Tight binding of transition-state analogues to a peptidyl-aminoacyl-L/D-isomerase from frog skin.

d-Amino acids in peptide linkage are a noteworthy exception from the universal homochirality of proteins and peptides.[1] The first animal peptide that was found to contain a d-amino acid as the second residue was dermorphin, isolated from the skin of a South American tree frog.[2] This peptide binds with high affinity to μ-opiate receptors. The d-residue in this and related amphibian opioid peptides is essential for the biological function, whereas the all-L-isomers lack activity.