A citrate is an ionic form of citric acid, such as C3H5O(COO)33-, that is, citric acid minus three hydrogen ions.

Chemical strucutre of citric acid.

Citrates are compounds containing this group, either ionic compounds, the salts, or analogous covalent compounds, esters. An example of a salt is sodium citrate and an ester is trimethyl citrate. See category for a bigger list.

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.

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.

Citrate is a key component in the commonly used SSC 20X hybridization buffer. There exists authoritative literature (Maniatis) that incorrectly instructs the preparation of this buffer to include 3M NaCl and 0.3M Sodium Citrate, to be titrated up with NaOH to a pH of 7. When the two components are actually mixed together, the pH is slightly basic. Therefore, the pH of the solution should instead be titrated down to 7 with either citric acid or HCl.

Citrate is also an intermediate in the TCA (Krebs) Cycle. After pyruvate dehydrogenase forms acetyl CoA (from pyruvate using five cofactors: TPP, lipoamide, FAD, NAD+, and CoA), Citrate Synthase catalyzes the condensation of OAA with Acetyl CoA to form citrate. The rest of the cycle entails subsequent conversion into isocitrate, alpha-keto glutarate, succinyl-CoA, succinate, fumarate, malate, and back to OAA. These reactions are catalyzed by isocitrate dehydrogenase, alpha-keto glutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, and malate dehydrogenase, respectively.


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