Ribosomally produced peptides that contain D-amino acids have been isolated from a number of vertebrate and invertebrate sources. In each case, the D-amino acids are introduced by a posttranslational modification of a parent peptide containing only amino acids of the L-configuration. The only known enzyme to catalyze such a reaction is the peptide epimerase (also known as peptide isomerase) from the venom of the funnel web spider, Agelenopsis aperta. This enzyme interconverts two 48-amino acid long peptide toxins, 1 and 2, that differ only by the stereochemistry at a single serine residue (Figure 3).
Figure 3. Reaction Catalyzed by the Peptide Epimerase.
In a recent paper we reported the synthesis and testing of two pentapeptide analogs that contain modified amino acids at the site normally occupied by the substrate serine residue, J. Org. Chem.2002, 8389. When the L-chloroalanine-containing peptide 3 was incubated with the epimerase it was converted into the dehydroalanine-containing peptide 4 via an elimination of HCl (Figure 4). The dehydroalanine peptide 4 was independently synthesized and found to act as a potent inhibitor of the epimerase (IC50 = 0.5 mM). These results support a direct deprotonation/reprotonation mechanism in which a carbanionic intermediate is formed (Figure 3). The observed inhibition by 4 can be attributed to the sp2-hybridization of the a-carbon in the dehydroalanine unit that mimics the planar geometry of the anionic intermediate.
Figure 4. Reaction of Chloroalanine Pentapetide with Peptide Epimerase.
Andrew Murkin, B. Sc. UBC, Ph.D. Candidate, e-mail: