Isotope-Edited NMR of Cyclosporin A Bound to Cyclophilin: Evidence for a Trans 9,10 Amide Bond

The binding of a $^13$C-labeled cyclosporin A (CsA) analog to cyclophilin (peptidyl prolyl isomerase) was examined by means of isotope-edited nuclear magnetic resonance (NMR) techniques. A trans 9,10 peptide bond was adopted when CsA was bound to cyclophilin, in contrast to the cis 9,10 peptide bond found in the crystalline and solution conformations of CsA. Furthermore, nuclear Overhauser effects (NOEs) were observed between the $\zeta_3$ and $\epsilon_3$ protons of the methylleucine (MeLeu) residue at position 9 of CsA and tryptophan$^{121}$ (Trp$^{121}$) and phenylalanine (Phe) protons of cyclophilin, suggesting that the MeLeu$^9$ residue of CsA interacts with cyclophilin. These results illustrate the power of isotope-edited NMR techniques for rapidly providing useful information about the conformations and active site environment of inhibitors bound to their target enzymes.