Citrate

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

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Citrate family

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 for a bigger list.

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.

pH

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.

Buffering

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 HCl.

Citric acid can act as a mild chelating agent.

Metabolism

TCA cycle

Citrate is an intermediate in the TCA (Krebs) Cycle. 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.

See also TCA cycle

Role in Glycolysis

High concentration of citrate can inhibit phosphofructokinase, the pace-maker of glycolysis.

See also