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A citrate can refer either to the conjugate base of citric acid, (C₃H₅O(COO)₃³⁻), or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is trimethyl citrate.

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 peroxide 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. It is also permissible for most applications simply dissolve the NaCl and Na₃-citrate without subsequently adjusting the pH.

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.

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. 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 committment step in fatty acid synthesis). In short: Citrate is transported to the cytoplasm, converted to acetyl coa which is converted into malonyl coa by acetyl coa carboxylase enzyme which is allosterically modulated by citrate.

See also TCA cycle

Role in Glycolysis

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