Enantiomers Can Be Distinguished by Biological Molecules

Enzymes and receptors can tell the difference between enantiomers because enzymes and receptors are proteins, and proteins are chiral molecules. 

An achiral reagent reacts identically with both enantiomers. A sock, which is achiral, fits on either foot. 

A chiral reagent reacts differently with each enantiomer. A shoe, which is chiral, fits on only one foot. 

An achiral reagent, such as hydroxide ion, cannot distinguish between enantiomers. Thus, it reacts with (i )-2-bromobutane at the same rate that it reacts with (5)-2-bromobutane. 

Because an enzyme is chiral, not only can it distinguish between cis-trans isomers, such as maleate and fumarate (Section 6.16), but it can also distinguish between enantiomers and catalyze the reaction of only one of them. 

---