4-Hydroxybenzoic acid, also known as p-hydroxybenzoic acid (PHBA), is a monohydroxybenzoic acid, a phenolic derivative of benzoic acid. It is a white crystalline solid that is slightly soluble in water and chloroform but more soluble in polar organic solvents such as alcohols and acetone. 4-Hydroxybenzoic acid is primarily known as the basis for the preparation of its esters, known as parabens, which are used as preservatives in cosmetics and some ophthalmic solutions. It is isomeric with 2-hydroxybenzoic acid, known as salicylic acid, a precursor to aspirin, and with 3-hydroxybenzoic acid. Natural occurrences It is found in plants of the genus Vitex such as V. agnus-castus or V. negundo, and in Hypericum perforatum (St John's wort). It is also found in Spongiochloris spongiosa, a freshwater green alga. The compound is also found in Ganoderma lucidum, a medicinal mushroom with the longest record of use. Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol via 4-hydroxybenzoate. Occurrences in food 4-Hydroxybenzoic acid can be found naturally in coconut. It is one of the main catechins metabolites found in humans after consumption of green tea infusions. It is also found in wine, in vanilla, in Macrotyloma uniflorum (horse gram), carob and in Phyllanthus acidus (Otaheite gooseberry). Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in p-hydroxybenzoic acid (). It is also found in cloudy olive oil and in the edible mushroom Russula virescens (green-cracking russula). Related compounds p-Hydroxybenzoic acid glucoside can be found in mycorrhizal and non-mycorrhizal roots of Norway spruces (Picea abies). Violdelphin is an anthocyanin, a type of plant pigments, found in blue flowers and incorporating two p-hydroxybenzoic acid residues, one rutinoside and two glucosides associated with a delphinidin. Agnuside is the ester of aucubin and p-hydroxybenzoic acid. Metabolism Biosynthesis Chorismate lyase is an enzyme that transforms chorismate into 4-hydroxybenzoate and pyruvate. This enzyme catalyses the first step in ubiquinone biosynthesis in Escherichia coli and other Gram-negative bacteria. Benzoate 4-monooxygenase is an enzyme that utilizes benzoate, NADPH, H+ and O2 to produce 4-hydroxybenzoate, NADP+ and H2O. This enzyme can be found in Aspergillus niger. As an intermediate The enzyme 4-methoxybenzoate monooxygenase (O-demethylating) transforms 4-methoxybenzoate, an electron acceptor AH2 and O2 into 4-hydroxybenzoate, formaldehyde, the reduction product A and H2O. This enzyme participates in 2,4-dichlorobenzoate degradation in Pseudomonas putida. The enzyme 4-hydroxybenzaldehyde dehydrogenase uses 4-hydroxybenzaldehyde, NAD+ and H2O to produce 4-hydroxybenzoate, NADH and H+. This