From a mechanistic point of view, isomerases are enzymes that catalyze the structural rearrangement of isomers. Five subclasses are recognized by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology on the nomenclature and classification of enzymes. Cyclophilin A (CypA; EC 220.127.116.11) is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family, which catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and accelerates the folding of proteins. They are known to bind to cyclosporine, an immunosuppressant which is usually used to suppress rejection after internal organ transplants. More specifically, the cyclosporin-cyclophilin A complex inhibits a calcium/calmodulin-dependent phosphatase, calcineurin, the inhibition of which is thought to halt the production of the pro-inflammatory molecules interleukin 2 and TNF alpha.
 The cyclophilins. P. Wang, J. Heitman. Genome Biol. 2005, 6, 226.