4-Hydroxybenzoic acid

4-Hydroxybenzoic acid can be found naturally in coconut.[3] It is one of the main catechins metabolites found in humans after consumption of green tea infusions.[4] It is also found in wine,[5] in vanilla, in Macrotyloma uniflorum (horse gram), carob[6] and in Phyllanthus acidus (Otaheite gooseberry).

Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in p-hydroxybenzoic acid (892±52 mg/kg).[7] It is also found in cloudy olive oil and in the edible mushroom Russula virescens (green-cracking russula).

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.

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 enzyme participates in toluene and xylene degradation in bacteria such as Pseudomonas mendocina. It is also found in carrots (Daucus carota).

The enzyme that 2,4'-dihydroxyacetophenone dioxygenase transforms 2,4'-dihydroxyacetophenone and O₂ into 4-hydroxybenzoate and formate. This enzyme participates in bisphenol A degradation. It can be found in Alcaligenes species.

The enzyme 4-chlorobenzoate dehalogenase uses 4-chlorobenzoate and H₂O to produce 4-hydroxybenzoate and chloride. It can be found in Pseudomonas species.

The enzyme 4-hydroxybenzoyl-CoA thioesterase utilizes 4-hydroxybenzoyl-CoA and H₂O to produce 4-hydroxybenzoate and CoA. This enzyme participates in 2,4-dichlorobenzoate degradation. It can be found in Pseudomonas species.

The enzyme 4-hydroxybenzoate polyprenyltransferase uses a polyprenyl diphosphate and 4-hydroxybenzoate to produce diphosphate and 4-hydroxy-3-polyprenylbenzoate. This enzyme participates in ubiquinone biosynthesis.

The enzyme 4-hydroxybenzoate geranyltransferase utilizes geranyl diphosphate and 4-hydroxybenzoate to produce 3-geranyl-4-hydroxybenzoate and diphosphate. Biosynthetically, alkannin is produced in plants from the intermediates 4-hydroxybenzoic acid and geranyl pyrophosphate. This enzyme is involved in shikonin biosynthesis. It can be found in Lithospermum erythrorhizon.

The enzyme 3-hydroxybenzoate—CoA ligase uses ATP, 3-hydroxybenzoate and CoA to produce AMP, diphosphate and 3-hydroxybenzoyl-CoA. The enzyme works equally well with 4-hydroxybenzoate. It can be found in Thauera aromatica.

The enzyme 4-hydroxybenzoate 1-hydroxylase transforms 4-hydroxybenzoate, NAD(P)H, 2 H⁺ and O₂ into hydroquinone, NAD(P)⁺, H₂O and CO₂. This enzyme participates in 2,4-dichlorobenzoate degradation. It can be found in Candida parapsilosis.

The enzyme 4-hydroxybenzoate 3-monooxygenase transforms 4-hydroxybenzoate, NADPH, H⁺ and O₂ into protocatechuate, NADP⁺ and H₂O. This enzyme participates in benzoate degradation via hydroxylation and 2,4-dichlorobenzoate degradation. It can be found in Pseudomonas putida and Pseudomonas fluorescens.

The enzyme 4-hydroxybenzoate 3-monooxygenase (NAD(P)H) utilizes 4-hydroxybenzoate, NADH, NADPH, H⁺ and O₂ to produce 3,4-dihydroxybenzoate (protocatechuic acid), NAD⁺, NADP⁺ and H₂O. This enzyme participates in benzoate degradation via hydroxylation and 2,4-dichlorobenzoate degradation. It can be found in Corynebacterium cyclohexanicum and in Pseudomonas sp.

The enzyme 4-hydroxybenzoate decarboxylase uses 4-hydroxybenzoate to produce phenol and CO₂. This enzyme participates in benzoate degradation via coenzyme A (CoA) ligation. It can be found in Klebsiella aerogenes (Aerobacter aerogenes).

The enzyme 4-hydroxybenzoate—CoA ligase transforms ATP, 4-hydroxybenzoate and CoA to produce AMP, diphosphate and 4-hydroxybenzoyl-CoA. This enzyme participates in benzoate degradation via CoA ligation. It can be found in Rhodopseudomonas palustris.

Coniochaeta hoffmannii is a plant pathogen that commonly inhabits fertile soil. It is known to metabolize aromatic compounds of low molecular weight, such as p-hydroxybenzoic acid.

The enzyme 4-hydroxybenzoate 4-O-beta-D-glucosyltransferase transforms UDP-glucose and 4-hydroxybenzoate into UDP and 4-(beta-D-glucosyloxy)benzoate. It can be found in the pollen of Pinus densiflora.

4-Hydroxybenzoic acid is produced commercially from potassium phenoxide and carbon dioxide in the Kolbe-Schmitt reaction.[10] It can also be produced in the laboratory by heating potassium salicylate with potassium carbonate to 240 °C, followed by treating with acid.[11]

4-Hydroxybenzoic acid has about one tenth the acidity of benzoic acid, having an acid dissociation constant K_a = 3.3×10⁻⁵ M at 19 °C. Its acid dissociation follows this equation:

HOC₆H₄CO₂H ⇌ HOC₆H₄CO−2 + H⁺

Vectran is a manufactured fiber, spun from a liquid crystal polymer. Chemically it is an aromatic polyester produced by the polycondensation of 4-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid. The fiber has been shown to exhibit strong radiation shielding used by Bigelow Aerospace and produced by StemRad.[12]

4,4′-Dihydroxybenzophenone is generally prepared by the rearrangement of p-hydroxyphenylbenzoate. Alternatively, p-hydroxybenzoic acid can be converted to p-acetoxybenzoyl chloride. This acid chloride reacts with phenol to give, after deacetylation, 4,4′-dihydroxybenzophenone.

Examples of drugs made from PHBA include nifuroxazide, orthocaine, ormeloxifene and proxymetacaine.