Peptidylprolyl isomerase D (cyclophilin D), also known as PPID, is an enzyme which in humans is encoded by the PPID gene on chromosome 4. As a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family, this protein catalyzes the cis-trans isomerization of proline imidic peptide bonds, which allows it to facilitate folding or repair of proteins. In addition, PPID participates in many biological processes, including mitochondrial metabolism, apoptosis, redox, and inflammation, as well as in related diseases and conditions, such as ischemic reperfusion injury, AIDS, and cancer. Structure Like other cyclophilins, PPID forms a β-barrel structure with a hydrophobic core. This β-barrel is composed of eight anti-parallel β-strands and capped by two α-helices at the top and bottom. In addition, the β-turns and loops in the strands contribute to the flexibility of the barrel. PPID in particular is composed of 370 residues and shares structural homology with PPIF, FKBP4, and FKBP5, including an N-terminal immunophilin-like domain and a C-terminal tetratricopeptide repeat (TPR) domain. Function The protein encoded by this gene is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family. PPIases catalyze the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and accelerate the folding of proteins. Generally, PPIases are found in all eubacteria and eukaryotes, as well as in a few archaea, and thus are highly conserved. The PPIase family is further divided into three structurally distinct subfamilies: cyclophilin (CyP), FK506-binding protein (FKBP), and parvulin (Pvn). As a cyclophilin, PPID binds cyclosporin A (CsA) and can be found within in the cell or secreted by the cell. In eukaryotes, cyclophilins localize ubiquitously to many cell and tissue types. In addition to PPIase and protein chaperone activities, cyclophilins also function in mitochondrial metabolism, apoptosis, immunological response, inflammation, and cell growth and proliferation. PPID in particular helps chaperone the assembly of heat shock protein Hsp90, as well as the nuclear localization of glucocorticoid, estrogen and progesterone receptors. Along with PPIF, PPID regulates mitochondrial apoptosis. In response to elevated reactive oxygen species (ROS) and calcium ion levels, PPID interacts with Bax to promote mitochondrial pore formation, thus releasing pro-apoptotic factors such as cytochrome C and AIF. Clinical Significance As a cyclophilin, PPID binds the immunosuppressive drug CsA to form a CsA-cyclophilin complex, which then targets calcineurin to inhibit the signaling pathway for T-cell activation. In cardiac myogenic cells, cyclophilins have been observed to be activated by heat shock and hypoxia-reoxygenation as well