Uronic acid

Uronic acids are a class of sugar acids with both carbonyl and carboxylic acid functional groups. They are derived from monosaccharides, where the terminal carbon's aldehyde group is oxidized to a carboxylic acid. Uronic acids play a crucial role in the metabolism of carbohydrates and in the structural components of polysaccharides in both plants and animals. They are involved in various biological processes, including detoxification, the formation of glycosaminoglycans, and the metabolism of glycoproteins and glycolipids.

Structure and Classification

Uronic acids retain the same number of carbon atoms as the monosaccharides from which they are derived. The most common uronic acids include glucuronic acid, derived from glucose, and galacturonic acid, derived from galactose. Other notable examples are mannuronic acid and iduronic acid, which are components of the polysaccharides alginate and heparan sulfate, respectively.

Biosynthesis

The biosynthesis of uronic acids involves the oxidation of the aldehyde group at the terminal carbon of a monosaccharide to a carboxylic acid. This reaction is catalyzed by specific enzymes known as uronate dehydrogenases. For example, UDP-glucose dehydrogenase catalyzes the conversion of UDP-glucose to UDP-glucuronic acid, a key intermediate in the synthesis of many glycosaminoglycans and glycoconjugates.

Function

Uronic acids serve several important functions in living organisms:

• Detoxification: Glucuronic acid, for instance, conjugates with various hydrophobic molecules, such as bilirubin, drugs, and toxins, making them more water-soluble and easier to excrete.
• Structural Components: Galacturonic acid is a major component of pectin, a structural polysaccharide found in plant cell walls. Mannuronic and iduronic acids are found in alginate and heparan sulfate, respectively, which are important for the structural integrity and function of various tissues.
• Biological Signaling: Certain uronic acid-containing polysaccharides, such as heparan sulfate, are involved in cell signaling and regulation processes.

Metabolism

The metabolism of uronic acids involves their conversion back into monosaccharides or into other metabolites that can enter various biochemical pathways. For example, glucuronic acid can be converted back into glucose or into xylulose, which can then enter the pentose phosphate pathway.

Clinical Significance

Abnormalities in the metabolism of uronic acids can lead to various diseases. For instance, deficiencies in enzymes responsible for the degradation of glycosaminoglycans can lead to mucopolysaccharidoses, a group of lysosomal storage diseases characterized by the accumulation of glycosaminoglycans in tissues.

See Also

• Monosaccharide
• Polysaccharide
• Glycosaminoglycan
• Detoxification