FK506-binding protein molecule. Computer model showing the primary (rods) and secondary (alpha- helices, blue, and beta-sheets, purple) structures of the N-terminal of FK506-binding protein 52 (FKBP52). This is a peptidyl-prolyl isomerase enzyme, that is it catalyses the conversion of one peptidyl-prolyl isomer to another. FKBPs are involved in many cellular processes, including cell signalling, protein trafficking and transcription. They are also the target for immunosuppressive drugs, such as FK506, also known as tacrolimus.