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|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−.
Other citric acid ions
Salts of the hydrogen citrate ions are weakly acidic, while salts of the citrate ion itself (with an inert cation such as sodium ion) are weakly basic.
As a weak acid, citrate can be used as a component in buffer solutions, including the commonly used SSC 20X hybridization buffer. 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 describe solutions of citrate and acid for buffer pHs of between 3.0 and 6.2.
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.
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. 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]
- 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
- Maniatis, T.; Fritsch, E. F.; Sambrook, J. 1982. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.
- Gomori, G. (1955). "Methods in Enzymology Volume 1". Methods in Enzymology 1. p. 138. doi:10.1016/0076-6879(55)01020-3. ISBN 9780121818012.
|Citric Acid Cycle Metabolic Pathway|
|Acetyl-CoA||NADH + H+||NAD+||H2O||FADH2||FAD||CoA + ATP(GTP)||Pi + ADP(GDP)|
|+||H2O||NADH + H+ + CO2|
|H2O||H2O||NAD(P)+||NAD(P)H + H+||CO2|