Glucuronates

Glucuronic acid (from Greek γλυκύς 'sweet' and οὖρον 'urine') is a uronic acid that was first isolated from urine (hence the name). It is found in many gums such as gum arabic (c. 18%), xanthan, and Kombucha tea and is important for the metabolism of microorganisms, plants and animals. Glucuronic acid (from Greek γλυκύς 'sweet' and οὖρον 'urine') is a uronic acid that was first isolated from urine (hence the name). It is found in many gums such as gum arabic (c. 18%), xanthan, and Kombucha tea and is important for the metabolism of microorganisms, plants and animals. Glucuronic acid is a sugar acid derived from glucose, with its sixth carbon atom oxidized to a carboxylic acid. In living beings, this primary oxidation occurs with UDP-α-D-glucose (UDPG), not with the free sugar. Glucuronic acid, like its precursor glucose, can exist as a linear (carboxo-)aldohexose (<1%), or as a cyclic hemiacetal (furanose or pyranose). Aldohexoses such as D-glucose are capable of forming two furanose forms (α and β) and two pyranose forms (α and β). By the Fischer convention, glucuronic acid has two stereoisomers (enantiomers), D- and L-glucuronic acid, depending on its configuration at C-5. Most physiological sugars are of the D-configuration. Due to ring closure, cyclic sugars have another asymmetric carbon atom (C-1), resulting in two more stereoisomers, named anomers. Depending on the configuration at C-1, there are two anomers of glucuronic acid, α- and β-form. In β-D-glucuronic acid the C-1 hydroxy group is on the same side of the pyranose ring as the carboxyl group. In the free sugar acid, the β-form is prevalent (~64%), whereas in the organism, the α-form UDP-α-D-glucuronic acid (UDPGA) predominates. Carbohydrate stereoisomers, which differ in configuration at only one (other) asymmetric C-atom, are named epimers. For example, D-mannuronic (C-2), D-alluronic (C-3), D-galacturonic (C-4), and L-iduronic acid (C-5) are epimers of glucuronic acid. The nonplanar pyranose rings can assume either chair (in 2 variants) or boat conformation. The preferred conformation depends on spatial interference or other interactions of the substituents. The pyranose form of D-glucose and its derivative D-glucuronic acid prefer the chair 4C1. Additional oxidation at C-1 to the carboxyl level yields the dicarboxylic glucaric acid. Glucuronolactone is the self-ester (lactone) of glucuronic acid. Direct oxidation of an aldose affects the aldehyde group first. A laboratory synthesis of a uronic acid from an aldose requires protecting the aldehyde and hydroxy groups from oxidation, for example by conversion to cyclic acetals (e. g., acetonides).