epimers

Meaning: Epimers are a subset of diastereomers. Two molecules are epimers when they have opposite configuration at one stereocenter and identical configuration at all the others. They are not mirror images, so they are different compounds with different physical properties, and enzymes treat them as distinct substrates. D-glucose and D-galactose differ only at C4. D-glucose and D-mannose differ only at C2. Anomers are epimers at the anomeric carbon (C1 in glucose), which becomes a new stereocenter when the sugar closes into a ring. The two forms are α and β.

On the MCAT: You are usually given Fischer projections and asked how two sugars are related. Compare the left or right position of each OH group, carbon by carbon. One difference means epimers. Every center different means enantiomers. Two or more differences, but not all, means diastereomers that are not epimers. Passages may also describe mutarotation, in which α and β anomers interconvert in solution through the open-chain form, or an epimerase enzyme that converts one sugar into its epimer.

Common confusion: Students call D-glucose and L-glucose epimers. They differ at every stereocenter, so they are enantiomers. Count the differing centers before naming the relationship.