Lactose

Lactose is a disaccharide present in milk. It is made up of the monosaccharides glucose and galactose.  The two monosaccarides join via a condensation reaction forming a β1-4 glycosidic bond. Commonly associated with the Lac Operon which is a model complex for understanding selective gene transcription control in E. coli.

In humans the disaccharide is is hydrolysed in humans by the enzyme lactase and by β-galactosidase in bacteria.

Many adults are unable to drink milk as they are unable to hydrolyse the disaccharide into the two monosaccharides. This condition is called lactose intolerance, or hypolactasia. A deficiency of the enzyme lactase is usually the cause of hypolactasia.