Citrate

Citrate refers to the tricarboxylate anion derived from citric acid, a weak organic acid abundant in citrus fruits. In aqueous solution it exists primarily as the citrate ion (C6H5O7^3−) and forms salts such as trisodium citrate and tripotassium citrate, as well as citrate esters. The three carboxyl groups confer metal-chelating properties and a high capacity to buffer pH around physiological values. The pKa values of citric acid are about 3.13, 4.76, and 6.40, so citrate dominates at neutral pH.

In metabolism, citrate is a key intermediate of the citric acid cycle. Citrate synthase condenses acetyl-CoA with oxaloacetate to form citrate, which can be further oxidized to regenerate oxaloacetate. Citrate can also be transported from mitochondria to the cytosol, where it serves as a source of acetyl-CoA for fatty acid and cholesterol synthesis when cellular energy is high.

Citrate and citrate salts have wide applications. They function as buffering and chelating agents in biochemical laboratories and as sequestrants for metal ions. In food and beverage production they serve as acidity regulators (labeled as E331 for sodium, potassium, or ammonium citrate) and as sequestrants to improve texture and stability; emulsifying salts are used in processed cheeses. In medicine, trisodium citrate is used as an anticoagulant in blood collection tubes and in some oral rehydration formulations. Citrates are also employed in cleaning products and industrial processes to bind metals.

Citrate is generally regarded as safe in approved uses. Excessive intake can cause gastrointestinal upset or metabolic disturbances, and sensitive individuals should follow product labels.