This is a crystal structure of chymotrypsin. It (1afq.pdb) was retrieved from the Protein Data Bank. Chymotrypsin cleaves peptide linkages on the carboxyl side of phenylalanine, tyrosine, or tryptophan. Here, you can see an inhibitor, the small wire frame molecule, bound to the active site. The enantionmer of this inhibitor will not fit into the active site. It would be like shoving your right hand into a left-handed glove.