 Although parvulin is as active as the larger prolyl isomerases against a short proline-containing test peptide, it has lower relative activity against biological substrates, possibly because the larger molecules have a higher ability to bind the substrate peptide.Parvulin itself contains proline residues and its folding can be accelerated by the presence of cyclophilin; parvulin folding can also be autocatalytic.A eukaryotic homolog of parvulin known as Pin1 is required to execute the transition from G2 phase to M phase in the cell cycle.