CRYSTAL-STRUCTURE OF CYCLOPHILIN-A COMPLEXED WITH SUBSTRATE ALA-PRO SUGGESTS A SOLVENT-ASSISTED MECHANISM OF CIS-TRANS ISOMERIZATION

Cyclophilin is a binding protein for the immunosuppressive drug cyclos porin A and is also an enzyme with peptidyl-prolyl cis-trans isomerase activity. The crystal structure of cyclophilin A complexed with the s ubstrate Ala-Pro has been determined and refined to an R factor of 0.1 96 at 1.64-angstrom resolution. The structure shows that only the cis form of Ala-Pro binds cyclophilin A despite the fact that Ala-Pro has an equilibrium majority of the trans form in solution. Simulation of t he cis-trans isomerization in an ESV10 graphics system suggests a solv ent-assisted mechanism in which first the peptidyl-prolyl bond is deso lvated at the ground state by binding to the hydrophobic pocket of the active site, and later the intermediate state is stabilized by a hydr ogen bond between the carbonyl oxygen of the amide bond and a bound wa ter molecule.