Editing Citric acid (section)

==Biochemistry==
===Citric acid cycle===
{{Main|Citric acid cycle}}

Citrate is an intermediate in the [[citric acid cycle]], also known as the ''tricarboxylic acid'' (''TCA'') ''cycle'' or the ''Krebs cycle'', a central metabolic pathway for animals, plants, and bacteria. In the Krebs cycle, [[citrate synthase]] catalyzes the condensation of [[oxaloacetate]] with acetyl CoA to form citrate. Citrate then acts as the substrate for [[aconitase]] and is converted into [[aconitic acid]]. The cycle ends with regeneration of oxaloacetate. This series of chemical reactions is the source of two-thirds of the food-derived energy in higher organisms. The chemical energy released is available under the form of [[Adenosine triphosphate]] (ATP). [[Hans Adolf Krebs]] received the 1953 [[Nobel Prize in Physiology or Medicine]] for the discovery.

===Other biological roles===
Citrate can 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 into the cytoplasm, converted into acetyl-CoA, which is then converted into malonyl-CoA by acetyl-CoA carboxylase, which is allosterically modulated by citrate.

High concentrations of cytosolic citrate can inhibit [[phosphofructokinase]], the catalyst of a rate-limiting step of [[glycolysis]]. This effect is advantageous: high concentrations of citrate indicate that there is a large supply of biosynthetic precursor molecules, so there is no need for phosphofructokinase to continue to send molecules of its substrate, [[fructose 6-phosphate]], into glycolysis. Citrate acts by augmenting the inhibitory effect of high concentrations of [[Adenosine triphosphate|ATP]], another sign that there is no need to carry out glycolysis.<ref name="Tightly Controlled">{{cite book|last1=Stryer|first1=Lubert|last2=Berg|first2=Jeremy|last3=Tymoczko|first3=John|title=Biochemistry|date=2003|publisher=Freeman|location=New York|isbn=978-0716746843|edition=5. ed., international ed., 3. printing|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK22395/|chapter=Section 16.2: The Glycolytic Pathway Is Tightly Controlled}}</ref>

Citrate is a vital component of bone, helping to regulate the size of [[apatite]] crystals.<ref>{{cite journal|last1= Hu |first1= Y.-Y. |last2= Rawal |first2= A. |last3= Schmidt-Rohr |first3= K. |title= Strongly bound citrate stabilizes the apatite nanocrystals in bone |journal= Proceedings of the National Academy of Sciences |volume= 107 |pages= 22425–22429 |date=December 2010 |doi= 10.1073/pnas.1009219107 |pmid= 21127269 |pmc= 3012505 |issue=52|bibcode= 2010PNAS..10722425H |doi-access= free }}</ref>