Epimers are diastereomers that are related by the inversion of configuration at a single chiral center (1). This definition extends the original meaning of epimer, which was used to identify sugars that differed in configuration at C2 (2). This definition intentionally excludes enantiomers, such as D- and L-alanine, since they are not diastereomers. It also excludes diastereomers that are related by the inversion of more than a single chiral center. Thus, D-glucose and D-mannose are epimers, as are D-glucose and D-galactose. D-mannose and D-galactose are not epimers, however, because they are related by inversion at two chiral centers, C2 and C4 (Fig. 1)
Figure 1. Stereochemical drawings of glucose, mannose, and galactose, with their four chiral carbons. The configurations at C2 and C4 are labeled and distinguish these three sugars. Glucose is an epimer of both mannose and galactose because they differ by the configuration of a single chiral center. Mannose and galactose have different configurations at both C2 and C4 and are not epimers.
The chemical conversion of one epimer to another is called epimerization. If this interconversion is catalyzed by an enzyme, the enzyme is an epimerase. As an example, UDP-glucose-4-epimerase catalyzes the epimerization of the C4 carbon of glucose. In the reaction, UDP-glucose is epimerized to UDP-galactose. When the inversion of configuration occurs to interconvert enantiomers instead of diastereomers, the reaction is a racemization.