Conformation of an FK506 analog in aqueous solution is similar to the FKBP-bound conformation of FK506

Abstract

The conformation of a water-soluble analog of the immunosuppressant FK506 was determined in aqueous solution using NMR spectroscopy. The three-dimensional structures for both the 7,8-cis and trans isomers of [32-Arg] ascomycin which are present in a 1:1 ratio were found to be quite different from FK506 in the crystal state or in chloroform solution but more closely resemble the structure of FK506 when bound to the FK506-binding protein (FKBP). These results suggest that the FKBP-bound conformation of FK506 largely preexists in aqueous solution and is not induced by the protein as previously postulated. In addition, using uniformly 15 N- 13 C-labeled FKBP and isotope-filtering NMR techniques, it is shown that FKBP binds exclusively the trans form of [32-Arg]ascomycin and does not catalyze the cis/trans interconversion of the inhibitor