Mapping the stereospecificity of peptidyl prolyl cis/trans isomerases

The stereospecificity of peptidyl prolyl cis/trans isomerases (PPIases) was studied using tetrapeptide substrate analogs in which one amino acid residue was replaced by the cognate D-amino acid in various positions of the peptide chain. Reversed stereocenters around proline markedly increased the rate of the spontaneous trans to cis isomerization of the prolyl bond whereas cis to trans isomerizations were less sensitive. PPIases like human cyclophilin18, human FKBP12, Escherichia coli parvulin10 and the PPIase domain of E. coli trigger factor exhibited stereoselectivity demanding at the P1 to P2' position of the substrate chain. The discriminating factor for stereoselectivity was the lack of formation of the Michaelis complexes of the diastereomeric substrates. However, D-alanine at the P1 position preserved considerable affinity to the active site, and largely prevented activation of the catalytic machinery for all PPIases investigated.