Enantiomers

A special type of isomerism is found in the pairs of structures that are mirror images of each other. These mirror images are called enantiomers, and the two members of the pair are designated as a D- and an L-sugar (Fig.1). The vast majority of the sugars in humans are D-isomers. In the Disomeric form, the –OH group on the asymmetric carbon (a carbon linked to four different atoms or groups) farthest from the carbonyl carbon is on the right, whereas in the L-isomer, it is on the left. Most enzymes are specific for either the D or the L form, but enzymes known as isomerases are able to interconvert D- and L-isomers.

Figure 1: Enantiomers (mirror images) of glucose. Designation of D and L is by comparison to a triose, glyceraldehyde. [Note: The asymmetric carbons are shown in green.]