffers may use a mixture of sodium citrate and citric acid – canonical buffer tables compiled for biochemical studies[2] describe solutions of citrate and acid for buffer pHs of between 3.0 and 6.2.

Chelating

Citric acid can act as a mild chelating agent; citrate, usually in the form of trisodium citrate, may be given as an anticoagulant, because it chelates calcium ions, and therefore inhibits coagulation. Another application is in the form of iron(II) citrate as a nutritional supplement. Here, the benefit is the solubility as a chelate of the otherwise mostly insoluble iron.

Metabolism

Citrate is an intermediate in the TCA (Krebs) cycle, a central metabolic pathway for both eukaryotes such as animals and plants and prokaryotes such as bacteria. After the pyruvate dehydrogenase complex forms acetyl-CoA, from pyruvate and five cofactors (thiamine pyrophosphate, lipoamide, FAD, NAD+, and CoA), citrate synthase catalyzes the condensation of oxaloacetate with acetyl CoA to form citrate. Citrate continues in the TCA cycle via aconitase with the eventual regeneration of oxaloacetate, which can combine with another molecule of acetyl CoA and continue cycling.

Some bacteria, notably E. coli, can produce and consume citrate internally as part of their TCA cycle, but are unable to use it as food because they lack the enzymes required to import it into the cell. The acquisition by these bacteria, after tens of thousands of generations, of the ability to use citrate as food was studied by Lenski et al.[3][4] to explore mechanisms of evolution under selective pressure (in this case, a citrate-containing culture medium with limited amounts of other foods). They found evidence that in this case the innovation occurred via an accumulation of several somewhat rare mutations, none of which by itself would confer the selective advantage, rather than by a single extremely rare mutation.

Interactive pathway map

Click on genes, proteins and metabolites below to link to respective articles. [§ 1]

}px; border:solid #ccc 1px; background-color:white;">
|}px|alt=TCA Cycle edit]]
TCA Cycle edit
  1. ^ The interactive pathway map can be edited at WikiPathways: "TCACycle_WP78". 

Fatty acid synthesis

Citrate can also be transported out of the mitochondria and into the cytoplasm, then broken down into acetyl-CoA for fatty acid synthesis and into oxaloacetate. Citrate is a positive modulator of this conversion, and allosterically regulates the enzyme acetyl-CoA carboxylase, which is the regulating enzyme in the conversion of acetyl-CoA into malonyl-CoA (the commitment step in fatty acid synthesis). In short, citrate is transported to the cytoplasm, converted to acetyl CoA, which is converted into malonyl CoA by the acetyl CoA carboxylase, which is allosterically modulated by citrate.

See also TCA cycle

Role in glycolysis

High concentrations of cytosolic citrate can inhibit phosphofructokinase, catalyst of one of the rate-limiting steps of glycolysis.

References

  1. ^ Maniatis, T.; Fritsch, E. F.; Sambrook, J. 1982. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.
  2. ^ Gomori, G. (1955). "Methods in Enzymology Volume 1". Methods in Enzymology 1. p. 138.  
  3. ^ http://phys.org/news/2014-02-bacteria-freezer-yield-startling-results.html
  4. ^ http://www.rpgroup.caltech.edu/courses/Evolution_GIST_2013/files_2013/articles/PNAS-2008-Blount-7899-906.pdf