4-Hydroxybenzoic acid, also known as p-hydroxybenzoate or 4-carboxyphenol, belongs to the class of organic compounds known as hydroxybenzoic acid derivatives. Hydroxybenzoic acid derivatives are compounds containing a hydroxybenzoic acid (or a derivative), which is a benzene ring bearing a carboxyl and a hydroxyl groups. 4-Hydroxybenzoic acid 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. It is a nutty and phenolic tasting compound. 4-Hydroxybenzoic acid exists in all living species, ranging from bacteria to plants to humans. 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 A√ßa√≠ oil, obtained from the fruit of the a√ßa√≠ palm (Euterpe oleracea), at relatively high concetrations (892¬±52 mg/kg). It is also found in cloudy olive oil and in the edible mushroom Russula virescens. It has been detected in red huckleberries, rabbiteye blueberries, and corianders and in a lower concentration in olives, red raspberries, and almonds. In humans, 4-hydroxybenzoic acid is involved in ubiquinone biosynthesis. In particular, 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. 4-Hydroxybenzoic acid can be biosynthesized by the enzyme Chorismate lyase. 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. 4-Hydroxybenzoate is an intermediate in many enzyme-mediated reactions in microbes. For instance, 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. 4-hydroxybenzaldehyde dehydrogenase is also found in carrots. 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 and is 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 and is found in Pseudomonas putida and Pseudomonas fluorescens. 4-Hydroxybenzoic acid is a popular antioxidant in part because of its low toxicity. 4-Hydroxybenzoic acid has estrogenic activity both in vitro and in vivo (PMID 9417843).