Figure 1

A schematic illustration of the trans and cis isomers of the peptide bond between proline (on the left of each structure shown) and another amino acid (P₁, on the right). The interconversion between the two forms is catalyzed by cyclophilins and other peptidyl-prolyl isomerases (PPIases) [7]. The carbon atoms of the proline are indicated by Greek letters; P₂ indicates a third amino acid on the other side of the proline. The peptide bond has some double-bond character and is planar.