New Results on the Conformations of Potent DP IV (CD26) Inhibitors bearing the N-terminal MWP Structural Motif

Conclusion

Conformational analysis by NMR spectroscopy and molecular modeling revealed a left-handed PPII helix-like structure for Trp²-Tat(1–9) (cis and trans) and an even more flexible structure for TXA2-R(1–9).

PPII helices form a well-defined structural class comparable with the other structures defined in proteins and are characterized by exposed, mobile structures with 4–8 residues, mostly found on the protein surface. Polyproline II helices are mainly identified by their torsion angles of φ∼−75° and Ψ∼145−. They do not form regular interchain hydrogen bonds, but are hydrogen bonded with water molecules. PPII helices have a strong preference for the amino acid proline, although it is not necessarily present. These features were also reported for the parent peptide Tat(1–9)⁴ as well as for the well known DP IV substrates neuropeptide Y and pancreatic polypeptide⁵ suggesting that PPII-like helical structures represent a favored structural class for the interaction with DP IV.

Thus, the considerable enhancement of the inhibition capacity of both Trp²-Tat(1–9) and TXA2-R(1–9) compared to the moderate inhibitor Tat(1–9)², K_i=2.68±0.01 10⁻⁴ M, can only be due to tryptophan in the second position suggesting that its side chain is favored to exhibit attractive hydrophobic interactions with DP IV compared with aspartic acid.

On the other hand, we could show recently that Tat(1–9) and its analogues as well as TXA2-R(1–9) inhibit DP IV according to different inhibition mechanisms (Lorey et al., manuscript submitted). One possible explanation for these findings might be enzyme-ligand interactions relying on multiple weak binding sites as described for PPII helices⁵ rather than specific lock and key binding. Certainly, only an X-ray structure of DP IV would help to understand the interaction of DP IV with inhibitors.