(A) Overview of the nomenclature of the different parts of the sequence of hDPPI. One-letter amino acid codes are used. (B) Schematic drawing of the hDPPI structure in complex with a generic peptide substrate. The amino acids on the peptide bound to the enzyme are numbered P2 and P1 on the N-terminal side of the scissile bond and P1′, P2′, etc. on the C-terminal side. The corresponding substrate binding sites on the enzyme are numbered S2, S1, S1′, S2′, etc. [32,34].