d-Galacturonic acid is a sugar acid, an oxidized form of d-galactose. It is the main component of pectin, in which it exists as the polymer polygalacturonic acid. In its open form, it has an aldehyde group at C1 and a carboxylic acid group at C6. Other oxidized forms of d-galactose are d-galactonic acid (carboxylic group at C1) and meso-galactaric acid (mucic acid) (carboxylic groups at C1 and C6). It is also a uronic acid or hexuronic acid. Naturally occurring uronic acids are d-glucuronic acid, d-galacturonic acid, l-iduronic acid and d-mannuronic acid.