Examples for artificial Diels—Alderase. (a) tHisF protein with the copper binding site. (b) ACCO protein with the copper-bound cavity and docked substrate. (c) FhuA protein and the copper binding site. (d) Oligonucleotide and the conjugated copper complexes. (e) LmrR protein with the conjugation sites for copper complexes shown in red. (f) αRep A3 protein with the central pocket to conjugate copper complexes. (g) Neocarzinostatin protein with the copper complex noncovalently docked in the pocket via the testosterone tail. (h) FhuA protein with the copper complex conjugated. (i) apo-nitrobindin protein with the covalently conjugated copper complex. (j) Schematic representation that the copper complex with the adenosine tail may bind to the cell-surface receptor and catalyze the D—A reaction. (k) Bovine pancreatic polypeptide and the unnatural amino acid residues introduced to form a copper binding site. Reprinted from references 367, 369, 477-485. Copyright 1021, 2009, 2010, 2019, 2019, 2011, 2017, 2016, 2016, 2016, and 2018 Wiley-VCH and American Chemical Society.