From Proteopediaproteopedia link
Crystal structure of human dipeptidyl peptidase IV (DPPIV) complexed with cyanopyrrolidine (C5-pro-pro) inhibitor 21ag
[DPP4_HUMAN] Cell surface glycoprotein receptor involved in the costimulatory signal essential for T-cell receptor (TCR)-mediated T-cell activation. Acts as a positive regulator of T-cell coactivation, by binding at least ADA, CAV1, IGF2R, and PTPRC. Its binding to CAV1 and CARD11 induces T-cell proliferation and NF-kappa-B activation in a T-cell receptor/CD3-dependent manner. Its interaction with ADA also regulates lymphocyte-epithelial cell adhesion. In association with FAP is involved in the pericellular proteolysis of the extracellular matrix (ECM), the migration and invasion of endothelial cells into the ECM. May be involved in the promotion of lymphatic endothelial cells adhesion, migration and tube formation. When overexpressed, enhanced cell proliferation, a process inhibited by GPC3. Acts also as a serine exopeptidase with a dipeptidyl peptidase activity that regulates various physiological processes by cleaving peptides in the circulation, including many chemokines, mitogenic growth factors, neuropeptides and peptide hormones. Removes N-terminal dipeptides sequentially from polypeptides having unsubstituted N-termini provided that the penultimate residue is proline.        
Publication Abstract from PubMed
A series of (5-substituted pyrrolidinyl-2-carbonyl)-2-cyanopyrrolidine (C5-Pro-Pro) analogues was discovered as dipeptidyl peptidase IV (DPPIV) inhibitors as a potential treatment of diabetes and obesity. X-ray crystallography data show that these inhibitors bind to the catalytic site of DPPIV with the cyano group forming a covalent bond with the serine residue of DPPIV. The C5-substituents make various interactions with the enzyme and affect potency, chemical stability, selectivity, and PK properties of the inhibitors. Optimized analogues are extremely potent with subnanomolar K(i)'s, are chemically stable, show very little potency decrease in the presence of plasma, and exhibit more than 1,000-fold selectivity against related peptidases. The best compounds also possess good PK and are efficacious in lowering blood glucose in an oral glucose tolerance test in ZDF rats.
Discovery, structure-activity relationship, and pharmacological evaluation of (5-substituted-pyrrolidinyl-2-carbonyl)-2-cyanopyrrolidines as potent dipeptidyl peptidase IV inhibitors.,Pei Z, Li X, Longenecker K, von Geldern TW, Wiedeman PE, Lubben TH, Zinker BA, Stewart K, Ballaron SJ, Stashko MA, Mika AK, Beno DW, Long M, Wells H, Kempf-Grote AJ, Madar DJ, McDermott TS, Bhagavatula L, Fickes MG, Pireh D, Solomon LR, Lake MR, Edalji R, Fry EH, Sham HL, Trevillyan JM J Med Chem. 2006 Jun 15;49(12):3520-35. PMID:16759095
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.